Pass variables by reference in JavaScript

Question

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?

Answer 1

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.)

Answer 2

1. primitive type variables like strings and numbers are always passed by value.

2. 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 {item:"laptop"}
console.log(object2); //output: Object {item:"bike"}
console.log(number1); //ouput: 10

Answer 3

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();

Answer 4

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);

Answer 5

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/

Answer 6

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. :)

Answer 7

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);
});

Answer 8

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"]

Answer 9

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()

Answer 10

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));

Answer 11

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]

Answer 12

JavaScript not being strong type. It allows you to resolve problems in many different ways, as it seem in this question.

However, for a maintainability point of view, I would have to agree with Bart Hofland. A function should get arguments to do something with and return the result. Making them easily reusable.

If you feel that variables need to be passed by reference, you may be better served building them into objects, IMHO.

Answer 13

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
}

Answer 14

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

Answer 15

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>

Answer 16

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

Answer 17

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)