Is JavaScript a pass-by-reference or pass-by-value language? – Dev

The best answers to the question “Is JavaScript a pass-by-reference or pass-by-value language?” in the category Dev.

QUESTION:

The primitive types (number, string, etc.) are passed by value, but objects are unknown, because they can be both passed-by-value (in case we consider that a variable holding an object is in fact a reference to the object) and passed-by-reference (when we consider that the variable to the object holds the object itself).

Although it doesn’t really matter at the end, I want to know what is the correct way to present the arguments passing conventions. Is there an excerpt from JavaScript specification, which defines what should be the semantics regarding this?

ANSWER:

It’s always pass by value, but for objects the value of the variable is a reference. Because of this, when you pass an object and change its members, those changes persist outside of the function. This makes it look like pass by reference. But if you actually change the value of the object variable you will see that the change does not persist, proving it’s really pass by value.

Example:

function changeObject(x) {
  x = { member: "bar" };
  console.log("in changeObject: " + x.member);
}

function changeMember(x) {
  x.member = "bar";
  console.log("in changeMember: " + x.member);
}

var x = { member: "foo" };

console.log("before changeObject: " + x.member);
changeObject(x);
console.log("after changeObject: " + x.member); /* change did not persist */

console.log("before changeMember: " + x.member);
changeMember(x);
console.log("after changeMember: " + x.member); /* change persists */

Output:

before changeObject: foo
in changeObject: bar
after changeObject: foo

before changeMember: foo
in changeMember: bar
after changeMember: bar

ANSWER:

It’s interesting in JavaScript. Consider this example:

function changeStuff(a, b, c)
{
  a = a * 10;
  b.item = "changed";
  c = {item: "changed"};
}

var num = 10;
var obj1 = {item: "unchanged"};
var obj2 = {item: "unchanged"};

changeStuff(num, obj1, obj2);

console.log(num);
console.log(obj1.item);
console.log(obj2.item);

This produces the output:

10
changed
unchanged
  • If obj1 was not a reference at all, then changing obj1.item would have no effect on the obj1 outside of the function.
  • If the argument was a proper reference, then everything would have changed. num would be 100, and obj2.item would read "changed". Instead, num stays 10 and obj2.item remains "unchanged“.

Instead, the situation is that the item passed in is passed by value. But the item that is passed by value is itself a reference.
Technically, this is called call-by-sharing.

In practical terms, this means that if you change the parameter itself (as with num and obj2), that won’t affect the item that was fed into the parameter. But if you change the internals of the parameter, that will propagate back up (as with obj1).

ANSWER:

My two cents… This is the way I understand it. (Feel free to correct me if I’m wrong)

It’s time to throw out everything you know about pass by value / reference.

Because in JavaScript, it doesn’t matter whether it’s passed by value or by reference or whatever.
What matters is mutation vs assignment of the parameters passed into a function.

OK, let me do my best to explain what I mean. Let’s say you have a few objects.

var object1 = {};
var object2 = {};

What we have done is “assignment”… We’ve assigned 2 separate empty objects to the variables “object1” and “object2”.

Now, let’s say that we like object1 better… So, we “assign” a new variable.

var favoriteObject = object1;

Next, for whatever reason, we decide that we like object 2 better. So, we do a little re-assignment.

favoriteObject = object2;

Nothing happened to object1 or to object2. We haven’t changed any data at all. All we did was re-assign what our favorite object is. It is important to know that object2 and favoriteObject are both assigned to the same object. We can change that object via either of those variables.

object2.name="Fred";
console.log(favoriteObject.name) // Logs Fred
favoriteObject.name="Joe";
console.log(object2.name); // Logs Joe

OK, now let’s look at primitives like strings for example

var string1 = 'Hello world';
var string2 = 'Goodbye world';

Again, we pick a favorite.

var favoriteString = string1;

Both our favoriteString and string1 variables are assigned to ‘Hello world’. Now, what if we want to change our favoriteString??? What will happen???

favoriteString = 'Hello everyone';
console.log(favoriteString); // Logs 'Hello everyone'
console.log(string1); // Logs 'Hello world'

Uh oh…. What has happened. We couldn’t change string1 by changing favoriteString… Why?? Because we didn’t change our string object. All we did was “RE ASSIGN” the favoriteString variable to a new string. This essentially disconnected it from string1. In the previous example, when we renamed our object, we didn’t assign anything. (Well, not to the variable itself, … we did, however, assign the name property to a new string.) Instead, we mutated the object which keeps the connections between the 2 variables and the underlying objects. (Even if we had wanted to modify or mutate the string object itself, we couldn’t have, because strings are actually immutable in JavaScript.)

Now, on to functions and passing parameters…. When you call a function, and pass a parameter, what you are essentially doing is an “assignment” to a new variable, and it works exactly the same as if you assigned using the equal (=) sign.

Take these examples.

var myString = 'hello';

// Assign to a new variable (just like when you pass to a function)
var param1 = myString;
param1 = 'world'; // Re assignment

console.log(myString); // Logs 'hello'
console.log(param1);   // Logs 'world'

Now, the same thing, but with a function

function myFunc(param1) {
    param1 = 'world';

    console.log(param1);   // Logs 'world'
}

var myString = 'hello';
// Calls myFunc and assigns param1 to myString just like param1 = myString
myFunc(myString);

console.log(myString); // logs 'hello'

OK, now let’s give a few examples using objects instead… first, without the function.

var myObject = {
    firstName: 'Joe',
    lastName: 'Smith'
};

// Assign to a new variable (just like when you pass to a function)
var otherObj = myObject;

// Let's mutate our object
otherObj.firstName="Sue"; // I guess Joe decided to be a girl

console.log(myObject.firstName); // Logs 'Sue'
console.log(otherObj.firstName); // Logs 'Sue'

// Now, let's reassign the variable
otherObj = {
    firstName: 'Jack',
    lastName: 'Frost'
};

// Now, otherObj and myObject are assigned to 2 very different objects
// And mutating one object has no influence on the other
console.log(myObject.firstName); // Logs 'Sue'
console.log(otherObj.firstName); // Logs 'Jack';

Now, the same thing, but with a function call

function myFunc(otherObj) {

    // Let's mutate our object
    otherObj.firstName="Sue";
    console.log(otherObj.firstName); // Logs 'Sue'

    // Now let's re-assign
    otherObj = {
        firstName: 'Jack',
        lastName: 'Frost'
    };
    console.log(otherObj.firstName); // Logs 'Jack'

    // Again, otherObj and myObject are assigned to 2 very different objects
    // And mutating one object doesn't magically mutate the other
}

var myObject = {
    firstName: 'Joe',
    lastName: 'Smith'
};

// Calls myFunc and assigns otherObj to myObject just like otherObj = myObject
myFunc(myObject);

console.log(myObject.firstName); // Logs 'Sue', just like before

OK, if you read through this entire post, perhaps you now have a better understanding of how function calls work in JavaScript. It doesn’t matter whether something is passed by reference or by value… What matters is assignment vs mutation.

Every time you pass a variable to a function, you are “Assigning” to whatever the name of the parameter variable is, just like if you used the equal (=) sign.

Always remember that the equals sign (=) means assignment.
Always remember that passing a parameter to a function in JavaScript also means assignment.
They are the same and the 2 variables are connected in exactly the same way (which is to say they aren’t, unless you count that they are assigned to the same object).

The only time that “modifying a variable” affects a different variable is when the underlying object is mutated (in which case you haven’t modified the variable, but the object itself.

There is no point in making a distinction between objects and primitives, because it works the same exact way as if you didn’t have a function and just used the equal sign to assign to a new variable.

The only gotcha is when the name of the variable you pass into the function is the same as the name of the function parameter. When this happens, you have to treat the parameter inside the function as if it was a whole new variable private to the function (because it is)

function myFunc(myString) {
    // myString is private and does not affect the outer variable
    myString = 'hello';
}

var myString = 'test';
myString = myString; // Does nothing, myString is still 'test';

myFunc(myString);
console.log(myString); // Logs 'test'

ANSWER:

The variable doesn’t “hold” the object; it holds a reference. You can assign that reference to another variable, and now both reference the same object. It’s always pass by value (even when that value is a reference…).

There’s no way to alter the value held by a variable passed as a parameter, which would be possible if JavaScript supported passing by reference.