/javascript-pass-by-value-and-reference

JavaScript-pass-by-value-and-reference

Is Javascript a pass-by-value, or a pass-by-reference?

  • In practice the answer is actually: 'neither'
  • Now, technically, Javascript is pass-by-value.

Lets look at the simple example for primitive values

function changeThisValue(param) {  
    param = 2;
}

var myNumber = 9;  
console.log(myNumber);  
changeThisValue(myNumber);  
console.log(myNumber);

// Output:
// 9
// 9

But take a look at this code

function changeThisValue(param) {  
    param.value = 2;
}

var obj = {value: 9};
console.log(obj.value);
changeThisValue(obj);
console.log(obj.value);

// Output:
// 9
// 2

So, ok, clearly it's pass-by-reference, 'NO'.. But why??

If it were pass-by-value, we would be unable to change the value of the passed-in object, we would be changing a copy of that value inside changeThisValue().

Well, now that you're convinced Javascript is pass-by-reference, look at this:

function changeThisValue(param) {  
    param = {value: 2};
}

var obj = {value: 9};  
console.log(obj.value);  
changeThisValue(obj);  
console.log(obj.value);

// Output:
// 9
// 9

In this case, we've assigned a new object to param.

In pass-by-reference, one would expect obj to reflect this change, but it does not.

So, here's what's happening:

Javascript is actually passing a reference to obj by value. (Say that again...?)

When you call changeThisValue(obj), you're creating two (completely independent) references to the same object. Outside of the function, you have obj, and inside the function, you have param. They both refer the same object in memory, but the reference variables themselves are independent.

So, what's the answer? Technically (language internals), it's pass-by-value. In practice, I'd put it like this: Javascript is pass-by-reference for objects,

Another simple example

Pass by value

let a = 1;
let b = a;
b = b + 2;
console.log(a, b); // 1 3

The first statement let a = 1 defines a variable a initialized with the number 1.

The second statement let b = a defines another variable b and initializes it with the value of a variable — which is passing by value.

Pass by reference

let a = [1];
let b = a;
b.push(2);
console.log(a, b); // [1, 2]  [1, 2]

When creating an object you’re given a reference to that object. If two variables hold the same reference, then changing the object reflects in both variables.