Function Declaration and Function Expression in JavaScript

Function Declaration

function keyword is the first word in statement then it is a function declaration.

Just as variable declaration must start with "var", function declaration must begin with "function".

	function square(x) { // function declaration
	return x*x;
	}
	console.log(square(5)); // 25

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Function Expression

function keyword is not the first word in statement then it is a function expression.

function expression can be Anonymous function expression or named function expression

	var square = function(x) { // Anonymous function expression
	return x*x;
	};
	console.log(square(5)); // 25

view raw gistfile1.js hosted with ❤ by GitHub

	var a = function square(x) { // Named function expression
	return x*x;
	};
	console.log(a(5));

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Link 1 - Function Declaration
Link 2 - Anonymous Function Expression
Link 3 - Named Function Expression