Why Can You Use JavaScript Functions Before They are Defined?

Understanding the Unexpected Function Usage in JavaScript

Despite its unconventional appearance, JavaScript allows you to use functions before they are explicitly defined. This seemingly magical behavior stems from the special nature of function declarations.

The Magic of Function Declarations

In JavaScript, function declarations behave differently from assignments with function expressions. When a function is declared using the function keyword, its identifier is bound before any code within the function block is executed.

For instance, consider the following code:

function fooCheck() {
  alert(internalFoo()); // Uses internalFoo() before definition

  return internalFoo(); // Still uses it

  function internalFoo() { return true; } // Defined later
}

fooCheck();

In this code, the fooCheck function can access the internalFoo function even though it is defined later within the same block. This is because the function declaration itself causes the identifier internalFoo to be bound and made available.

Function Expressions and Assignment

In contrast, if you change the definition to a function expression assigned to a variable, the code will no longer work:

var internalFoo = function() { return true; };

This is because function expressions are evaluated from top to bottom, and the assignment statement executes only after fooCheck has been called.

The Distinction between Declarations and Expressions

Function declarations and function expressions are syntactically distinct entities. Function declarations are statements, while function expressions are expressions. This difference leads to the different behaviors observed in the code examples.

This behavior is outlined in the ECMAScript standard (section 10.1.3). While the standard may not be the most accessible read, it provides a comprehensive explanation of this intricate aspect of JavaScript.

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