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javascriptfunctionhoisting
Nikhil Pai
Nikhil Pai

Reputation: 125

Javascript - Precedence in hoisting

In hoisting, do variables take precedence over function definition or the other way round? Please see the code below:

function a()
{
    var x = 10;

    function x() {
        return 20;
    }

    return x;
}

Upvotes: 5

Views: 1575

Answers (2)

JLRishe
JLRishe

Reputation: 101710

It's not a matter of one taking precedence over the other (there is precedence taking place, but that's largely just a matter of semantics).

What matters here is that the assignment part of the variable declaration is not hoisted, whereas the entire function definition is.

Functions are hoisted before variable declarations, but the net effect is the same.

After hoisting, your function will act like this:

function a()
{
    var x = function x() {  // hoisted function declaration/definition
        return 20;
    };
    var x;                  // hoisted variable declaration
    x = 10;                 // unhoisted part of variable declaration
    return x;
}

the x = 10 takes place after all the hoisting is done, so that is the value that remains in x.

To respond to @thefourtheye's request (I think this is what s/he is asking for), if your original function looked like this: 
function a() {
    function x() {
        return 20;
    }
    var x = 10;
    return x;
}

Then after hoisting, it would look like this (same as above):

function a() {
    var x = function x() {  // hoisted function declaration/definition
        return 20;
    }
    var x;                  // hoisted variable declaration (does nothing)
    x = 10;                 // unhoisted variable assignment
    return x;
}

As one last illustration, try this:

function a() {
    console.log(x);
    var x = 10;
    console.log(x);
    function x() { return 20; };
}

When called, this prints out:

function x() { return 20; }
10

The reason for this is that hoisting is causing the function to behave like this:

function a() {
    var x = function x() { return 20; };
    var x;
    console.log(x);
    x = 10;
    console.log(x);
}

Upvotes: 4

Leo
Leo

Reputation: 13848

You code would be equal to the following:

function a() {
  var x;
  function x() { // this function is assigned to variable indicator "x"
    return 20;
  }
  x = 10; // this overrides the variable indicator "x"
  return x;
}

So when you call the function:

a() // 10

Upvotes: 1

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