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What is difference between var functionName = function vs function functionName

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The difference between: var functionName = function() {} and function functionName() {} in Javascript?

There are two ways of declaring functions in JS. Which are called function expression and a function declaration.

The first example is a function declaration:

function abc(){}

The second example is a function expression:

var abc = function() {};

Let’s see the difference between them:

What is difference between var functionName function vs function functionName JavaScript

function expression example code

The function expression is defined when that line is reached.

The function expression is often called an “anonymous function” because it does not have to have a name,

<!DOCTYPE html>
<html>
<body>

    <script>
        // TypeError: functionOne is not a function
        functionOne();

        var functionOne = function() {
          console.log("Hello!");
      };
  </script>

</body>
</html>

function declaration example code

A function declaration is defined as soon as its surrounding function or script is executed (due to hoisting).

<!DOCTYPE html>
<html>
<body>

    <script>
        // Outputs: "Hello!"
        functionTwo();

        function functionTwo() {
          console.log("Hello!");
      }
  </script>

</body>
</html>

Note: Hoisting is the JavaScript interpreter’s action of moving all variable and function declarations to the top of the current scope.

Performance test

Function declarations are faster (source: stackoverflow.com).

Function declarations and expression performance

Do comment if you have any suggestions or questions on this topic.

Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.

OS: Windows 10

Code: HTML 5 Version

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