How do I pass variables by reference in JavaScript?
I have three variables that I want to perform several operations to, so I want to put them in a for loop and perform the operations to each one.
Pseudocode:
myArray = new Array(var1, var2, var3);
for (var x = 0; x < myArray.length; x++){
// Do stuff to the array
makePretty(myArray[x]);
}
// Now do stuff to the updated variables
What is the best way to do this?
There is no "pass by reference" available in JavaScript. You can pass an object (which is to say, you can pass-by-value a reference to an object) and then have a function modify the object contents:
function alterObject(obj) {
obj.foo = "goodbye";
}
var myObj = { foo: "hello world" };
alterObject(myObj);
alert(myObj.foo); // "goodbye" instead of "hello world"
You can iterate over the properties of an array with a numeric index and modify each cell of the array, if you want.
var arr = [1, 2, 3];
for (var i = 0; i < arr.length; i++) {
arr[i] = arr[i] + 1;
}
It's important to note that "pass-by-reference" is a very specific term. It does not mean simply that it's possible to pass a reference to a modifiable object. Instead, it means that it's possible to pass a simple variable in such a way as to allow a function to modify that value in the calling context. So:
function swap(a, b) {
var tmp = a;
a = b;
b = tmp; //assign tmp to b
}
var x = 1, y = 2;
swap(x, y);
alert("x is " + x + ", y is " + y); // "x is 1, y is 2"
In a language like C++, it's possible to do that because that language does (sort-of) have pass-by-reference.
edit — this recently (March 2015) blew up on Reddit again over a blog post similar to mine mentioned below, though in this case about Java. It occurred to me while reading the back-and-forth in the Reddit comments that a big part of the confusion stems from the unfortunate collision involving the word "reference". The terminology "pass by reference" and "pass by value" predates the concept of having "objects" to work with in programming languages. It's really not about objects at all; it's about function parameters, and specifically how function parameters are "connected" (or not) to the calling environment. In particular, note that in a true pass-by-reference language — one that does involve objects — one would still have the ability to modify object contents, and it would look pretty much exactly like it does in JavaScript. However, one would also be able to modify the object reference in the calling environment, and that's the key thing that you can't do in JavaScript. A pass-by-reference language would pass not the reference itself, but a reference to the reference.
edit — here is a blog post on the topic. (Note the comment to that post that explains that C++ doesn't really have pass-by-reference. That is true. What C++ does have, however, is the ability to create references to plain variables, either explicitly at the point of function invocation to create a pointer, or implicitly when calling functions whose argument type signature calls for that to be done. Those are the key things JavaScript doesn't support.)
Primitive type variables like strings and numbers are always passed by value.
Arrays and Objects are passed by reference or by value based on these conditions:
if you are setting the value of an object or array it is Pass by Value.
object1 = { prop: "car" };
array1 = [1,2,3];
if you are changing a property value of an object or array then it is Pass by Reference.
object1.prop = "car";
array1[0] = 9;
Code
function passVar(obj1, obj2, num) {
obj1.prop = "laptop"; // will CHANGE original
obj2 = { prop: "computer" }; //will NOT affect original
num = num + 1; // will NOT affect original
}
var object1 = {
prop: "car"
};
var object2 = {
prop: "bike"
};
var number1 = 10;
passVar(object1, object2, number1);
console.log(object1); // output: Object { prop: "laptop" }
console.log(object2); // output: Object { prop: "bike" }
console.log(number1); // ouput: 10
Workaround to pass variable like by reference:
var a = 1;
inc = function(variableName) {
window[variableName] += 1;
};
inc('a');
alert(a); // 2
And yup, actually you can do it without access a global variable:
inc = (function () {
var variableName = 0;
var init = function () {
variableName += 1;
alert(variableName);
}
return init;
})();
inc();
Simple Object
function foo(x) {
// Function with other context
// Modify `x` property, increasing the value
x.value++;
}
// Initialize `ref` as object
var ref = {
// The `value` is inside `ref` variable object
// The initial value is `1`
value: 1
};
// Call function with object value
foo(ref);
// Call function with object value again
foo(ref);
console.log(ref.value); // Prints "3"
Custom Object
Object rvar
/**
* Aux function to create by-references variables
*/
function rvar(name, value, context) {
// If `this` is a `rvar` instance
if (this instanceof rvar) {
// Inside `rvar` context...
// Internal object value
this.value = value;
// Object `name` property
Object.defineProperty(this, 'name', { value: name });
// Object `hasValue` property
Object.defineProperty(this, 'hasValue', {
get: function () {
// If the internal object value is not `undefined`
return this.value !== undefined;
}
});
// Copy value constructor for type-check
if ((value !== undefined) && (value !== null)) {
this.constructor = value.constructor;
}
// To String method
this.toString = function () {
// Convert the internal value to string
return this.value + '';
};
} else {
// Outside `rvar` context...
// Initialice `rvar` object
if (!rvar.refs) {
rvar.refs = {};
}
// Initialize context if it is not defined
if (!context) {
context = this;
}
// Store variable
rvar.refs[name] = new rvar(name, value, context);
// Define variable at context
Object.defineProperty(context, name, {
// Getter
get: function () { return rvar.refs[name]; },
// Setter
set: function (v) { rvar.refs[name].value = v; },
// Can be overrided?
configurable: true
});
// Return object reference
return context[name];
}
}
// Variable Declaration
// Declare `test_ref` variable
rvar('test_ref_1');
// Assign value `5`
test_ref_1 = 5;
// Or
test_ref_1.value = 5;
// Or declare and initialize with `5`:
rvar('test_ref_2', 5);
// ------------------------------
// Test Code
// Test Function
function Fn1(v) { v.value = 100; }
// Test
function test(fn) { console.log(fn.toString()); console.info(fn()); }
// Declare
rvar('test_ref_number');
// First assign
test_ref_number = 5;
test(() => test_ref_number.value === 5);
// Call function with reference
Fn1(test_ref_number);
test(() => test_ref_number.value === 100);
// Increase value
test_ref_number++;
test(() => test_ref_number.value === 101);
// Update value
test_ref_number = test_ref_number - 10;
test(() => test_ref_number.value === 91);
Yet another approach to pass any (local, primitive) variables by reference is by wrapping variable with closure "on the fly" by eval. This also works with "use strict". (Note: be aware that eval is not friendly to JavaScript optimizers, and also missing quotes around variable name may cause unpredictive results)
"use strict"
// Return text that will reference variable by name (by capturing that variable to closure)
function byRef(varName){
return "({get value(){return "+varName+";}, set value(v){"+varName+"=v;}})";
}
// Demo
// Assign argument by reference
function modifyArgument(argRef, multiplier){
argRef.value = argRef.value * multiplier;
}
(function(){
var x = 10;
alert("x before: " + x);
modifyArgument(eval(byRef("x")), 42);
alert("x after: " + x);
})()
Live sample: https://jsfiddle.net/t3k4403w/
There's actually a pretty sollution:
function updateArray(context, targetName, callback) {
context[targetName] = context[targetName].map(callback);
}
var myArray = ['a', 'b', 'c'];
updateArray(this, 'myArray', item => {return '_' + item});
console.log(myArray); //(3) ["_a", "_b", "_c"]
I personally dislike the "pass by reference" functionality offered by various programming languages. Perhaps that's because I am just discovering the concepts of functional programming, but I always get goosebumps when I see functions that cause side effects (like manipulating parameters passed by reference). I personally strongly embrace the "single responsibility" principle.
IMHO, a function should return just one result/value using the return keyword. Instead of modifying a parameter/argument, I would just return the modified parameter/argument value and leave any desired reassignments up to the calling code.
But sometimes (hopefully very rarely), it is necessary to return two or more result values from the same function. In that case, I would opt to include all those resulting values in a single structure or object. Again, processing any reassignments should be up to the calling code.
Example:
Suppose passing parameters would be supported by using a special keyword like 'ref' in the argument list. My code might look something like this:
//The Function
function doSomething(ref value) {
value = "Bar";
}
//The Calling Code
var value = "Foo";
doSomething(value);
console.log(value); //Bar
Instead, I would actually prefer to do something like this:
//The Function
function doSomething(value) {
value = "Bar";
return value;
}
//The Calling Code:
var value = "Foo";
value = doSomething(value); //Reassignment
console.log(value); //Bar
When I would need to write a function that returns multiple values, I would not use parameters passed by reference either. So I would avoid code like this:
//The Function
function doSomething(ref value) {
value = "Bar";
//Do other work
var otherValue = "Something else";
return otherValue;
}
//The Calling Code
var value = "Foo";
var otherValue = doSomething(value);
console.log(value); //Bar
console.log(otherValue); //Something else
Instead, I would actually prefer to return both new values inside an object, like this:
//The Function
function doSomething(value) {
value = "Bar";
//Do more work
var otherValue = "Something else";
return {
value: value,
otherValue: otherValue
};
}
//The Calling Code:
var value = "Foo";
var result = doSomething(value);
value = result.value; //Reassignment
console.log(value); //Bar
console.log(result.otherValue);
These code examples are quite simplified, but it roughly demonstrates how I personally would handle such stuff. It helps me to keep various responsibilities in the correct place.
Happy coding. :)
I've been playing around with syntax to do this sort of thing, but it requires some helpers that are a little unusual. It starts with not using 'var' at all, but a simple 'DECLARE' helper that creates a local variable and defines a scope for it via an anonymous callback. By controlling how variables are declared, we can choose to wrap them into objects so that they can always be passed by reference, essentially. This is similar to one of the Eduardo Cuomo's answer above, but the solution below does not require using strings as variable identifiers. Here's some minimal code to show the concept.
function Wrapper(val){
this.VAL = val;
}
Wrapper.prototype.toString = function(){
return this.VAL.toString();
}
function DECLARE(val, callback){
var valWrapped = new Wrapper(val);
callback(valWrapped);
}
function INC(ref){
if(ref && ref.hasOwnProperty('VAL')){
ref.VAL++;
}
else{
ref++;//or maybe throw here instead?
}
return ref;
}
DECLARE(5, function(five){ //consider this line the same as 'let five = 5'
console.log("five is now " + five);
INC(five); // increment
console.log("five is incremented to " + five);
});
Actually it is really easy. The problem is understanding that once passing classic arguments, you are scoped into another, read-only zone.
The solution is to pass the arguments using JavaScript's object-oriented design. It is the same as putting the arguments in a global/scoped variable, but better...
function action(){
/* Process this.arg, modification allowed */
}
action.arg = [["empty-array"], "some string", 0x100, "last argument"];
action();
You can also promise stuff up to enjoy the well-known chain:
Here is the whole thing, with promise-like structure
function action(){
/* Process this.arg, modification allowed */
this.arg = ["a", "b"];
}
action.setArg = function(){this.arg = arguments; return this;}
action.setArg(["empty-array"], "some string", 0x100, "last argument")()
Or better yet...
action.setArg(["empty-array"],"some string",0x100,"last argument").call()
JavaScript can modify array items inside a function (it is passed as a reference to the object/array).
function makeAllPretty(items) {
for (var x = 0; x < myArray.length; x++){
// Do stuff to the array
items[x] = makePretty(items[x]);
}
}
myArray = new Array(var1, var2, var3);
makeAllPretty(myArray);
Here's another example:
function inc(items) {
for (let i=0; i < items.length; i++) {
items[i]++;
}
}
let values = [1,2,3];
inc(values);
console.log(values);
// Prints [2,3,4]
Putting aside the pass-by-reference discussion, those still looking for a solution to the stated question could use:
const myArray = new Array(var1, var2, var3);
myArray.forEach(var => var = makePretty(var));
As we don't have javascript pass by reference functionality, the only way to do this is to make the function return the value and let the caller assign it:
So
"makePretty(myArray[x]);"
should be
"myArray[x] = makePretty(myArray[x]);"
This is in case you need assignment inside the function, if only mutation is necessary, then passing the object and mutating it should be enough
I know exactly what you mean. The same thing in Swift will be no problem. The bottom line is use let, not var.
The fact that primitives are passed by value, but the fact that the value of var i at the point of iteration is not copied into the anonymous function is quite surprising to say the least.
for (let i = 0; i < boxArray.length; i++) {
boxArray[i].onclick = function() { console.log(i) }; // Correctly prints the index
}
If you want to pass variables by reference, a better way to do that is by passing your arguments in an object and then start changing the value by using window:
window["varName"] = value;
Example:
// Variables with first values
var x = 1, b = 0, f = 15;
function asByReference (
argumentHasVars = {}, // Passing variables in object
newValues = []) // Pass new values in array
{
let VarsNames = [];
// Getting variables names one by one
for(let name in argumentHasVars)
VarsNames.push(name);
// Accessing variables by using window one by one
for(let i = 0; i < VarsNames.length; i += 1)
window[VarsNames[i]] = newValues[i]; // Set new value
}
console.log(x, b, f); // Output with first values
asByReference({x, b, f}, [5, 5, 5]); // Passing as by reference
console.log(x, b, f); // Output after changing values
I like to solve the lack of by reference in JavaScript like this example shows.
The essence of this is that you don't try to create a by reference. You instead use the return functionality and make it able to return multiple values. So there isn't any need to insert your values in arrays or objects.
var x = "First";
var y = "Second";
var z = "Third";
log('Before call:',x,y,z);
with (myFunc(x, y, z)) {x = a; y = b; z = c;} // <-- Way to call it
log('After call :',x,y,z);
function myFunc(a, b, c) {
a = "Changed first parameter";
b = "Changed second parameter";
c = "Changed third parameter";
return {a:a, b:b, c:c}; // <-- Return multiple values
}
function log(txt,p1,p2,p3) {
document.getElementById('msg').innerHTML += txt + '<br>' + p1 + '<br>' + p2 + '<br>' + p3 + '<br><br>'
}
<div id='msg'></div>
Using Destructuring here is an example where I have 3 variables, and on each I do the multiple operations:
If value is less than 0 then change to 0,
If greater than 255 then change to 1,
Otherwise dived the number by 255 to convert from a range of 0-255 to a range of 0-1.
let a = 52.4, b = -25.1, c = 534.5;
[a, b, c] = [a, b, c].map(n => n < 0 ? 0 : n > 255 ? 1 : n / 255);
console.log(a, b, c); // 0.20549019607843136 0 1
I am trying to get around understanding javascript closures from a practical scenario.I know from a theoretical perspective , With the help of closures inner functions can have access to the variables in the enclosing function i.e parent function.
I have read a couple of questions on stackOverflow as well.
i am really missing the point of what is happening here?
var foo = [];
for(var i=0;i<10;i++){
foo[i] = function(){
return i;
}
}
console.log(foo[0]());
This gives me out a 10. Most of the articles say that by the time it reaches the inner anonymous function, The for loop is getting executed as a result the last value that is present in the loop which is 10 is being printed.
But i am still not able to get to the bottom of this.
On Contrary, If i use something like:
var foo = [];
for(var i=0;i<10;i++){
(function(){
var y =i;
foo[i] = function(){
return y;
}
})();
}
console.log(foo[0]());
I am getting the output.Any help would be highly appreciated.
maybe this code block helps
var foo = [];
for(var i = 0; i < 10; i++) {
foo[i] = function() {
return i; // is a reference and will always be the value, which 'i' have on function execution
}
}
// 'i' is 10 here!
console.log(foo[0]()); // executing the function will return the current value of 'i'
///////////////////////////////////////
var foo = [];
for(var i=0;i<10;i++) {
/* thats a IIFE (immediately invoked function expression) */
(function(a) { // 'a' is now a local variable
foo[a] = function() { // defines a function
return a; // is a reference to local variable 'a'
};
})(i); // <- passing the current value of i as parameter to the invoked function
}
// 'i' is 10 here
console.log(foo[0]()); // returns the reference to 'a' within the same scope, where the function was defined
In your first scenario, all of your functions added to the foo array are referencing the same var i. All functions will return whatever i was set to last, which is 10 because during the last iteration of the loop that's what it's value was set to.
In the second scenario, you are Immediately Invoking this function:
(function(){
var y =i;
foo[i] = function(){
return y;
}
})();
By immediately invoking it you are effectively locking in the local state of var y, for each iteration of the loop - it provides a unique scope for each function added to the array.
var x = 3;
(function (){
console.log('before', x);
var x = 7;
console.log('after', x);
return ;
})();
In the above code var X is initialized globally. So inside the function the first console.log should print "before 3" but i don't get it. The reason is that i am trying to re-declare the global variable.
Can somebody explain why this is happening?
In the above code var X is initialized globally. so inside the function the first console.log should print "before 3".
No, it should print before undefined, because var takes effect from the beginning of the function regardless of where you write it.
Your code is exactly the same as this:
var x = 3;
(function (){
var x;
console.log('before', x);
x = 7;
console.log('after', x);
return ;
})();
And of course, variables start with the value undefined.
Details: Poor misunderstood var
The JavaScript parser does Variable Hoisting when parsing your code. This means that any variable declaration will be moved to the top of the current scope, thus in your case, this code will get executed:
var x = 3;
(function (){
var x;
console.log('before', x);
x = 7;
console.log('after', x);
return ;
})();
So your local variable x gets declared at first with an initial value of undefined.
This should explain, why you get an "beforeundefined" for the first console.log().
The scope of a variable is much simpler than in other languages. It doesn't start at declaration but is either :
the function in which you have the declaration
the global scope if the declaration isn't in a function
MDN :
The scope of a variable declared with var is the enclosing function
or, for variables declared outside a function, the global scope (which
is bound to the global object).
You can imagine that all variable declarations are moved to the start of the scope (the function). So this is exactly like
var x = 3;
(function (){
var x;
console.log('before', x); // now undefined
x = 7;
console.log('after', x); // now 7
return ;
})();
Be careful to understand what is the exact scope (the function, not the block) :
var x = 3;
(function (){
console.log('before', x); // this is undefined !
if (true) {
var x = 7;
}
return ;
})();
If I declare a function literal:
var x = function(){
alert('hi');
};
console.log(x); // returns the function code.
However:
var x = (function(){
alert('hi');
})();
console.log(x); // returns undefined?
I don't understand why this happens. Isn't the point of writing a function as a literal is to still be able to access it by its variable reference name? I know this may be silly but I'm just learning javascript so don't judge too harshly.
Your function does not return anything, so its return value is undefined.
A self-executing function is executed and the function is not stored anywhere - only its return value survives (and any external variables the function sets/modifies).
For example, this code would be equivalent to var x = 'hi';:
var x = (function(){
return 'hi';
})();
The purpose of self-invoking functions is usually to create a new scope, e.g. when creating callback functions in a loop:
for(var i = 0; i < 5; i++) {
window.setTimeout(function(){ alert('i = ' + i); }, 1000 * i);
}
This would use the same i in all callbacks so it would alert i = 5 5 times.
for(var i = 0; i < 5; i++) {
(function(i) {
window.setTimeout(function(){ alert('i = ' + i); }, 1000 * i);
})(i);
}
By using a self-executing function we create a new scope and thus a new i in each loop.
Another use of self-executing functions is to create a new scope where certain variables are ensured to be available and set to the correct value:
(function($, window, undefined) {
// here the following always applies:
// $ === jQuery
// window === the global object [assuming the function was executed in the global scope]
// undefined is well, undefined - in some js engines someone could have redefined it
})(jQuery, this);
If you:
var foo = somefunction;
… then you assign a function to foo.
If you:
var foo = somefunction();
… then you assign the return value of a function call to foo
Your function:
function(){
alert('hi');
}
… has no return statement, so it will return undefined.
I had a bug, it went like this
for(var i=0; i<arr.length; i++){
var v = arr[i]
var obj=new Thing
obj.TheCallback = function(e) { blah = v; domorestuff(); ... }
obj.runCodeToExecuteTheCallback()
}
The problem is v inside the function is using the v from the last loop. It's a closure thing where you can access variables in your parent scope. But my question is...
Why is javascript reusing v in each iteration? When it goes out of scope (at the end of the loop) i dont except it to be MODIFIED from any other scope unless it was passed in (such as the inner function). Why the heck is javascript clobbering my variable? Is there some kind of reason for this design? Is it a bug that has been decided never to be fixed?
This is a very common issue people encounter.
JavaScript doesn't have block scope, just function scope. So each function you create in the loop is created in the same variable scope, so they're referencing the same v variable.
To create a new scope, you need to invoke a function, and pass in whatever you want to exist in a new scope.
function createCallback( x ) {
return function(e) { blah = x; domorestuff(); ... }
}
for(var i=0; i<arr.length; i++){
var v = arr[v]
var obj=new Thing
obj.TheCallback = createCallback( v );
obj.runCodeToExecuteTheCallback()
}
Here I invoked the createCallback() function, pass in v, and had createCallback() return a function that references the local variable (named x in the function, though you could name it v as well).
The returned function is of course assigned to obj.TheCallback.
Its variable scope exists as long as the function exists, so it will continue to have reference to any variables that it needs that were in the variable scope when it was created.
Another solution:
var i, obj;
for ( i = 0; i < arr.length; i++ ) {
obj = new Thing;
(function ( v ) {
obj.callback = function ( e ) {
// do stuff with v
};
})( arr[i] );
}