Here's a sample of a simple Javascript class with a public and private method (fiddle: http://jsfiddle.net/gY4mh/).
function Example() {
function privateFunction() {
// "this" is window when called.
console.log(this);
}
this.publicFunction = function() {
privateFunction();
}
}
ex = new Example;
ex.publicFunction();
Calling the private function from the public one results in "this" being the window object. How should I ensure my private methods are called with the class context and not window? Would this be undesirable?
Using closure. Basically any variable declared in function, remains available to functions inside that function :
var Example = (function() {
function Example() {
var self = this; // variable in function Example
function privateFunction() {
// The variable self is available to this function even after Example returns.
console.log(self);
}
self.publicFunction = function() {
privateFunction();
}
}
return Example;
})();
ex = new Example;
ex.publicFunction();
Another approach is to use "apply" to explicitly set what the methods "this" should be bound to.
function Test() {
this.name = 'test';
this.logName = function() {
console.log(this.name);
}
}
var foo = {name: 'foo'};
var test = new Test();
test.logName()
// => test
test.logName.apply(foo, null);
// => foo
Yet another approach is to use "call":
function Test() {
this.name = 'test';
this.logName = function() {
console.log(this.name);
}
}
var foo = {name: 'foo'};
var test = new Test();
test.logName()
// => test
test.logName.call(foo, null);
// => foo
both "apply" and "call" take the object that you want to bind "this" to as the first argument and an array of arguments to pass in to the method you are calling as the second arg.
It is worth understanding how the value of this in javascript is determined in addition to just having someone tell you a code fix. In javascript, this is determined the following ways:
If you call a function via an object property as in object.method(), then this will be set to the object inside the method.
If you call a function directly without any object reference such as function(), then this will be set to either the global object (window in a browser) or in strict mode, it will be set to undefined.
If you create a new object with the new operator, then the constructor function for that object will be called with the value of this set to the newly created object instance. You can think of this as the same as item 1 above, the object is created and then the constructor method on it is called.
If you call a function with .call() or .apply() as in function.call(xxx), then you can determine exactly what this is set to by what argument you pass to .call() or .apply(). You can read more about .call() here and .apply() here on MDN.
If you use function.bind(xxx) this creates a small stub function that makes sure your function is called with the desired value of this. Internally, this likely just uses .apply(), but it's a shortcut for when you want a single callback function that will have the right value of this when it's called (when you aren't the direct caller of the function).
In a callback function, the caller of the callback function is responsible for determining the desired value of this. For example, in an event handler callback function, the browser generally sets this to be the DOM object that is handling the event.
There's a nice summary of these various methods here on MDN.
So, in your case, you are making a normal function call when you call privateFunction(). So, as expected the value of this is set as in option 2 above.
If you want to explictly set it to the current value of this in your method, then you can do so like this:
var Example = (function() {
function Example() {
function privateFunction() {
// "this" is window when called.
console.log(this);
}
this.publicFunction = function() {
privateFunction.call(this);
}
}
return Example;
})();
ex = new Example;
ex.publicFunction();
Other methods such as using a closure and defined var that = this are best used for the case of callback functions when you are not the caller of the function and thus can't use 1-4. There is no reason to do it that way in your particular case. I would say that using .call() is a better practice. Then, your function can actually use this and can behave like a private method which appears to be the behavior you seek.
I guess most used way to get this done is by simply caching (storing) the value of this in a local context variable
function Example() {
var that = this;
// ...
function privateFunction() {
console.log(that);
}
this.publicFunction = function() {
privateFunction();
}
}
a more convenient way is to invoke Function.prototype.bind to bind a context to a function (forever). However, the only restriction here is that this requires a ES5-ready browser and bound functions are slightly slower.
var privateFunction = function() {
console.log(this);
}.bind(this);
I would say the proper way is to use prototyping since it was after all how Javascript was designed. So:
var Example = function(){
this.prop = 'whatever';
}
Example.prototype.fn_1 = function(){
console.log(this.prop);
return this
}
Example.prototype.fn_2 = function(){
this.prop = 'not whatever';
return this
}
var e = new Example();
e.fn_1() //whatever
e.fn_2().fn_1() //not whatever
Here's a fiddle http://jsfiddle.net/BFm2V/
If you're not using EcmaScript5, I'd recommend using Underscore's (or LoDash's) bind function.
In addition to the other answers given here, if you don't have an ES5-ready browser, you can create your own "permanently-bound function" quite simply with code like so:
function boundFn(thisobj, fn) {
return function() {
fn.apply(thisobj, arguments);
};
}
Then use it like this:
var Example = (function() {
function Example() {
var privateFunction = boundFn(this, function() {
// "this" inside here is the same "this" that was passed to boundFn.
console.log(this);
});
this.publicFunction = function() {
privateFunction();
}
}
return Example;
}()); // I prefer this order of parentheses
Voilà -- this is magically the outer context's this instead of the inner one!
You can even get ES5-like functionality if it's missing in your browser like so (this does nothing if you already have it):
if (!Function.prototype.bind) {
Function.prototype.bind = function (thisobj) {
var that = this;
return function() {
that.apply(thisobj, arguments);
};
}:
}
Then use var yourFunction = function() {}.bind(thisobj); exactly the same way.
ES5-like code that is fully compliant (as possible), checking parameter types and so on, can be found at mozilla Function.prototype.bind. There are some differences that could trip you up if you're doing a few different advanced things with functions, so read up on it at the link if you want to go that route.
I would say assigning self to this is a common technique:
function Example() {
var self = this;
function privateFunction() {
console.log(self);
}
self.publicFunction = function() {
privateFunction();
};
}
Using apply (as others have suggested) also works, though it's a bit more complex in my opinion.
It might be beyond the scope of this question, but I would also recommend considering a different approach to JavaScript where you actually don't use the this keyword at all. A former colleague of mine at ThoughtWorks, Pete Hodgson, wrote a really helpful article, Class-less JavaScript, explaining one way to do this.
Related
I'm new to JavaScript so this is possibly a trivial question:
I'm trying to construct an object that stores a mapping from a set of integers to some of its methods, i.e. something like this:
'use strict';
function Foo() {
this.funcs = {
1: this.func1,
2: this.func2,
}
}
Foo.prototype.func1 = function() {
this.prop = 1;
}
Foo.prototype.func2 = function() {
this.prop = 2;
}
I'd then like to be able to call methods of Foo like this:
foo = new Foo();
var func = foo.funcs[1];
func();
But this results in: Cannot set property 'prop' of undefined, i.e. this does not refer to foo.
What's the problem here and is there a better way to implement this?
Your problem is this line:
var func = foo.funcs[1];
JavaScript determines the value of this based on how a function is called. If you use dot notation, such as foo.funcs[1](); then the value of this will associated with the foo object. But when you run func(), that's just a plain function and this will have the default value of undefined.
It would be worth your time to read the two chapters of You Don't Know JS that discuss this. It should take less than an hour to learn, and you'll be way ahead of most JS programmers once you learn it.
The rules might not make sense until you read the chapter, but they are summarized below:
Determining the this binding for an executing function requires
finding the direct call-site of that function. Once examined, four
rules can be applied to the call-site, in this order of precedence:
Called with new? Use the newly constructed object.
Called with call or apply (or bind)? Use the specified object.
Called with a context object owning the call? Use that context object.
Default: undefined in strict mode, global object otherwise.
Based on the above rules, the code below is the simplest way you could get it to work the way you are expecting it to:
'use strict';
function Foo() {
this.funcs = {
1: this.func1,
2: this.func2,
}
}
Foo.prototype.func1 = function() {
this.prop = 1;
console.log('called func1. this.prop =', this.prop);
}
Foo.prototype.func2 = function() {
this.prop = 2;
console.log('called func2. this.prop =', this.prop);
}
const foo = new Foo();
foo.funcs[1]();
There are a few ways to achieve what you require, however the most robust approach is to bind() each function to the instance of Foo() that is being instantiated.
This can be done by passing this to bind() of each function:
this.func1.bind(this)
Using bind() in this way ensures that this, for func1 and func2 is defined as the instance of Foo(). This in turn ensures that this.prop can be accessed and assigned as expected:
'use strict';
function Foo() {
this.funcs = {
/* Bind the functions to this Foo() instance */
1: this.func1.bind(this),
2: this.func2.bind(this),
}
}
Foo.prototype.func1 = function() {
this.prop = 1;
console.log('called func1. this.prop =', this.prop);
}
Foo.prototype.func2 = function() {
this.prop = 2;
console.log('called func2. this.prop =', this.prop);
}
const foo = new Foo();
var func = foo.funcs[1];
func();
foo.funcs[2]();
Another key thing to note is the bind() based approach above ensures that, if you acquire and call a reference to one of the functions on the funcs field as shown in your original post, that it will work as expected:
/* As per original post - doing this is not possible without .bind() */
var func = foo.funcs[1];
func();
Without the use of bind(), this method of acquiring and calling func will fail due to func not being bound to the instance of Foo.
Background
I want a function keeping track of its own state:
var myObject = {
myFunction: function () {
var myself = this.myFunction;
var firstTime = Boolean(!myself.lastRetry);
if (firstTime) {
myself.lastRetry = Date.now();
return true;
}
// some more code
}
}
The problem with the above code is that the value of this will depend on the site of the function call. I want the function to be able to refer to itself without using:
myObject.myFunction
.bind()
.apply()
.call()
Question
Is it possible to give a function this kind of self awareness independent of its call site and without any help from external references to it?
If you want to store that state on the function instance, give the function a name, and use that name within it:
var myObject = {
myFunction: function theFunctionName() {
// ^^^^^^^^^^^^^^^--------------------- name
var firstTime = Boolean(!theFunctionName.lastRetry);
// ^--------------------------- using it
if (firstTime) {
theFunctionName.lastRetry = Date.now();
// ^------------------------------------------------ using it
return true;
}
// some more code
}
};
You'd do that whenever you want to use a function recursively as well. When you give a name to a function that way (putting the name after function and before (), that name is in-scope within the function's own code. (It's not in-scope for the code containing the function if it's a function expression, but it is if it's a function declaration. Yours is an expression.)
That's a named function expression (where previously you had an anonymous function expression). You may hear warnings about NFEs, but the issues various JavaScript implementations had with them are essentially in the past. (IE8 still handles them incorrectly, though: More in this post on my blog.)
You might consider keeping that state somewhere private, though, via an IIFE:
var myObject = (function(){
var lastRetry = null;
return {
myFunction: function() {
var firstTime = Boolean(!lastRetry);
if (firstTime) {
lastRetry = Date.now();
return true;
}
// some more code
}
};
})();
Now, nothing outside that outer anonymous function can see lastRetry at all. (And you don't have to worry about IE8, if you're supporting stubborn XP users. :-) )
Side note: The unary ! operator always returns a boolean, so your
var firstTime = Boolean(!theFunctionName.lastRetry);
...is exactly equivalent to:
var firstTime = !theFunctionName.lastRetry;
...but with an extra unnecessary function call. (Not that it hurts anything.)
Of course you can, simply give your function an internal named representation and it can refer to itself from there. For example...
var obj = {
doThings:function doThingsInternal(arg1, arg2) {
console.log(arg1, arg2);
for (var arg in doThingsInternal.arguments) {
console.log(arg);
}
}
};
obj.doThings('John', 'Doe');
You could use a simple Closure, if you are not too bent on keeping state existence knowledge within the function. But I guess you don't want that. Another way to do this could be changing the function itself on the first call. Benefits, no/less state variables needed and no costly checks on subsequent calls! -
var myObject = {
myFunction: function () {
// Whatever you wanna do on the first call...
// ...
// And then...
this.myFunction = function(){
// Change the definition to whatever it should do
// in the subsequent calls.
}
// return the first call value.
}
};
You can extend this model to any states by changing the function definition per your state.
I have target object
function Foo() {
this.someVar = 'some var';
};
Foo.prototype.callback() {
console.log(this);
};
And object, that will call this callback
function Bar(callback) {
this.callback = callback;
};
Bar.prototype.onSomeAction = function() {
this.callback();
};
And initial code
foo = new Foo();
bar = new Bar();
bar.callback = foo.callback;
bar.onSomeAction();
Result: i have logged to console Bar()'s context instead of Foo().
How can i get context of Foo() in the Foo() callback?
PS: I tried closures
Foo.prototype.callback() {
var foo = this;
return function(foo) {
console.log(foo);
};
};
but it does nothing. I have not fully understanding of the closures :(
The reason your original code didn't work is that the value of this inside of a method call is the value of the object it's being called on. That means when you say:
bar.callback = foo.callback;
And then you call:
bar.callback();
The code defined here:
Foo.prototype.callback = function () {
console.log(this);
};
gets called with this being a reference to bar because bar is to the left of the . on the method call. So whenever you assign a function as an object property, calling it on that object will call it with the object as this.
You could also have written:
function callback() {
console.log(this);
}
bar.callback = callback;
bar.callback();
And you would find that this still references bar.
In fact, if you call the plain function callback(); as defined above, you'll find that this is a reference to the global object, usually window in web browsers. That's because all global variables and functions are properties of window, so callback(); is implicitly window.callback();
The fact that the value of this depends on what object is calling a function can be a problem when passing callbacks around, since sometimes you want this to reference the original object the function was a property of. The bind method was design to solve this problem, and Yuri Sulyma gave the right answer:
bar.callback = foo.callback.bind(foo);
However, the way you would do this using closures is to capture an instance of Foo within an anonymous function that calls the correct method on the correct object:
foo = new Foo();
bar = new Bar();
bar.callback = function () {
foo.callback();
};
bar.onSomeAction();
Which is essentially what bind does. In fact, we call write our own naive version of bind using a closure:
Function.prototype.bind = function (obj) {
var fn = this;
return function () {
fn.call(obj);
};
};
call let's you call a function with the value of this explicitly defined. This allows you to "set the context" the function is called in so that it's the same as calling obj.fn() when you call bar.callback(). Since when we call foo.callback.bind(foo);, obj is foo and fn is foo.callback, the result is that calling bar.callback() becomes the same as calling foo.callback().
That's where Dalorzo's answer comes from. He uses call to explicitly set the context.
There's also another function for setting the context called apply that also takes an array representing the arguments for the function as its second argument. This allows us to write a more complete version of bind by taking advantage of the special arguments variable:
Function.prototype.bind = function (obj) {
var fn = this;
return function () {
fn.apply(obj, arguments);
};
};
bar.callback = foo.callback.bind(foo);
You can polyfill Function.prototype.bind() if necessary: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind#Compatibility
Try using these changes:
Use call to set context:
bar.onSomeAction.call(foo);
And I think your callback function needs to change to:
Foo.prototype.callback=function() {
console.log(this);
};
I have this Javascript constructor-
function TestEngine() {
this.id='Foo';
}
TestEngine.prototype.fooBar = function() {
this.id='bar';
return true;
}
TestEngine.prototype.start = function() {
this.fooBar();
}
TestEngine.prototype.startMethod = function() {
inter = setInterval(this.start, 200);
}
var test = new TestEngine();
test.startMethod();
Gives me this error -
Uncaught TypeError: Object [object global] has no method 'fooBar'
I tried console.log and found out that when I call this.start from within setInterval, this points to the window object. Why is this so?
The this pointer can point to one of many things depending upon the context:
In constructor functions (function calls preceded by new) this points to the newly created instance of the constructor.
When a function is called as a method of an object (e.g. obj.funct()) then the this pointer inside the function points to the object.
You can explicitly set what this points to by using call, apply or bind.
If none of the above then the this pointer points to the global object by default. In browsers this is the window object.
In your case you're calling this.start inside setInterval. Now consider this dummy implementation of setInterval:
function setInterval(funct, delay) {
// native code
}
It's important to understand that start is not being called as this.start. It's being called as funct. It's like doing something like this:
var funct = this.start;
funct();
Now both these functions would normally execute the same, but there's one tiny problem - the this pointer points to the global object in the second case while it points to the current this in the first.
An important distinction to make is that we're talking about the this pointer inside start. Consider:
this.start(); // this inside start points to this
var funct = this.start;
funct(); // this inside funct (start) point to window
This is not a bug. This is the way JavaScript works. When you call a function as a method of an object (see my second point above) the this pointer inside the function points to that object.
In the second case since funct is not being called as a method of an object the fourth rule is applied by default. Hence this points to window.
You can solve this problem by binding start to the current this pointer and then passing it to setInterval as follows:
setInterval(this.start.bind(this), 200);
That's it. Hope this explanation helped you understand a little bit more about the awesomeness of JavaScript.
Here is a neat way to do OOP with javascript:
//Global Namespace:
var MyNamespace = MyNamespace || {};
//Classes:
MyNamespace.MyObject = function () {
this.PublicVar = 'public'; //Public variable
var _privatVar = 'private'; //Private variable
//Public methods:
this.PublicMethod = function () {
}
//Private methods:
function PrivateMethod() {
}
}
//USAGE EXAMPLE:
var myObj = new MyNamespace.MyObject();
myObj.PublicMethod();
This way you encapsulate your methods and variables into a namespace/class to make it much easier use and maintain.
Therefore you could write your code like this:
var MyNamespace = MyNamespace || {};
//Class: TestEngine
MyNamespace.TestEngine = function () {
this.ID = null;
var _inter = null;
//Public methods:
this.StartMethod = function (id) {
this.ID = id;
_inter = setInterval(Start, 1000);
}
//Private methods:
function Start() {
FooBar();
console.log(this.ID);
}
function FooBar() {
this.ID = 'bar';
return true;
}
}
//USAGE EXAMPLE:
var testEngine = new MyNamespace.TestEngine();
testEngine.StartMethod('Foo');
console.log(testEngine.ID);
Initially, the ID is set to 'Foo'
After 1 second the ID is set to 'bar'
Notice all variables and methods are encapsulated inside the TestEngine class.
Try this:
function TestEngine() {
this.id='Foo';
}
TestEngine.prototype.fooBar = function() {
this.id='bar';
return true;
}
TestEngine.prototype.start = function() {
this.fooBar();
}
TestEngine.prototype.startMethod = function() {
var self = this;
var inter = setInterval(function() {
self.start();
}, 200);
}
var test = new TestEngine();
test.startMethod();
setInterval calls start function with window context. It means when start gets executed, this inside start function points to window object. And window object don't have any method called fooBar & you get the error.
Anonymous function approach:
It is a good practice to pass anonymous function to setInterval and call your function from it. This will be useful if your function makes use of this.
What I did is, created a temp variable self & assigned this to it when it is pointing your TestEngine instance & calling self.start() function with it.
Now inside start function, this will be pointing to your testInstance & everything will work as expected.
Bind approach:
Bind will make your life easier & also increase readability of your code.
TestEngine.prototype.startMethod = function() {
setInterval(this.start.bind(this), 200);
}
var name = 'The Window';
var object = {
name: 'My Object',
getNameFunc: function(){
return function() {
return this.name;
};
}
};
console.log( object.getNameFunc()() );
when i tested the code. the result is The Window. but i think this.name; should be My Object. what's wrong with my thinking.
when i add var before the name : "My Object", it show's an error.? why?
this inside a function is the "receiver" it was invoked upon.
That is,
for the construct x.f(), this inside the function (f) will evaluate to the value of x.
for all other cases, this will evaluate to window inside the invoked function. (The functions call, apply, and bind can also alter this... but that's another story.)
In the posted example the second function (the one with this.name) is not invoked using the x.f() form and so this is the window object.
The "simple fix" is to use a closure: (The first function is invoked in the x.f() form and thus this is the same as object, which is as expected. We capture the value of this in the current scope via a closure created with self and the returned function.)
getNameFunc : function () {
var self = this
return function () {
return self.name
}
}
However, I may consider another design, depending :)
Happy coding.
Additional clarification, for comment:
...that is because you are using circle.getArea() which is of the form x.f(). Thus this inside the getArea function evaluates to circle.
In the code posted you are invoking two different functions in a row. Imagine writing the code like this:
var nameFunc = object.getNameFunc()
nameFunc()
The first function call is in the form of x.f() and thus this inside getNameFunc is the evaluation of object. However, in the second line, the function (nameFunc) is not invoked in the form x.f(). Therefore, the this inside nameFunc (the function returned from getNameFunc) will evaluate to window, as discussed above.
var myObject = {
name:'My Object'
};
console.log(myObject.name);
console.log(myObject['name']);
There are various other ways to make objects in javascript.
this is a hidden argument that is automatically passed from the calling function to the callee. The traditional way is to do:
function MyObject() {
this.name = 'My Object';
}
myObject = new MyObject();
console.log(myObject.name);
Nowadays you might just use closures:
[**edit**: redacted because not a good method]
Nowadays you might just use closures, correctly:
function makeObject() {
var THIS = {};
THIS.name = 'My Object';
THIS.sayMyName = function () {
return THIS.name+" is my name";
}
return THIS;
}
There are many libraries that support "smarter" ways to make objects as well.
You need to use .bind() to set the right context for the method, so the this keyword will be what you want it to actually be.
The default is in such a scenario for the this keyword is to point to the window object, because...this is how the JS engine works.
var name = "The Window";
var object = {
name : "My Object",
getNameFunc : function(){
return function(){
return this.name;
}.bind(this); // <-- sets the context of "this" to "object"
}
};
console.log( object.getNameFunc()() );
As the others have written, you need to target this. I believe this piece of code will help you to understand how this in javascript works
var name = "The Window";
var object = {
name : "My Object",
getNameFunc : function(){
that = this; // targeting this
return function() {
return that.name;
};
}
};
alert(object.getNameFunc()()); // it is My Object now
var object = {
name : "My Object",
getNameFunc : function(){
return (function(){
return this.name;
}).bind(this);
}
};
.bind, use the ES5-shim for browser support
The problem lies in the way you have declared your function.
The important point we need to remember while placing function inside a method is to use arrow function (if our function block is going to have a this keyword).
Instead of declaring a new variable and assigning this keyword to the variable, we can easily solve this problem using Arrow Functions.
Just convert the normal function into arrow function and boom it will work.
var name = 'The Window';
var object = {
name: 'My Object',
getNameFunc: function(){
return () => {
return this.name;
};
}
};
console.log( object.getNameFunc()() );
This works because arrow functions are always lexically binded and not dynamically binded like any other functions.