Question

I just can’t understand how variables are passed, why are some passed by reference while other by value?

Example:

var a=4;
var b=a;
b=b++;
alert(a);//unmodified 4

var c=["t","ttt"];
var d=c;
d=d.sort(function(x,y){return (y.length-x.length);});
alert(c);//modified ["ttt","t"]

Where can I find a list of what variables will work like the first example and which like the second? (booleans, strings, etc… there are too many to test them all by miself)

Answer 1

JavaScript is always pass by value.

It’s very common to say that objects in JavaScript are pass by reference, however that is not true. In a true pass by reference language, you could pass a reference to an object’s reference, and have it point to another object. This is not possible in JavaScript.

Take, for example, C#. By default everything in C# is pass by value, just like JavaScript.

void foo(string s) {
    s = "passbyvalue";
}

string a = "original value";
foo(a);
// a is still "original value"

However, if you alter foo to use pass by reference, the behavior changes:

void foo(ref string s) {
    s = "passbyreference";
}
string a = "original value";
foo(ref a);
// a is now "passbyreference"  

In JavaScript, only the first example is possible.

In JavaScript, everything is pass by value. This includes the object reference (yes, it is confusing). The reference itself is a value (it’s very much like a pointer). It merely contains an id that allows the runtime to look up the object that is most likely stored in its heap. When you pass an object to a function, you are actually copying its reference into the function

It may be a seemingly small, and even anal, difference. But it’s a key difference. Languages that have true pass by reference such as C# and C++ allow you to do things that are simply not possible in languages like JavaScript. The above trite example is one.