CODEWITHSUNDEEP
javascriptscope
Mohsen
Mohsen

Reputation: 65835

Why doesn't {var foo = foo} throw a ReferenceError?

Why doesn't var foo = foo throw a ReferenceError?

Note: foo = foo does throw a ReferenceError.

Upvotes: 3

Views: 103

Answers (4)

Mahesha999
Mahesha999

Reputation: 24801

When you use var keyword

var foo = foo

JavaScript hoisting creates foo and assigns it undefined value before code executes. So you can assign it any value which is foo here and foo itself is undefined so in fact you are again assigning undefined to foo through the same variable

When you are doing

foo = foo

you don't have left side foo defined earlier to assign a value to it.

When you are doing

var foo = bar

you don't have bar defined earlier.

Upvotes: 0

Denys Séguret
Denys Séguret

Reputation: 382394

When you declare

var foo = ...

you declare the variable for the entire scope (that is your function if not global), not just the code afterwards, contrary to other languages.

So in the right part of the assignment, foo is already declared, even if it is still undefined. There is no reference error.

Note that this property of var declaration in javascript can be a source of error. Because you might very well have (in more complex) this kind of code :

if (true) {
    var a = 3; // do you think this is "local" ?
}
var a;
alert(a); // a is 3, did you expect it ?

Upvotes: 5

millimoose
millimoose

Reputation: 39980

Presumably, variable declarations are hoisted in Javascript. Which means the code

function bar() {
    // some other code
    var foo = foo;
}

is equivalent to:

function bar() {
    var foo;
    // some other code
    foo = foo;
}

In fact, even the following works:

function bar() {
    return foo;
    var foo;
}

(And returns undefined.)

Upvotes: 2

lrsjng
lrsjng

Reputation: 2625

JavaScript sorts var declaration to top, so at assignment time it is already declared (even if undefined):

var foo;
foo = foo;

Upvotes: 1

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