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Difference between function expression vs declaration in JavaScript

  • Last Updated : 23 May, 2019

Function Declaration:
A Function Declaration( or a Function Statement) defines a function with the specified parameters without requiring a variable assignment. They exist on their own, i.e, they are standalone constructs and cannot be nested within a non-function block. A function is declared using the function keyword.

  • Syntax:
    function gfg(parameter1, parameter2) {
     //A set of statements
     }
    

Function Expression:
A Function Expression works just like a function declaration or a function statement, the only difference is that a function name is NOT started in a function expression, that is, anonymous functions are created in function expressions. The function expressions run as soon as they are defined.

  • Syntax:
    var gfg = function(parameter1, parameter2) {
     //A set of statements
     }
    

Example 1: Using a Function Declaration






<!DOCTYPE html>
<html>
  
<head>
    <title>Function Declaration</title>
</head>
  
<body>
    <center>
        <h1 style="color:green">GeeksforGeeks</h1>
        <h3>Function Declaration</h3>
        <script>
            function gfg(a, b) {
                return a * b;
            }
            var result = gfg(5, 5);
            document.write(result);
        </script>
    </center>
</body>
  
</html>

Output:

25

Example 2: Using a Function Expression




<!DOCTYPE html>
<html>
  
<head>
    <title>Function Expression</title>
</head>
  
<body>
    <center>
        <h1 style="color:green">GeeksforGeeks</h1>
        <h3>Function Expression</h3>
        <script>
            var gfg = function(a, b) {
                return a * b;
            }
            document.write(gfg(5, 5));
        </script>
    </center>
</body>
  
</html>

Output:

25



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