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Variables passed by reference or value (clarification)

This is my example code:

var foo = {a : 1};
var bar = foo;
console.log(bar.a);//1, as expected
foo.a = 2;
console.log(bar.a);//2, as expected, as objects are passed by reference
foo = {a : 10};
console.log(bar.a);//2, not expected, I expected 10

The last log does not give the expected result. Thinking what is the foo = {a : value}

same foo.a = value

, I expected the last result to be 10. What is wrong with my expectation? I think I am missing a big tutorial here.

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5 answers


First, you set foo

to one object:

var foo = {a : 1};

and then overwrite it with a completely new object:



foo = {a : 10};

In this case, foo

and are bar

no longer connected, since the link is now broken.

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When you reach this line of code:

foo = {a : 10};


foo is assigned the variable new ; from now on there are two different variables on foo and bar, and for this reason the last line prints 2 as the bar still points to the old value.

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You are reassigning foo

to a new object literal on the second line. Bar

is still pointing to the old object, so you get 2 (which you changed on the 4th line).

If this explanation doesn't clear things up, it might be helpful to step back and try to understand variables, objects, and references at a higher level. Eloquent JavaScript The Data Structures chapter might be a good place to start.

edit: Point to be clarified: it's important to understand that you are not ignoring anything, you just changed the link foo

. Now they point to different things in memory. That's why Bar

it's the same.

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var foo = {a : 1};  //foo refering to object {a : 1}
var bar = foo;      //bar refering to same object as foo
console.log(bar.a); //1, as expected
foo.a = 2;          //The object to which bar and foo are pointing gets changed
console.log(bar.a); //2, as expected, as objects are passed by reference
foo = {a : 10};     //foo starts pointing to the newly created object, whereas bar is still refering the old object(no change in bar) and the earlier object `{a : 1}` exists and poinyted to by bar
console.log(bar.a); //2, not expected, I expected 10 //hence this result
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first you assigned foo to the bar at which time ref foo is sent to bar.

now if you change the value of bar.a then foo will be changed

foo = {
    a: 2
}

bar = foo
console.log(bar.a) //gives 2

bar.a = 30

console.log(foo.a) //gives 30

but when you rewrite foo the connection will be lost and both will act as two separate variables

so now

foo.a = 20
console.log(bar.a) //gives 30
console.log(foo.a) //gives 20
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