Javascript – Is JavaScript a pass-by-reference or pass-by-value language


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?

Best 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);


This produces the output:

  • 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).

Edited for clarification:

JavaScript is pass-by-value. Passing-by-reference and passing a reference are two different things. Pass-by refers to the mechanism for passing a parameter to a function. It's either the value of the variable (pass-by-value) or a pointer (reference) to the location of the variable (pass-by-reference). There's no mechanism within JavaScript to do the latter. It's not as complicated or nuanced as some of these answers try to make it.

Think of a variable as a box that contains information. When you pass it by value, you're just passing a new box with a copy of the information contained in the box. If you change that information within the new box in the function, the original value in the original box is unchanged.

Passing by reference is passing the actual box. If you change what's inside, it will be changed even after the function exits.

In the case of objects, think of the information contained within the box as a map to the locations of other boxes. You can change the contents of those boxes which will be permanent changes after the function exits, but you can't change the contents of the original map.

Assigning a new object to c just put a new map into the new box that is only local to the function. This is also what happened with a.