The Difference Between Function Expression and Function Declaration in JavaScript

Function expressions and declarations have two major differences. One is that for function expressions, you are assigning the function to a variable. In a function declaration, you don't:

// Function expression
// Will throw a TypeError
console.log(func());
var func = function() { return 5; }

// Function declaration
// Will run without a problem, because it is hoisted
console.log(func());
function func() { return 5; }

The biggest difference however is that expressions are not hoisted. If you call the function before it was initialized, you will get a type error:

Whichever you choose to use throughout your code, make sure you stay consistent to avoid unexpected behaviors.

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Resources:

• Function expression
• Function declaration