Why does this work? Object references in Javascript

Question

I've finally been curious enough to find out why javascript does its voodoo magic to learn why not all object references are created equal.

Given the example:

var a, b, c, d;
a = 100; b = a;

c = {}; d = c;

b = 10; d.e = 'f';

console.log(a, b); // outputs 100, 10
console.log(c, d); // outputs object => e = 'f', object => e = 'f'

If all variables in javascript are objects, then what makes the use case with c and d cast explicitly as an Object so different than defining a and b as Number? Or, why will c and d be linked to one another, and not a and b?

Anurag · Accepted Answer · 2011-06-09 16:48:43Z

8

All variables in JavaScript are not objects. There are native types as well.

c and d are not linked to one another. They are pointing to the same object reference. If you were to reassign d to something else, it will not affect c.

var c = {};
var d = c;
d = { foo: "bar" };

c === d // false

However, if you were to modify the object being referenced by c or d, it will modify the same object since c and d are both referring to the same object as in your example.

edited Jun 9, 2011 at 16:48

answered Jun 9, 2011 at 16:42

Anurag

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1 Comment

buzzedword Over a year ago

That makes a LOT more sense seeing it like that. Thanks!

JAB · Accepted Answer · 2011-06-09 16:41:37Z

3

It looks to me that the difference is with b, you're reassigning the variable to a new object/value, while with d, you're modifying the existing object.

answered Jun 9, 2011 at 16:41

JAB

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8 Comments

buzzedword Over a year ago

So what you're saying then is, that (as Anurag pointed out) using a Native type will implicitly create a new object, whereas reassigning the object to another variable simply makes a reference-- unless the new keyword is explicitly used?

lawnsea Over a year ago

This is not strictly correct. The values assigned to a and b are not objects.

JAB Over a year ago

@Buzzedword: Something like that.

JAB Over a year ago

@lawnsea: Hence why I changed it to "object/value" rather than just "object".

lawnsea Over a year ago

@Buzzedword @JAB - No. See Anurag's explanation and my annotation of your code sample. The new operator has nothing to do with this.

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lawnsea · Accepted Answer · 2011-06-09 16:49:47Z

2

The value of a that is assigned to b is a Number. The value assigned from c to d is a reference to an Object.

var a, b, c, d;
a = 100; // a has value 100, a number
b = a; // b has value 100, a number

c = {}; // c has value p, a reference to some object P
d = c; // d has value p, a reference to P

b = 10; // b has value 10, a number
d.e = 'f'; // P.e has value 'f', a string

edited Jun 9, 2011 at 16:49

answered Jun 9, 2011 at 16:43

lawnsea

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