Why Can You Call JavaScript Functions Before They Are Defined?

Understanding the Function Hoisting in JavaScript

Why can we invoke functions before their declaration in JavaScript?

In JavaScript, a puzzling phenomenon occurs where functions can be invoked even before they're formally defined. This behavior is known as function hoisting.

Hoisting Explained

Hoisting separates the declaration of a function from its execution. Function declarations are processed during the compilation phase, and their identifiers are bound to the global context or the scope where they're defined. This means that functions can be referenced and called even before they're explicitly defined within the script.

Consider the following code:

<code class="javascript">function fooCheck() {
  alert(internalFoo()); // Invoking internalFoo() before its definition

  return internalFoo(); // Same here

  function internalFoo() { return true; } // Function definition comes later
}

fooCheck();</code>

In this code, internalFoo() is invoked before its definition. Surprisingly, it works because during hoisting, the function declaration is lifted to the top of the scope, making its identifier available for reference throughout the script.

Function Declaration vs. Expression

This behavior is only applicable to function declarations. Function expressions, such as anonymous functions defined with the var keyword, do not exhibit the same hoisting behavior. If we change the above code to use a function expression, the script will break:

<code class="javascript">var internalFoo = function () { return true; };</code>

Conclusion

Function hoisting is a fundamental aspect of JavaScript that can be confusing at first. With this understanding, you can now navigate the complexities of asynchronous programming and maintain efficient code flow.

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