Related
What is the use of bind() in JavaScript?
Bind creates a new function that will force the this inside the function to be the parameter passed to bind().
Here's an example that shows how to use bind to pass a member method around that has the correct this:
var myButton = {
content: 'OK',
click() {
console.log(this.content + ' clicked');
}
};
myButton.click();
var looseClick = myButton.click;
looseClick(); // not bound, 'this' is not myButton - it is the globalThis
var boundClick = myButton.click.bind(myButton);
boundClick(); // bound, 'this' is myButton
Which prints out:
OK clicked
undefined clicked
OK clicked
You can also add extra parameters after the 1st (this) parameter and bind will pass in those values to the original function. Any additional parameters you later pass to the bound function will be passed in after the bound parameters:
// Example showing binding some parameters
var sum = function(a, b) {
return a + b;
};
var add5 = sum.bind(null, 5);
console.log(add5(10));
Which prints out:
15
Check out JavaScript Function bind for more info and interactive examples.
Update: ECMAScript 2015 adds support for => functions. => functions are more compact and do not change the this pointer from their defining scope, so you may not need to use bind() as often. For example, if you wanted a function on Button from the first example to hook up the click callback to a DOM event, the following are all valid ways of doing that:
var myButton = {
... // As above
hookEvent(element) {
// Use bind() to ensure 'this' is the 'this' inside click()
element.addEventListener('click', this.click.bind(this));
}
};
Or:
var myButton = {
... // As above
hookEvent(element) {
// Use a new variable for 'this' since 'this' inside the function
// will not be the 'this' inside hookEvent()
var me = this;
element.addEventListener('click', function() { me.click() });
}
};
Or:
var myButton = {
... // As above
hookEvent(element) {
// => functions do not change 'this', so you can use it directly
element.addEventListener('click', () => this.click());
}
};
The simplest use of bind() is to make a function that, no matter
how it is called, is called with a particular this value.
x = 9;
var module = {
x: 81,
getX: function () {
return this.x;
}
};
module.getX(); // 81
var getX = module.getX;
getX(); // 9, because in this case, "this" refers to the global object
// create a new function with 'this' bound to module
var boundGetX = getX.bind(module);
boundGetX(); // 81
Please refer to this link on MDN Web Docs for more information:
Function.prototype.bind()
bind allows-
set the value of "this" to an specific object. This becomes very helpful as sometimes this is not what is intended.
reuse methods
curry a function
For example, you have a function to deduct monthly club fees
function getMonthlyFee(fee){
var remaining = this.total - fee;
this.total = remaining;
return this.name +' remaining balance:'+remaining;
}
Now you want to reuse this function for a different club member. Note that the monthly fee will vary from member to member.
Let's imagine Rachel has a balance of 500, and a monthly membership fee of 90.
var rachel = {name:'Rachel Green', total:500};
Now, create a function that can be used again and again to deduct the fee from her account every month
//bind
var getRachelFee = getMonthlyFee.bind(rachel, 90);
//deduct
getRachelFee();//Rachel Green remaining balance:410
getRachelFee();//Rachel Green remaining balance:320
Now, the same getMonthlyFee function could be used for another member with a different membership fee. For Example, Ross Geller has a 250 balance and a monthly fee of 25
var ross = {name:'Ross Geller', total:250};
//bind
var getRossFee = getMonthlyFee.bind(ross, 25);
//deduct
getRossFee(); //Ross Geller remaining balance:225
getRossFee(); //Ross Geller remaining balance:200
From the MDN docs on Function.prototype.bind() :
The bind() method creates a new function that, when called, has its
this keyword set to the provided value, with a given sequence of
arguments preceding any provided when the new function is called.
So, what does that mean?!
Well, let's take a function that looks like this :
var logProp = function(prop) {
console.log(this[prop]);
};
Now, let's take an object that looks like this :
var Obj = {
x : 5,
y : 10
};
We can bind our function to our object like this :
Obj.log = logProp.bind(Obj);
Now, we can run Obj.log anywhere in our code :
Obj.log('x'); // Output : 5
Obj.log('y'); // Output : 10
This works, because we bound the value of this to our object Obj.
Where it really gets interesting, is when you not only bind a value for this, but also for its argument prop :
Obj.logX = logProp.bind(Obj, 'x');
Obj.logY = logProp.bind(Obj, 'y');
We can now do this :
Obj.logX(); // Output : 5
Obj.logY(); // Output : 10
Unlike with Obj.log, we do not have to pass x or y, because we passed those values when we did our binding.
Variables has local and global scopes. Let's suppose that we have two variables with the same name. One is globally defined and the other is defined inside a function closure and we want to get the variable value which is inside the function closure. In that case we use this bind() method. Please see the simple example below:
var x = 9; // this refers to global "window" object here in the browser
var person = {
x: 81,
getX: function() {
return this.x;
}
};
var y = person.getX; // It will return 9, because it will call global value of x(var x=9).
var x2 = y.bind(person); // It will return 81, because it will call local value of x, which is defined in the object called person(x=81).
document.getElementById("demo1").innerHTML = y();
document.getElementById("demo2").innerHTML = x2();
<p id="demo1">0</p>
<p id="demo2">0</p>
Summary:
The bind() method takes an object as an first argument and creates a new function. When the function is invoked the value of this in the function body will be the object which was passed in as an argument in the bind() function.
How does this work in JS anyway
The value of this in javascript is dependent always depends on what Object the function is called. The value of this always refers to the object left of the dot from where is the function is called. In case of the global scope this is window (or global in nodeJS). Only call, apply and bind can alter the this binding differently. Here is an example to show how the this keyword works:
let obj = {
prop1: 1,
func: function () { console.log(this); }
}
obj.func(); // obj left of the dot so this refers to obj
const customFunc = obj.func; // we store the function in the customFunc obj
customFunc(); // now the object left of the dot is window,
// customFunc() is shorthand for window.customFunc()
// Therefore window will be logged
How is bind used?
Bind can help in overcoming difficulties with the this keyword by having a fixed object where this will refer to. For example:
var name = 'globalName';
const obj = {
name: 'myName',
sayName: function () { console.log(this.name);}
}
const say = obj.sayName; // we are merely storing the function the value of this isn't magically transferred
say(); // now because this function is executed in global scope this will refer to the global var
const boundSay = obj.sayName.bind(obj); // now the value of this is bound to the obj object
boundSay(); // Now this will refer to the name in the obj object: 'myName'
Once the function is bound to a particular this value we can pass it around and even put it on properties on other objects. The value of this will remain the same.
The bind() method creates a new function instance whose this value is bound to the value that was passed into bind().
For example:
window.color = "red";
var o = { color: "blue" };
function sayColor(){
alert(this.color);
}
var objectSayColor = sayColor.bind(o);
objectSayColor(); //blue
Here, a new function called objectSayColor() is created from sayColor() by calling bind() and passing in the object o. The objectSayColor() function has a this value equivalent to o, so calling the function, even as a global call, results in the string “blue” being displayed.
Reference : Nicholas C. Zakas - PROFESSIONAL JAVASCRIPT® FOR WEB DEVELOPERS
I will explain bind theoretically as well as practically
bind in javascript is a method -- Function.prototype.bind . bind is a method. It is called on function prototype. This method creates a function whose body is similar to the function on which it is called but the 'this' refers to the first parameter passed to the bind method. Its syntax is
var bindedFunc = Func.bind(thisObj,optionsArg1,optionalArg2,optionalArg3,...);
Example:--
var checkRange = function(value){
if(typeof value !== "number"){
return false;
}
else {
return value >= this.minimum && value <= this.maximum;
}
}
var range = {minimum:10,maximum:20};
var boundedFunc = checkRange.bind(range); //bounded Function. this refers to range
var result = boundedFunc(15); //passing value
console.log(result) // will give true;
Creating a new Function by Binding Arguments to Values
The bind method creates a new function from another function with one or more arguments bound to specific values, including the implicit this argument.
Partial Application
This is an example of partial application. Normally we supply a function with all of its arguments which yields a value. This is known as function application. We are applying the function to its arguments.
A Higher Order Function (HOF)
Partial application is an example of a higher order function (HOF) because it yields a new function with a fewer number of argument.
Binding Multiple Arguments
You can use bind to transform functions with multiple arguments into new functions.
function multiply(x, y) {
return x * y;
}
let multiplyBy10 = multiply.bind(null, 10);
console.log(multiplyBy10(5));
Converting from Instance Method to Static Function
In the most common use case, when called with one argument the bind method will create a new function that has the this value bound to a specific value. In effect this transforms an instance method to a static method.
function Multiplier(factor) {
this.factor = factor;
}
Multiplier.prototype.multiply = function(x) {
return this.factor * x;
}
function ApplyFunction(func, value) {
return func(value);
}
var mul = new Multiplier(5);
// Produces garbage (NaN) because multiplying "undefined" by 10
console.log(ApplyFunction(mul.multiply, 10));
// Produces expected result: 50
console.log(ApplyFunction(mul.multiply.bind(mul), 10));
Implementing a Stateful CallBack
The following example shows how using binding of this can enable an object method to act as a callback that can easily update the state of an object.
function ButtonPressedLogger()
{
this.count = 0;
this.onPressed = function() {
this.count++;
console.log("pressed a button " + this.count + " times");
}
for (let d of document.getElementsByTagName("button"))
d.onclick = this.onPressed.bind(this);
}
new ButtonPressedLogger();
<button>press me</button>
<button>no press me</button>
As mentioned, Function.bind() lets you specify the context that the function will execute in (that is, it lets you pass in what object the this keyword will resolve to in the body of the function.
A couple of analogous toolkit API methods that perform a similar service:
jQuery.proxy()
Dojo.hitch()
Bind Method
A bind implementation might look something like so:
Function.prototype.bind = function () {
const self = this;
const args = [...arguments];
const context = args.shift();
return function () {
return self.apply(context, args.concat([...arguments]));
};
};
The bind function can take any number of arguments and return a new function.
The new function will call the original function using the JS Function.prototype.apply method.The apply method will use the first argument passed to the target function as its context (this), and the second array argument of the apply method will be a combination of the rest of the arguments from the target function, concat with the arguments used to call the return function (in that order).
An example can look something like so:
function Fruit(emoji) {
this.emoji = emoji;
}
Fruit.prototype.show = function () {
console.log(this.emoji);
};
const apple = new Fruit('🍎');
const orange = new Fruit('🍊');
apple.show(); // 🍎
orange.show(); // 🍊
const fruit1 = apple.show;
const fruit2 = apple.show.bind();
const fruit3 = apple.show.bind(apple);
const fruit4 = apple.show.bind(orange);
fruit1(); // undefined
fruit2(); // undefined
fruit3(); // 🍎
fruit4(); // 🍊
/**
* Bind is a method inherited from Function.prototype same like call and apply
* It basically helps to bind a function to an object's context during initialisation
*
* */
window.myname = "Jineesh";
var foo = function(){
return this.myname;
};
//IE < 8 has issues with this, supported in ecmascript 5
var obj = {
myname : "John",
fn:foo.bind(window)// binds to window object
};
console.log( obj.fn() ); // Returns Jineesh
Consider the Simple Program listed below,
//we create object user
let User = { name: 'Justin' };
//a Hello Function is created to Alert the object User
function Hello() {
alert(this.name);
}
//since there the value of this is lost we need to bind user to use this keyword
let user = Hello.bind(User);
user();
//we create an instance to refer the this keyword (this.name);
Simple Explanation:
bind() create a new function, a new reference at a function it returns to you.
In parameter after this keyword, you pass in the parameter you want to preconfigure. Actually it does not execute immediately, just prepares for execution.
You can preconfigure as many parameters as you want.
Simple Example to understand bind:
function calculate(operation) {
if (operation === 'ADD') {
alert('The Operation is Addition');
} else if (operation === 'SUBTRACT') {
alert('The Operation is Subtraction');
}
}
addBtn.addEventListener('click', calculate.bind(this, 'ADD'));
subtractBtn.addEventListener('click', calculate.bind(this, 'SUBTRACT'));
The bind function creates a new function with the same function body as the function it is calling .It is called with the this argument .why we use bind fun. : when every time a new instance is created and we have to use first initial instance then we use bind fun.We can't override the bind fun.simply it stores the initial object of the class.
setInterval(this.animate_to.bind(this), 1000/this.difference);
function.prototype.bind() accepts an Object.
It binds the calling function to the passed Object and the returns
the same.
When an object is bound to a function, it means you will be able to
access the values of that object from within the function using
'this' keyword.
It can also be said as,
function.prototype.bind() is used to provide/change the context of a
function.
let powerOfNumber = function(number) {
let product = 1;
for(let i=1; i<= this.power; i++) {
product*=number;
}
return product;
}
let powerOfTwo = powerOfNumber.bind({power:2});
alert(powerOfTwo(2));
let powerOfThree = powerOfNumber.bind({power:3});
alert(powerOfThree(2));
let powerOfFour = powerOfNumber.bind({power:4});
alert(powerOfFour(2));
Let us try to understand this.
let powerOfNumber = function(number) {
let product = 1;
for (let i = 1; i <= this.power; i++) {
product *= number;
}
return product;
}
Here, in this function, this corresponds to the object bound to the function powerOfNumber. Currently we don't have any function that is bound to this function.
Let us create a function powerOfTwo which will find the second power of a number using the above function.
let powerOfTwo = powerOfNumber.bind({power:2});
alert(powerOfTwo(2));
Here the object {power : 2} is passed to powerOfNumber function using bind.
The bind function binds this object to the powerOfNumber() and returns the below function to powerOfTwo. Now, powerOfTwo looks like,
let powerOfNumber = function(number) {
let product = 1;
for(let i=1; i<=2; i++) {
product*=number;
}
return product;
}
Hence, powerOfTwo will find the second power.
Feel free to check this out.
bind() function in Javascript
The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.
An example for the first part
grabbed from react package useSt8
import { useState } from "react"
function st8() {
switch(arguments.length) {
case 0: return this[0]
case 1: return void this[1](arguments[0])
default: throw new Error("Expected 0 or 1 arguments")
}
}
function useSt8(initial) {
// this in st8 will be something like [state, setSatate]
return st8.bind(useState(initial))
}
// usage
function Counter() {
const count = useSt8(0);
return (
<>
Count: {count()}
<button onClick={() => count(0)}>Reset</button>
<button onClick={() => count(prevCount => prevCount + 1)}>inc</button>
</>
);
}
An example for the second part
const add = (a, b) => a+b
someThis = this
// new function with this value equal to someThis
add5 = add.bind(someThis, 5)
add5(10) // 15
// we don't use this in add decelartion so this will work too.
add10 = add.bind(null, 10)
add10(5) // 15
Here's the simplest possible explanation:
Say you have a function
function _loop(n) { console.log("so: " + n) }
obviously you can call it like _loop(69) as usual.
Rewrite like this:
var _loop = function() { console.log("so: " + this.n) }
Notice there are now
no arguments as such
you use "this. " to get to the named arguments
You can now call the function like this:
_loop.bind( {"n": 420} )
That's it.
Most typical use case:
A really typical use is when you need to add an argument to a callback.
Callbacks can't have arguments.
So just "rewrite" the callback as above.
Simple example
function lol(second, third) {
console.log(this.first, second, third);
}
lol(); // undefined, undefined, undefined
lol('1'); // undefined, "1", undefined
lol('1', '2'); // undefined, "1", "2"
lol.call({first: '1'}); // "1", undefined, undefined
lol.call({first: '1'}, '2'); // "1", "2", undefined
lol.call({first: '1'}, '2', '3'); // "1", "2", "3"
lol.apply({first: '1'}); // "1", undefined, undefined
lol.apply({first: '1'}, ['2', '3']); // "1", "2", "3"
const newLol = lol.bind({first: '1'});
newLol(); // "1", undefined, undefined
newLol('2'); // "1", "2", undefined
newLol('2', '3'); // "1", "2", "3"
const newOmg = lol.bind({first: '1'}, '2');
newOmg(); // "1", "2", undefined
newOmg('3'); // "1", "2", "3"
const newWtf = lol.bind({first: '1'}, '2', '3');
newWtf(); // "1", "2", "3"
Another usage is that you can pass binded function as an argument to another function which is operating under another execution context.
var name = "sample";
function sample(){
console.log(this.name);
}
var cb = sample.bind(this);
function somefunction(cb){
//other code
cb();
}
somefunction.call({}, cb);
In addition to what have been said, the bind() method allows an object to borrow a method from another object without making a copy of that method. This is known as function borrowing in JavaScript.
i did not read above code but i learn something in simple so want to share here about bind method after bind method we can use it as any normal method.
<pre> note: do not use arrow function it will show error undefined </pre>
let solarSystem = {
sun: 'red',
moon : 'white',
sunmoon : function(){
let dayNight = this.sun + ' is the sun color and present in day and '+this.moon + ' is the moon color and prenet in night';
return dayNight;
}
}
let work = function(work,sleep){
console.log(this.sunmoon()); // accessing the solatSystem it show error undefine sunmmon untill now because we can't access directly for that we use .bind()
console.log('i work in '+ work +' and sleep in '+sleep);
}
let outPut = work.bind(solarSystem);
outPut('day','night')
bind is a function which is available in java script prototype, as the name suggest bind is used to bind your function call to the context whichever you are dealing with for eg:
var rateOfInterest='4%';
var axisBank=
{
rateOfInterest:'10%',
getRateOfInterest:function()
{
return this.rateOfInterest;
}
}
axisBank.getRateOfInterest() //'10%'
let knowAxisBankInterest=axisBank.getRateOfInterest // when you want to assign the function call to a varaible we use this syntax
knowAxisBankInterest(); // you will get output as '4%' here by default the function is called wrt global context
let knowExactAxisBankInterest=knowAxisBankInterest.bind(axisBank); //so here we need bind function call to its local context
knowExactAxisBankInterest() // '10%'
What is the use of bind() in JavaScript?
Bind creates a new function that will force the this inside the function to be the parameter passed to bind().
Here's an example that shows how to use bind to pass a member method around that has the correct this:
var myButton = {
content: 'OK',
click() {
console.log(this.content + ' clicked');
}
};
myButton.click();
var looseClick = myButton.click;
looseClick(); // not bound, 'this' is not myButton - it is the globalThis
var boundClick = myButton.click.bind(myButton);
boundClick(); // bound, 'this' is myButton
Which prints out:
OK clicked
undefined clicked
OK clicked
You can also add extra parameters after the 1st (this) parameter and bind will pass in those values to the original function. Any additional parameters you later pass to the bound function will be passed in after the bound parameters:
// Example showing binding some parameters
var sum = function(a, b) {
return a + b;
};
var add5 = sum.bind(null, 5);
console.log(add5(10));
Which prints out:
15
Check out JavaScript Function bind for more info and interactive examples.
Update: ECMAScript 2015 adds support for => functions. => functions are more compact and do not change the this pointer from their defining scope, so you may not need to use bind() as often. For example, if you wanted a function on Button from the first example to hook up the click callback to a DOM event, the following are all valid ways of doing that:
var myButton = {
... // As above
hookEvent(element) {
// Use bind() to ensure 'this' is the 'this' inside click()
element.addEventListener('click', this.click.bind(this));
}
};
Or:
var myButton = {
... // As above
hookEvent(element) {
// Use a new variable for 'this' since 'this' inside the function
// will not be the 'this' inside hookEvent()
var me = this;
element.addEventListener('click', function() { me.click() });
}
};
Or:
var myButton = {
... // As above
hookEvent(element) {
// => functions do not change 'this', so you can use it directly
element.addEventListener('click', () => this.click());
}
};
The simplest use of bind() is to make a function that, no matter
how it is called, is called with a particular this value.
x = 9;
var module = {
x: 81,
getX: function () {
return this.x;
}
};
module.getX(); // 81
var getX = module.getX;
getX(); // 9, because in this case, "this" refers to the global object
// create a new function with 'this' bound to module
var boundGetX = getX.bind(module);
boundGetX(); // 81
Please refer to this link on MDN Web Docs for more information:
Function.prototype.bind()
bind allows-
set the value of "this" to an specific object. This becomes very helpful as sometimes this is not what is intended.
reuse methods
curry a function
For example, you have a function to deduct monthly club fees
function getMonthlyFee(fee){
var remaining = this.total - fee;
this.total = remaining;
return this.name +' remaining balance:'+remaining;
}
Now you want to reuse this function for a different club member. Note that the monthly fee will vary from member to member.
Let's imagine Rachel has a balance of 500, and a monthly membership fee of 90.
var rachel = {name:'Rachel Green', total:500};
Now, create a function that can be used again and again to deduct the fee from her account every month
//bind
var getRachelFee = getMonthlyFee.bind(rachel, 90);
//deduct
getRachelFee();//Rachel Green remaining balance:410
getRachelFee();//Rachel Green remaining balance:320
Now, the same getMonthlyFee function could be used for another member with a different membership fee. For Example, Ross Geller has a 250 balance and a monthly fee of 25
var ross = {name:'Ross Geller', total:250};
//bind
var getRossFee = getMonthlyFee.bind(ross, 25);
//deduct
getRossFee(); //Ross Geller remaining balance:225
getRossFee(); //Ross Geller remaining balance:200
From the MDN docs on Function.prototype.bind() :
The bind() method creates a new function that, when called, has its
this keyword set to the provided value, with a given sequence of
arguments preceding any provided when the new function is called.
So, what does that mean?!
Well, let's take a function that looks like this :
var logProp = function(prop) {
console.log(this[prop]);
};
Now, let's take an object that looks like this :
var Obj = {
x : 5,
y : 10
};
We can bind our function to our object like this :
Obj.log = logProp.bind(Obj);
Now, we can run Obj.log anywhere in our code :
Obj.log('x'); // Output : 5
Obj.log('y'); // Output : 10
This works, because we bound the value of this to our object Obj.
Where it really gets interesting, is when you not only bind a value for this, but also for its argument prop :
Obj.logX = logProp.bind(Obj, 'x');
Obj.logY = logProp.bind(Obj, 'y');
We can now do this :
Obj.logX(); // Output : 5
Obj.logY(); // Output : 10
Unlike with Obj.log, we do not have to pass x or y, because we passed those values when we did our binding.
Variables has local and global scopes. Let's suppose that we have two variables with the same name. One is globally defined and the other is defined inside a function closure and we want to get the variable value which is inside the function closure. In that case we use this bind() method. Please see the simple example below:
var x = 9; // this refers to global "window" object here in the browser
var person = {
x: 81,
getX: function() {
return this.x;
}
};
var y = person.getX; // It will return 9, because it will call global value of x(var x=9).
var x2 = y.bind(person); // It will return 81, because it will call local value of x, which is defined in the object called person(x=81).
document.getElementById("demo1").innerHTML = y();
document.getElementById("demo2").innerHTML = x2();
<p id="demo1">0</p>
<p id="demo2">0</p>
Summary:
The bind() method takes an object as an first argument and creates a new function. When the function is invoked the value of this in the function body will be the object which was passed in as an argument in the bind() function.
How does this work in JS anyway
The value of this in javascript is dependent always depends on what Object the function is called. The value of this always refers to the object left of the dot from where is the function is called. In case of the global scope this is window (or global in nodeJS). Only call, apply and bind can alter the this binding differently. Here is an example to show how the this keyword works:
let obj = {
prop1: 1,
func: function () { console.log(this); }
}
obj.func(); // obj left of the dot so this refers to obj
const customFunc = obj.func; // we store the function in the customFunc obj
customFunc(); // now the object left of the dot is window,
// customFunc() is shorthand for window.customFunc()
// Therefore window will be logged
How is bind used?
Bind can help in overcoming difficulties with the this keyword by having a fixed object where this will refer to. For example:
var name = 'globalName';
const obj = {
name: 'myName',
sayName: function () { console.log(this.name);}
}
const say = obj.sayName; // we are merely storing the function the value of this isn't magically transferred
say(); // now because this function is executed in global scope this will refer to the global var
const boundSay = obj.sayName.bind(obj); // now the value of this is bound to the obj object
boundSay(); // Now this will refer to the name in the obj object: 'myName'
Once the function is bound to a particular this value we can pass it around and even put it on properties on other objects. The value of this will remain the same.
The bind() method creates a new function instance whose this value is bound to the value that was passed into bind().
For example:
window.color = "red";
var o = { color: "blue" };
function sayColor(){
alert(this.color);
}
var objectSayColor = sayColor.bind(o);
objectSayColor(); //blue
Here, a new function called objectSayColor() is created from sayColor() by calling bind() and passing in the object o. The objectSayColor() function has a this value equivalent to o, so calling the function, even as a global call, results in the string “blue” being displayed.
Reference : Nicholas C. Zakas - PROFESSIONAL JAVASCRIPT® FOR WEB DEVELOPERS
I will explain bind theoretically as well as practically
bind in javascript is a method -- Function.prototype.bind . bind is a method. It is called on function prototype. This method creates a function whose body is similar to the function on which it is called but the 'this' refers to the first parameter passed to the bind method. Its syntax is
var bindedFunc = Func.bind(thisObj,optionsArg1,optionalArg2,optionalArg3,...);
Example:--
var checkRange = function(value){
if(typeof value !== "number"){
return false;
}
else {
return value >= this.minimum && value <= this.maximum;
}
}
var range = {minimum:10,maximum:20};
var boundedFunc = checkRange.bind(range); //bounded Function. this refers to range
var result = boundedFunc(15); //passing value
console.log(result) // will give true;
Creating a new Function by Binding Arguments to Values
The bind method creates a new function from another function with one or more arguments bound to specific values, including the implicit this argument.
Partial Application
This is an example of partial application. Normally we supply a function with all of its arguments which yields a value. This is known as function application. We are applying the function to its arguments.
A Higher Order Function (HOF)
Partial application is an example of a higher order function (HOF) because it yields a new function with a fewer number of argument.
Binding Multiple Arguments
You can use bind to transform functions with multiple arguments into new functions.
function multiply(x, y) {
return x * y;
}
let multiplyBy10 = multiply.bind(null, 10);
console.log(multiplyBy10(5));
Converting from Instance Method to Static Function
In the most common use case, when called with one argument the bind method will create a new function that has the this value bound to a specific value. In effect this transforms an instance method to a static method.
function Multiplier(factor) {
this.factor = factor;
}
Multiplier.prototype.multiply = function(x) {
return this.factor * x;
}
function ApplyFunction(func, value) {
return func(value);
}
var mul = new Multiplier(5);
// Produces garbage (NaN) because multiplying "undefined" by 10
console.log(ApplyFunction(mul.multiply, 10));
// Produces expected result: 50
console.log(ApplyFunction(mul.multiply.bind(mul), 10));
Implementing a Stateful CallBack
The following example shows how using binding of this can enable an object method to act as a callback that can easily update the state of an object.
function ButtonPressedLogger()
{
this.count = 0;
this.onPressed = function() {
this.count++;
console.log("pressed a button " + this.count + " times");
}
for (let d of document.getElementsByTagName("button"))
d.onclick = this.onPressed.bind(this);
}
new ButtonPressedLogger();
<button>press me</button>
<button>no press me</button>
As mentioned, Function.bind() lets you specify the context that the function will execute in (that is, it lets you pass in what object the this keyword will resolve to in the body of the function.
A couple of analogous toolkit API methods that perform a similar service:
jQuery.proxy()
Dojo.hitch()
Bind Method
A bind implementation might look something like so:
Function.prototype.bind = function () {
const self = this;
const args = [...arguments];
const context = args.shift();
return function () {
return self.apply(context, args.concat([...arguments]));
};
};
The bind function can take any number of arguments and return a new function.
The new function will call the original function using the JS Function.prototype.apply method.The apply method will use the first argument passed to the target function as its context (this), and the second array argument of the apply method will be a combination of the rest of the arguments from the target function, concat with the arguments used to call the return function (in that order).
An example can look something like so:
function Fruit(emoji) {
this.emoji = emoji;
}
Fruit.prototype.show = function () {
console.log(this.emoji);
};
const apple = new Fruit('🍎');
const orange = new Fruit('🍊');
apple.show(); // 🍎
orange.show(); // 🍊
const fruit1 = apple.show;
const fruit2 = apple.show.bind();
const fruit3 = apple.show.bind(apple);
const fruit4 = apple.show.bind(orange);
fruit1(); // undefined
fruit2(); // undefined
fruit3(); // 🍎
fruit4(); // 🍊
/**
* Bind is a method inherited from Function.prototype same like call and apply
* It basically helps to bind a function to an object's context during initialisation
*
* */
window.myname = "Jineesh";
var foo = function(){
return this.myname;
};
//IE < 8 has issues with this, supported in ecmascript 5
var obj = {
myname : "John",
fn:foo.bind(window)// binds to window object
};
console.log( obj.fn() ); // Returns Jineesh
Consider the Simple Program listed below,
//we create object user
let User = { name: 'Justin' };
//a Hello Function is created to Alert the object User
function Hello() {
alert(this.name);
}
//since there the value of this is lost we need to bind user to use this keyword
let user = Hello.bind(User);
user();
//we create an instance to refer the this keyword (this.name);
Simple Explanation:
bind() create a new function, a new reference at a function it returns to you.
In parameter after this keyword, you pass in the parameter you want to preconfigure. Actually it does not execute immediately, just prepares for execution.
You can preconfigure as many parameters as you want.
Simple Example to understand bind:
function calculate(operation) {
if (operation === 'ADD') {
alert('The Operation is Addition');
} else if (operation === 'SUBTRACT') {
alert('The Operation is Subtraction');
}
}
addBtn.addEventListener('click', calculate.bind(this, 'ADD'));
subtractBtn.addEventListener('click', calculate.bind(this, 'SUBTRACT'));
The bind function creates a new function with the same function body as the function it is calling .It is called with the this argument .why we use bind fun. : when every time a new instance is created and we have to use first initial instance then we use bind fun.We can't override the bind fun.simply it stores the initial object of the class.
setInterval(this.animate_to.bind(this), 1000/this.difference);
function.prototype.bind() accepts an Object.
It binds the calling function to the passed Object and the returns
the same.
When an object is bound to a function, it means you will be able to
access the values of that object from within the function using
'this' keyword.
It can also be said as,
function.prototype.bind() is used to provide/change the context of a
function.
let powerOfNumber = function(number) {
let product = 1;
for(let i=1; i<= this.power; i++) {
product*=number;
}
return product;
}
let powerOfTwo = powerOfNumber.bind({power:2});
alert(powerOfTwo(2));
let powerOfThree = powerOfNumber.bind({power:3});
alert(powerOfThree(2));
let powerOfFour = powerOfNumber.bind({power:4});
alert(powerOfFour(2));
Let us try to understand this.
let powerOfNumber = function(number) {
let product = 1;
for (let i = 1; i <= this.power; i++) {
product *= number;
}
return product;
}
Here, in this function, this corresponds to the object bound to the function powerOfNumber. Currently we don't have any function that is bound to this function.
Let us create a function powerOfTwo which will find the second power of a number using the above function.
let powerOfTwo = powerOfNumber.bind({power:2});
alert(powerOfTwo(2));
Here the object {power : 2} is passed to powerOfNumber function using bind.
The bind function binds this object to the powerOfNumber() and returns the below function to powerOfTwo. Now, powerOfTwo looks like,
let powerOfNumber = function(number) {
let product = 1;
for(let i=1; i<=2; i++) {
product*=number;
}
return product;
}
Hence, powerOfTwo will find the second power.
Feel free to check this out.
bind() function in Javascript
The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.
An example for the first part
grabbed from react package useSt8
import { useState } from "react"
function st8() {
switch(arguments.length) {
case 0: return this[0]
case 1: return void this[1](arguments[0])
default: throw new Error("Expected 0 or 1 arguments")
}
}
function useSt8(initial) {
// this in st8 will be something like [state, setSatate]
return st8.bind(useState(initial))
}
// usage
function Counter() {
const count = useSt8(0);
return (
<>
Count: {count()}
<button onClick={() => count(0)}>Reset</button>
<button onClick={() => count(prevCount => prevCount + 1)}>inc</button>
</>
);
}
An example for the second part
const add = (a, b) => a+b
someThis = this
// new function with this value equal to someThis
add5 = add.bind(someThis, 5)
add5(10) // 15
// we don't use this in add decelartion so this will work too.
add10 = add.bind(null, 10)
add10(5) // 15
Here's the simplest possible explanation:
Say you have a function
function _loop(n) { console.log("so: " + n) }
obviously you can call it like _loop(69) as usual.
Rewrite like this:
var _loop = function() { console.log("so: " + this.n) }
Notice there are now
no arguments as such
you use "this. " to get to the named arguments
You can now call the function like this:
_loop.bind( {"n": 420} )
That's it.
Most typical use case:
A really typical use is when you need to add an argument to a callback.
Callbacks can't have arguments.
So just "rewrite" the callback as above.
Simple example
function lol(second, third) {
console.log(this.first, second, third);
}
lol(); // undefined, undefined, undefined
lol('1'); // undefined, "1", undefined
lol('1', '2'); // undefined, "1", "2"
lol.call({first: '1'}); // "1", undefined, undefined
lol.call({first: '1'}, '2'); // "1", "2", undefined
lol.call({first: '1'}, '2', '3'); // "1", "2", "3"
lol.apply({first: '1'}); // "1", undefined, undefined
lol.apply({first: '1'}, ['2', '3']); // "1", "2", "3"
const newLol = lol.bind({first: '1'});
newLol(); // "1", undefined, undefined
newLol('2'); // "1", "2", undefined
newLol('2', '3'); // "1", "2", "3"
const newOmg = lol.bind({first: '1'}, '2');
newOmg(); // "1", "2", undefined
newOmg('3'); // "1", "2", "3"
const newWtf = lol.bind({first: '1'}, '2', '3');
newWtf(); // "1", "2", "3"
Another usage is that you can pass binded function as an argument to another function which is operating under another execution context.
var name = "sample";
function sample(){
console.log(this.name);
}
var cb = sample.bind(this);
function somefunction(cb){
//other code
cb();
}
somefunction.call({}, cb);
In addition to what have been said, the bind() method allows an object to borrow a method from another object without making a copy of that method. This is known as function borrowing in JavaScript.
i did not read above code but i learn something in simple so want to share here about bind method after bind method we can use it as any normal method.
<pre> note: do not use arrow function it will show error undefined </pre>
let solarSystem = {
sun: 'red',
moon : 'white',
sunmoon : function(){
let dayNight = this.sun + ' is the sun color and present in day and '+this.moon + ' is the moon color and prenet in night';
return dayNight;
}
}
let work = function(work,sleep){
console.log(this.sunmoon()); // accessing the solatSystem it show error undefine sunmmon untill now because we can't access directly for that we use .bind()
console.log('i work in '+ work +' and sleep in '+sleep);
}
let outPut = work.bind(solarSystem);
outPut('day','night')
bind is a function which is available in java script prototype, as the name suggest bind is used to bind your function call to the context whichever you are dealing with for eg:
var rateOfInterest='4%';
var axisBank=
{
rateOfInterest:'10%',
getRateOfInterest:function()
{
return this.rateOfInterest;
}
}
axisBank.getRateOfInterest() //'10%'
let knowAxisBankInterest=axisBank.getRateOfInterest // when you want to assign the function call to a varaible we use this syntax
knowAxisBankInterest(); // you will get output as '4%' here by default the function is called wrt global context
let knowExactAxisBankInterest=knowAxisBankInterest.bind(axisBank); //so here we need bind function call to its local context
knowExactAxisBankInterest() // '10%'
Since I can determine the number of arguments a function expects to have by calling its Function.length property, is there any way for me to programmatically create the right number of parameters to insert into that function at runtime? Example:
var xyz = function(a,b) {};
var bcd = function(a,b,c,d,e,f) { }; // vararg example
var doc = document, func_length = xyz.length;
doc.xyz = (function() {
return function(a,b,c,d,e) { /* works for one but not the other */ } }).call(doc);
/* would prefer to `return function(a,b)` with only 2 parameters, if it is
used for function `xyz` (though returning 5 works fine in this case), and to
`return function(a,b,c,d,e,f)` with 6 if used for function `bcd` (which does
not work with only five params). */
// thinking about xyz.apply(null,arguments)...but which arguments..? :(
// Returning function(a,b,c,d,e) does not support functions with more than five
// parameters...which would mostly be varargs - hence my question
// I am also well aware that I can use an object or an array instead of
// using many params.
/* This is for incorporating a user-defined function for use in my code, while
* allowing for my function to do 'other stuff' afterward. (And allowing for
* varargs, of course).
* Because coding something like: doc.xyz = xyz is inflexible */
As you can see, I don't know how to do this, or if it is even possible. The search bar hasn't given me any other questions like this one, otherwise I would not have asked...
NOTE: This answer is a product of misunderstanding but
may help the future visitors of this site.
Another way:
Do you really need to add parameters? Writing the function this way would be enough:
function foo(){ //No arguments predefined
a = arguments[0] || ""; //first argument or (if not defined) empty string
b = arguments[1] || ""; //second argument etc.
c = arguments[2] || ""; //third argument etc.
alert(a+b+c);
}
foo("Hello ", "world!");
This alerts "Hello world".
The solution you want:
The simplest way:
This is what you've asked for but it's not as simple as the previous solution.
You can define a meta function with all the parameters and a handler function that changes over the time.
(function(){ //Wrapper
var foo_meta = function(a,b,c,d){ //Local meta of foo
alert(a+b+c+d); //Do the code
};
window.foo = function(a,b){ //Global foo
return foo_meta(a,b,"","");
};
window.redefine_foo = function(){ //Global foo-changer
//Rewrites foo
window.foo = function(a,b,c){
return foo_meta(a,b,c,"");
};
};
})(); //Wrapper
//Do some code
foo("a","b");
redefine_foo(); //Rewrite foo
foo("a","b","c");
//Note that foo_meta is not defined here
foo_meta == undefined; //It's safe in the wrapper :)
This will alert "ab" and then "abc". For the meaning of wrapper function, see the references.
Reference:
Arguments array: http://goo.gl/FaLM1H
Wrapping code: http://goo.gl/uQ5sd0
If you send two parameters 6 and 7 to a function doWork(a,b,c,d,e),a=7 and b=6 will be automatically set and rest of the parameters will be ignored.
Why not just pass one object into the function and use JQuery extend.
e.g.
var parm =
{ x: 1, y : 2};
f(p) {
p = $_.extend({...defaults here}, p);
...
}
This is an example for joining the arguments, regardless of the number of arguments, to show how function arguments can be turned into an array and then processed like any other array.
function foo(){ //No arguments predefined
// convert to real array
var args = Array.prototype.slice.call(arguments);
// or if Array generics are available
var args = Array.slice(arguments);
console.log(args.join(' '));
}
foo('Hello', 'world!');
foo('Hello', 'wonderful', 'world!');
Here is the fiddle
Ref: arguments MDN
Well, I think I've figured it out at last. I've realized that there may be no way to 'truly' add a parameter to a function the way that I was asking, but there is a way to emulate the same result:
var doc = document;
var xyz = function(a,b) {};
var bcd = function(a,b,c,d,e,f) {};
var obj = {};
// Now, here it is (mostly (sort of)):
obj.userFunc = function(args) {
var args_Array = [];
for (var i=0;i < arguments.length; i++ ) {
args_Array.push(arguments[i])
}
xyz.apply(null,args_Array); // or 'this'. or 'undefined'. Whatever you want.
// do other stuff
return this; // we know what to do to make 'this' scope explicit
} // end
var thisFunc = 'xyz'
doc[thisFunc] = obj.userFunc;
doc.xyz('h','i');
doc.xyz('h','i','j');
doc.xyz('h','i','j','k');
doc.xyz('h','i').xyz('j','l').xyz('j','q'); // etc.
The trick was to use the arguments object, which conveniently assimilated all the parameters into a readily available object, push each value into an array then apply the function.
In case you're wondering what the fuss was all about, I wanted to completely incorporate a user-defined function into another function, while still being able to do 'other stuff' afterward. My previous example worked to an extent, but did not have support for varargs. This does.
This approach is greatly more flexible than: doc[thisFunc] = userDefinedFunction
:) 4/26/2014
In some Javascript code (node.js specifically), I need to call a function with an unknown set of arguments without changing the context. For example:
function fn() {
var args = Array.prototype.slice.call(arguments);
otherFn.apply(this, args);
}
The problem in the above is that when I call apply, I'm change the context by passing this as the first argument. I'd like to pass args to the function being called without changing the context of the function being called. I essentially want to do this:
function fn() {
var args = Array.prototype.slice.call(arguments);
otherFn.apply(<otherFn's original context>, args);
}
Edit: Adding more detail regarding my specific question. I am creating a Client class that contains a socket (socket.io) object among other info pertaining to a connection. I am exposing the socket's event listeners via the client object itself.
class Client
constructor: (socket) ->
#socket = socket
#avatar = socket.handshake.avatar
#listeners = {}
addListener: (name, handler) ->
#listeners[name] ||= {}
#listeners[name][handler.clientListenerId] = wrapper = =>
# append client object as the first argument before passing to handler
args = Array.prototype.slice.call(arguments)
args.unshift(this)
handler.apply(this, args) # <---- HANDLER'S CONTEXT IS CHANGING HERE :(
#socket.addListener(name, wrapper)
removeListener: (name, handler) ->
try
obj = #listeners[name]
#socket.removeListener(obj[handler.clientListenerId])
delete obj[handler.clientListenerId]
Note that clientListenerId is a custom unique identifier property that is essentially the same as the answer found here.
If I understand you correctly:
changes context
| n | y |
accepts array n | func() | func.call() |
of arguments y | ???????? | func.apply() |
PHP has a function for this, call_user_func_array. Unfortunately, JavaScript is lacking in this regard. It looks like you simulate this behavior using eval().
Function.prototype.invoke = function(args) {
var i, code = 'this(';
for (i=0; i<args.length; i++) {
if (i) { code += ',' }
code += 'args[' + i + ']';
}
eval(code + ');');
}
Yes, I know. Nobody likes eval(). It's slow and dangerous. However, in this situation you probably don't have to worry about cross-site scripting, at least, as all variables are contained within the function. Really, it's too bad that JavaScript doesn't have a native function for this, but I suppose that it's for situations like this that we have eval.
Proof that it works:
function showArgs() {
for (x in arguments) {console.log(arguments[x]);}
}
showArgs.invoke(['foo',/bar/g]);
showArgs.invoke([window,[1,2,3]]);
Firefox console output:
--
[12:31:05.778] "foo"
[12:31:05.778] [object RegExp]
[12:31:05.778] [object Window]
[12:31:05.778] [object Array]
Simply put, just assign the this to what you want it to be, which is otherFn:
function fn() {
var args = Array.prototype.slice.call(arguments);
otherFn.apply(otherFn, args);
}
'this' is a reference to your function's context. That's really the point.
If you mean to call it in the context of a different object like this:
otherObj.otherFn(args)
then simply substitute that object in for the context:
otherObj.otherFn.apply(otherObj, args);
That should be it.
If you bind the function to an object and you use everywhere the bound function, you can call apply with null, but still get the correct context
var Person = function(name){
this.name = name;
}
Person.prototype.printName = function(){
console.log("Name: " + this.name);
}
var bob = new Person("Bob");
bob.printName.apply(null); //window.name
bob.printName.bind(bob).apply(null); //"Bob"
One way that you can work around the change of context that can happen in JavaScript when functions are called, is to use methods that are part of the object's constructor if you need them to be able to operate in a context where this is not going to mean the parent object, by effectively creating a local private variable to store the original this identifier.
I concede that - like most discussions of scope in JavaScript - this is not entirely clear, so here is an example of how I have done this:
function CounterType()
{
var counter=1;
var self=this; // 'self' will now be visible to all
var incrementCount = function()
{
// it doesn't matter that 'this' has changed because 'self' now points to CounterType()
self.counter++;
};
}
function SecondaryType()
{
var myCounter = new CounterType();
console.log("First Counter : "+myCounter.counter); // 0
myCounter.incrementCount.apply(this);
console.log("Second Counter: "+myCounter.counter); // 1
}
These days you can use rest parameters:
function fn(...args) {
otherFn(...args);
}
The only downside is, if you want to use some specific params in fn, you have to extract it from args:
function fn(...args) {
let importantParam = args[2]; //third param
// ...
otherFn(...args);
}
Here's an example to try (ES next version to keep it short):
// a one-line "sum any number of arguments" function
const sum = (...args) => args.reduce((sum, value) => sum + value);
// a "proxy" function to test:
var pass = (...args) => sum(...args);
console.log(pass(1, 2, 15));
I'm not going to accept this as an answer, as I'm still hoping for something more suitable. But here's the approach I'm using right now based upon the feedback on this question so far.
For any class that will be calling Client.prototype.addListener or Client.prototype.removeListener, I did added the following code to their constructor:
class ExampleClass
constructor: ->
# ...
for name, fn of this
this[name] = fn.bind(this) if typeof(fn) == 'function'
message: (recipient, body) ->
# ...
broadcast: (body) ->
# ...
In the above example, message and broadcast will always be bound to the new ExampleClass prototype object when it's instantiated, allowing the addListener code in my original question to work.
I'm sure some of you are wondering why I didn't just do something like the following:
example = new ExampleClass
client.addListener('message', example.bind(example))
# ...
client.removeListener('message', example.bind(example))
The problem is that every time .bind( ) is called, it's a new object. So that means that the following is true:
example.bind(example) != example.bind(example)
As such, the removeListener would never work successfully, thus my binding the method once when the object is instantiated.
Since you seem to want to be using the bind function as it is defined in Javascript 1.8.5, and be able to retrieve the original this object you pass the bind function, I recommend redefining the Function.prototype.bind function:
Function.prototype.bind = function (oThis) {
if (typeof this !== "function") {
throw new TypeError("Function.prototype.bind - what is trying to be bound is not callable");
}
var aArgs = Array.prototype.slice.call(arguments, 1),
fToBind = this,
fNOP = function () {},
fBound = function () {
return fToBind.apply(this instanceof fNOP && oThis
? this
: oThis,
aArgs.concat(Array.prototype.slice.call(arguments)));
};
fNOP.prototype = this.prototype;
fBound.prototype = new fNOP();
/** here's the additional code **/
fBound.getContext = function() {
return oThis;
};
/**/
return fBound;
};
Now you can retrieve the original context that you called the bind function with:
function A() {
return this.foo+' '+this.bar;
}
var HelloWorld = A.bind({
foo: 'hello',
bar: 'world',
});
HelloWorld(); // returns "hello world";
HelloWorld.getContext(); // returns {foo:"hello", bar:"world"};
I was just reminded of this question after a long time. Looking back now, I think what I was really trying to accomplish here was something similar to how the React library works with its automatic binding.
Essentially, each function is a wrapped bound function being called:
function SomeClass() {
};
SomeClass.prototype.whoami = function () {
return this;
};
SomeClass.createInstance = function () {
var obj = new SomeClass();
for (var fn in obj) {
if (typeof obj[fn] == 'function') {
var original = obj[fn];
obj[fn] = function () {
return original.apply(obj, arguments);
};
}
}
return obj;
};
var instance = SomeClass.createInstance();
instance.whoami() == instance; // true
instance.whoami.apply(null) == instance; // true
Just push properties directly to the function's object and call it with it's own "context".
function otherFn() {
console.log(this.foo+' '+this.bar); // prints: "hello world" when called from rootFn()
}
otherFn.foo = 'hello';
otherFn.bar = 'world';
function rootFn() {
// by the way, unless you are removing or adding elements to 'arguments',
// just pass the arguments object directly instead of casting it to Array
otherFn.apply(otherFn, arguments);
}
What is the use of bind() in JavaScript?
Bind creates a new function that will force the this inside the function to be the parameter passed to bind().
Here's an example that shows how to use bind to pass a member method around that has the correct this:
var myButton = {
content: 'OK',
click() {
console.log(this.content + ' clicked');
}
};
myButton.click();
var looseClick = myButton.click;
looseClick(); // not bound, 'this' is not myButton - it is the globalThis
var boundClick = myButton.click.bind(myButton);
boundClick(); // bound, 'this' is myButton
Which prints out:
OK clicked
undefined clicked
OK clicked
You can also add extra parameters after the 1st (this) parameter and bind will pass in those values to the original function. Any additional parameters you later pass to the bound function will be passed in after the bound parameters:
// Example showing binding some parameters
var sum = function(a, b) {
return a + b;
};
var add5 = sum.bind(null, 5);
console.log(add5(10));
Which prints out:
15
Check out JavaScript Function bind for more info and interactive examples.
Update: ECMAScript 2015 adds support for => functions. => functions are more compact and do not change the this pointer from their defining scope, so you may not need to use bind() as often. For example, if you wanted a function on Button from the first example to hook up the click callback to a DOM event, the following are all valid ways of doing that:
var myButton = {
... // As above
hookEvent(element) {
// Use bind() to ensure 'this' is the 'this' inside click()
element.addEventListener('click', this.click.bind(this));
}
};
Or:
var myButton = {
... // As above
hookEvent(element) {
// Use a new variable for 'this' since 'this' inside the function
// will not be the 'this' inside hookEvent()
var me = this;
element.addEventListener('click', function() { me.click() });
}
};
Or:
var myButton = {
... // As above
hookEvent(element) {
// => functions do not change 'this', so you can use it directly
element.addEventListener('click', () => this.click());
}
};
The simplest use of bind() is to make a function that, no matter
how it is called, is called with a particular this value.
x = 9;
var module = {
x: 81,
getX: function () {
return this.x;
}
};
module.getX(); // 81
var getX = module.getX;
getX(); // 9, because in this case, "this" refers to the global object
// create a new function with 'this' bound to module
var boundGetX = getX.bind(module);
boundGetX(); // 81
Please refer to this link on MDN Web Docs for more information:
Function.prototype.bind()
bind allows-
set the value of "this" to an specific object. This becomes very helpful as sometimes this is not what is intended.
reuse methods
curry a function
For example, you have a function to deduct monthly club fees
function getMonthlyFee(fee){
var remaining = this.total - fee;
this.total = remaining;
return this.name +' remaining balance:'+remaining;
}
Now you want to reuse this function for a different club member. Note that the monthly fee will vary from member to member.
Let's imagine Rachel has a balance of 500, and a monthly membership fee of 90.
var rachel = {name:'Rachel Green', total:500};
Now, create a function that can be used again and again to deduct the fee from her account every month
//bind
var getRachelFee = getMonthlyFee.bind(rachel, 90);
//deduct
getRachelFee();//Rachel Green remaining balance:410
getRachelFee();//Rachel Green remaining balance:320
Now, the same getMonthlyFee function could be used for another member with a different membership fee. For Example, Ross Geller has a 250 balance and a monthly fee of 25
var ross = {name:'Ross Geller', total:250};
//bind
var getRossFee = getMonthlyFee.bind(ross, 25);
//deduct
getRossFee(); //Ross Geller remaining balance:225
getRossFee(); //Ross Geller remaining balance:200
From the MDN docs on Function.prototype.bind() :
The bind() method creates a new function that, when called, has its
this keyword set to the provided value, with a given sequence of
arguments preceding any provided when the new function is called.
So, what does that mean?!
Well, let's take a function that looks like this :
var logProp = function(prop) {
console.log(this[prop]);
};
Now, let's take an object that looks like this :
var Obj = {
x : 5,
y : 10
};
We can bind our function to our object like this :
Obj.log = logProp.bind(Obj);
Now, we can run Obj.log anywhere in our code :
Obj.log('x'); // Output : 5
Obj.log('y'); // Output : 10
This works, because we bound the value of this to our object Obj.
Where it really gets interesting, is when you not only bind a value for this, but also for its argument prop :
Obj.logX = logProp.bind(Obj, 'x');
Obj.logY = logProp.bind(Obj, 'y');
We can now do this :
Obj.logX(); // Output : 5
Obj.logY(); // Output : 10
Unlike with Obj.log, we do not have to pass x or y, because we passed those values when we did our binding.
Variables has local and global scopes. Let's suppose that we have two variables with the same name. One is globally defined and the other is defined inside a function closure and we want to get the variable value which is inside the function closure. In that case we use this bind() method. Please see the simple example below:
var x = 9; // this refers to global "window" object here in the browser
var person = {
x: 81,
getX: function() {
return this.x;
}
};
var y = person.getX; // It will return 9, because it will call global value of x(var x=9).
var x2 = y.bind(person); // It will return 81, because it will call local value of x, which is defined in the object called person(x=81).
document.getElementById("demo1").innerHTML = y();
document.getElementById("demo2").innerHTML = x2();
<p id="demo1">0</p>
<p id="demo2">0</p>
Summary:
The bind() method takes an object as an first argument and creates a new function. When the function is invoked the value of this in the function body will be the object which was passed in as an argument in the bind() function.
How does this work in JS anyway
The value of this in javascript is dependent always depends on what Object the function is called. The value of this always refers to the object left of the dot from where is the function is called. In case of the global scope this is window (or global in nodeJS). Only call, apply and bind can alter the this binding differently. Here is an example to show how the this keyword works:
let obj = {
prop1: 1,
func: function () { console.log(this); }
}
obj.func(); // obj left of the dot so this refers to obj
const customFunc = obj.func; // we store the function in the customFunc obj
customFunc(); // now the object left of the dot is window,
// customFunc() is shorthand for window.customFunc()
// Therefore window will be logged
How is bind used?
Bind can help in overcoming difficulties with the this keyword by having a fixed object where this will refer to. For example:
var name = 'globalName';
const obj = {
name: 'myName',
sayName: function () { console.log(this.name);}
}
const say = obj.sayName; // we are merely storing the function the value of this isn't magically transferred
say(); // now because this function is executed in global scope this will refer to the global var
const boundSay = obj.sayName.bind(obj); // now the value of this is bound to the obj object
boundSay(); // Now this will refer to the name in the obj object: 'myName'
Once the function is bound to a particular this value we can pass it around and even put it on properties on other objects. The value of this will remain the same.
The bind() method creates a new function instance whose this value is bound to the value that was passed into bind().
For example:
window.color = "red";
var o = { color: "blue" };
function sayColor(){
alert(this.color);
}
var objectSayColor = sayColor.bind(o);
objectSayColor(); //blue
Here, a new function called objectSayColor() is created from sayColor() by calling bind() and passing in the object o. The objectSayColor() function has a this value equivalent to o, so calling the function, even as a global call, results in the string “blue” being displayed.
Reference : Nicholas C. Zakas - PROFESSIONAL JAVASCRIPT® FOR WEB DEVELOPERS
I will explain bind theoretically as well as practically
bind in javascript is a method -- Function.prototype.bind . bind is a method. It is called on function prototype. This method creates a function whose body is similar to the function on which it is called but the 'this' refers to the first parameter passed to the bind method. Its syntax is
var bindedFunc = Func.bind(thisObj,optionsArg1,optionalArg2,optionalArg3,...);
Example:--
var checkRange = function(value){
if(typeof value !== "number"){
return false;
}
else {
return value >= this.minimum && value <= this.maximum;
}
}
var range = {minimum:10,maximum:20};
var boundedFunc = checkRange.bind(range); //bounded Function. this refers to range
var result = boundedFunc(15); //passing value
console.log(result) // will give true;
Creating a new Function by Binding Arguments to Values
The bind method creates a new function from another function with one or more arguments bound to specific values, including the implicit this argument.
Partial Application
This is an example of partial application. Normally we supply a function with all of its arguments which yields a value. This is known as function application. We are applying the function to its arguments.
A Higher Order Function (HOF)
Partial application is an example of a higher order function (HOF) because it yields a new function with a fewer number of argument.
Binding Multiple Arguments
You can use bind to transform functions with multiple arguments into new functions.
function multiply(x, y) {
return x * y;
}
let multiplyBy10 = multiply.bind(null, 10);
console.log(multiplyBy10(5));
Converting from Instance Method to Static Function
In the most common use case, when called with one argument the bind method will create a new function that has the this value bound to a specific value. In effect this transforms an instance method to a static method.
function Multiplier(factor) {
this.factor = factor;
}
Multiplier.prototype.multiply = function(x) {
return this.factor * x;
}
function ApplyFunction(func, value) {
return func(value);
}
var mul = new Multiplier(5);
// Produces garbage (NaN) because multiplying "undefined" by 10
console.log(ApplyFunction(mul.multiply, 10));
// Produces expected result: 50
console.log(ApplyFunction(mul.multiply.bind(mul), 10));
Implementing a Stateful CallBack
The following example shows how using binding of this can enable an object method to act as a callback that can easily update the state of an object.
function ButtonPressedLogger()
{
this.count = 0;
this.onPressed = function() {
this.count++;
console.log("pressed a button " + this.count + " times");
}
for (let d of document.getElementsByTagName("button"))
d.onclick = this.onPressed.bind(this);
}
new ButtonPressedLogger();
<button>press me</button>
<button>no press me</button>
As mentioned, Function.bind() lets you specify the context that the function will execute in (that is, it lets you pass in what object the this keyword will resolve to in the body of the function.
A couple of analogous toolkit API methods that perform a similar service:
jQuery.proxy()
Dojo.hitch()
Bind Method
A bind implementation might look something like so:
Function.prototype.bind = function () {
const self = this;
const args = [...arguments];
const context = args.shift();
return function () {
return self.apply(context, args.concat([...arguments]));
};
};
The bind function can take any number of arguments and return a new function.
The new function will call the original function using the JS Function.prototype.apply method.The apply method will use the first argument passed to the target function as its context (this), and the second array argument of the apply method will be a combination of the rest of the arguments from the target function, concat with the arguments used to call the return function (in that order).
An example can look something like so:
function Fruit(emoji) {
this.emoji = emoji;
}
Fruit.prototype.show = function () {
console.log(this.emoji);
};
const apple = new Fruit('🍎');
const orange = new Fruit('🍊');
apple.show(); // 🍎
orange.show(); // 🍊
const fruit1 = apple.show;
const fruit2 = apple.show.bind();
const fruit3 = apple.show.bind(apple);
const fruit4 = apple.show.bind(orange);
fruit1(); // undefined
fruit2(); // undefined
fruit3(); // 🍎
fruit4(); // 🍊
/**
* Bind is a method inherited from Function.prototype same like call and apply
* It basically helps to bind a function to an object's context during initialisation
*
* */
window.myname = "Jineesh";
var foo = function(){
return this.myname;
};
//IE < 8 has issues with this, supported in ecmascript 5
var obj = {
myname : "John",
fn:foo.bind(window)// binds to window object
};
console.log( obj.fn() ); // Returns Jineesh
Consider the Simple Program listed below,
//we create object user
let User = { name: 'Justin' };
//a Hello Function is created to Alert the object User
function Hello() {
alert(this.name);
}
//since there the value of this is lost we need to bind user to use this keyword
let user = Hello.bind(User);
user();
//we create an instance to refer the this keyword (this.name);
Simple Explanation:
bind() create a new function, a new reference at a function it returns to you.
In parameter after this keyword, you pass in the parameter you want to preconfigure. Actually it does not execute immediately, just prepares for execution.
You can preconfigure as many parameters as you want.
Simple Example to understand bind:
function calculate(operation) {
if (operation === 'ADD') {
alert('The Operation is Addition');
} else if (operation === 'SUBTRACT') {
alert('The Operation is Subtraction');
}
}
addBtn.addEventListener('click', calculate.bind(this, 'ADD'));
subtractBtn.addEventListener('click', calculate.bind(this, 'SUBTRACT'));
The bind function creates a new function with the same function body as the function it is calling .It is called with the this argument .why we use bind fun. : when every time a new instance is created and we have to use first initial instance then we use bind fun.We can't override the bind fun.simply it stores the initial object of the class.
setInterval(this.animate_to.bind(this), 1000/this.difference);
function.prototype.bind() accepts an Object.
It binds the calling function to the passed Object and the returns
the same.
When an object is bound to a function, it means you will be able to
access the values of that object from within the function using
'this' keyword.
It can also be said as,
function.prototype.bind() is used to provide/change the context of a
function.
let powerOfNumber = function(number) {
let product = 1;
for(let i=1; i<= this.power; i++) {
product*=number;
}
return product;
}
let powerOfTwo = powerOfNumber.bind({power:2});
alert(powerOfTwo(2));
let powerOfThree = powerOfNumber.bind({power:3});
alert(powerOfThree(2));
let powerOfFour = powerOfNumber.bind({power:4});
alert(powerOfFour(2));
Let us try to understand this.
let powerOfNumber = function(number) {
let product = 1;
for (let i = 1; i <= this.power; i++) {
product *= number;
}
return product;
}
Here, in this function, this corresponds to the object bound to the function powerOfNumber. Currently we don't have any function that is bound to this function.
Let us create a function powerOfTwo which will find the second power of a number using the above function.
let powerOfTwo = powerOfNumber.bind({power:2});
alert(powerOfTwo(2));
Here the object {power : 2} is passed to powerOfNumber function using bind.
The bind function binds this object to the powerOfNumber() and returns the below function to powerOfTwo. Now, powerOfTwo looks like,
let powerOfNumber = function(number) {
let product = 1;
for(let i=1; i<=2; i++) {
product*=number;
}
return product;
}
Hence, powerOfTwo will find the second power.
Feel free to check this out.
bind() function in Javascript
The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.
An example for the first part
grabbed from react package useSt8
import { useState } from "react"
function st8() {
switch(arguments.length) {
case 0: return this[0]
case 1: return void this[1](arguments[0])
default: throw new Error("Expected 0 or 1 arguments")
}
}
function useSt8(initial) {
// this in st8 will be something like [state, setSatate]
return st8.bind(useState(initial))
}
// usage
function Counter() {
const count = useSt8(0);
return (
<>
Count: {count()}
<button onClick={() => count(0)}>Reset</button>
<button onClick={() => count(prevCount => prevCount + 1)}>inc</button>
</>
);
}
An example for the second part
const add = (a, b) => a+b
someThis = this
// new function with this value equal to someThis
add5 = add.bind(someThis, 5)
add5(10) // 15
// we don't use this in add decelartion so this will work too.
add10 = add.bind(null, 10)
add10(5) // 15
Here's the simplest possible explanation:
Say you have a function
function _loop(n) { console.log("so: " + n) }
obviously you can call it like _loop(69) as usual.
Rewrite like this:
var _loop = function() { console.log("so: " + this.n) }
Notice there are now
no arguments as such
you use "this. " to get to the named arguments
You can now call the function like this:
_loop.bind( {"n": 420} )
That's it.
Most typical use case:
A really typical use is when you need to add an argument to a callback.
Callbacks can't have arguments.
So just "rewrite" the callback as above.
Simple example
function lol(second, third) {
console.log(this.first, second, third);
}
lol(); // undefined, undefined, undefined
lol('1'); // undefined, "1", undefined
lol('1', '2'); // undefined, "1", "2"
lol.call({first: '1'}); // "1", undefined, undefined
lol.call({first: '1'}, '2'); // "1", "2", undefined
lol.call({first: '1'}, '2', '3'); // "1", "2", "3"
lol.apply({first: '1'}); // "1", undefined, undefined
lol.apply({first: '1'}, ['2', '3']); // "1", "2", "3"
const newLol = lol.bind({first: '1'});
newLol(); // "1", undefined, undefined
newLol('2'); // "1", "2", undefined
newLol('2', '3'); // "1", "2", "3"
const newOmg = lol.bind({first: '1'}, '2');
newOmg(); // "1", "2", undefined
newOmg('3'); // "1", "2", "3"
const newWtf = lol.bind({first: '1'}, '2', '3');
newWtf(); // "1", "2", "3"
Another usage is that you can pass binded function as an argument to another function which is operating under another execution context.
var name = "sample";
function sample(){
console.log(this.name);
}
var cb = sample.bind(this);
function somefunction(cb){
//other code
cb();
}
somefunction.call({}, cb);
In addition to what have been said, the bind() method allows an object to borrow a method from another object without making a copy of that method. This is known as function borrowing in JavaScript.
i did not read above code but i learn something in simple so want to share here about bind method after bind method we can use it as any normal method.
<pre> note: do not use arrow function it will show error undefined </pre>
let solarSystem = {
sun: 'red',
moon : 'white',
sunmoon : function(){
let dayNight = this.sun + ' is the sun color and present in day and '+this.moon + ' is the moon color and prenet in night';
return dayNight;
}
}
let work = function(work,sleep){
console.log(this.sunmoon()); // accessing the solatSystem it show error undefine sunmmon untill now because we can't access directly for that we use .bind()
console.log('i work in '+ work +' and sleep in '+sleep);
}
let outPut = work.bind(solarSystem);
outPut('day','night')
bind is a function which is available in java script prototype, as the name suggest bind is used to bind your function call to the context whichever you are dealing with for eg:
var rateOfInterest='4%';
var axisBank=
{
rateOfInterest:'10%',
getRateOfInterest:function()
{
return this.rateOfInterest;
}
}
axisBank.getRateOfInterest() //'10%'
let knowAxisBankInterest=axisBank.getRateOfInterest // when you want to assign the function call to a varaible we use this syntax
knowAxisBankInterest(); // you will get output as '4%' here by default the function is called wrt global context
let knowExactAxisBankInterest=knowAxisBankInterest.bind(axisBank); //so here we need bind function call to its local context
knowExactAxisBankInterest() // '10%'