How is it possible for JavaScript functions to be accessed prior to being defined?

Question

How is it possible for JavaScript functions to be accessed prior to being defined?

Similar Question:
Why can I access a function before it's declared in JavaScript?

Unexpectedly, the following code results in an error due to the undefined Foo:

window.Foo = Foo;

This code snippet also triggers the same error:

window.Foo = Foo;
Foo = function() {
    ...
};

Surprisingly, this piece of code runs without errors:

window.Foo = Foo;
function Foo(){
    ...
};

How is this behavior possible? Isn't JavaScript supposed to be interpreted line-by-line?

javascript function syntax

Answer 1

Answer №1

Hoisting is a key aspect to understand in JavaScript.

Let's break down hoisting:

When you use var to declare a variable in JavaScript, the variable is hoisted to the top of its scope. In JavaScript, there are only two scopes: Global and Function. This means that within a function,

function hoistAway() {
  console.log(a);
  var a = 5;
}

is essentially interpreted as

function hoistAway() {
  var a;
  console.log(a);
  a = 5;
}

In this scenario, the variable declaration is hoisted, but the initialization is not. Therefore, the code would output undefined instead of 5. However, it acknowledges the existence of a.

If you omit var, the declaration isn't explicit so it's not hoisted. For example:

function hoistAway() {
  console.log(a);
  a = 5;
}

would trigger an error.

In your examples:

window.Foo = Foo;

Foo was never declared, hence the error message.

window.Foo = Foo;
Foo = function() {
    ...
};

Once again, since Foo wasn't explicitly declared, there is no hoisting. It's worth noting that:

window.Foo = Foo;
var Foo = function() {
    ...
};

won't result in an error. Only the declaration gets hoisted, so if you did something like

var bob = {};
bob.Foo = Foo;
var Foo = function() {
    ...
};

then bob.Foo would be undefined. The case of window.Foo = Foo is peculiar because Foo and window.Foo are equivalent in the Global scope.

Lastly:

window.Foo = Foo;
function Foo(){
    ...
};

works fine, as function declarations (not expressions) are also hoisted; both the name and definition are lifted up in the code, making it operate as expected.

Answer 2

Hoisting is a key aspect to understand in JavaScript.

Let's break down hoisting:

When you use var to declare a variable in JavaScript, the variable is hoisted to the top of its scope. In JavaScript, there are only two scopes: Global and Function. This means that within a function,

function hoistAway() {
  console.log(a);
  var a = 5;
}

is essentially interpreted as

function hoistAway() {
  var a;
  console.log(a);
  a = 5;
}

In this scenario, the variable declaration is hoisted, but the initialization is not. Therefore, the code would output undefined instead of 5. However, it acknowledges the existence of a.

If you omit var, the declaration isn't explicit so it's not hoisted. For example:

function hoistAway() {
  console.log(a);
  a = 5;
}

would trigger an error.

In your examples:

window.Foo = Foo;

Foo was never declared, hence the error message.

window.Foo = Foo;
Foo = function() {
    ...
};

Once again, since Foo wasn't explicitly declared, there is no hoisting. It's worth noting that:

window.Foo = Foo;
var Foo = function() {
    ...
};

won't result in an error. Only the declaration gets hoisted, so if you did something like

var bob = {};
bob.Foo = Foo;
var Foo = function() {
    ...
};

then bob.Foo would be undefined. The case of window.Foo = Foo is peculiar because Foo and window.Foo are equivalent in the Global scope.

Lastly:

window.Foo = Foo;
function Foo(){
    ...
};

works fine, as function declarations (not expressions) are also hoisted; both the name and definition are lifted up in the code, making it operate as expected.

Answer 3

Answer №2

Prioritizing the parsing of the script enables your code to access functions that are declared at a later stage.

Answer 4

Prioritizing the parsing of the script enables your code to access functions that are declared at a later stage.

How is it possible for JavaScript functions to be accessed prior to being defined?

Answer №1

Answer №2

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