JavaScript Functions Tutorial

JavaScript Functions

Functions are one of the fundamental building blocks in JavaScript. A function is a JavaScript procedure - a set of statements that performs a task or calculates a value. To use a function, you must define it somewhere in the scope from which you wish to call it.

A JavaScript function is a block of code designed to perform a particular task. A JavaScript function is executed when "something" calls it.

Syntax

A JavaScript function is defined with the function keyword, followed by a name, followed by parentheses (). Function names can contain letters, digits, underscores, and dollar signs (same rules as variables).

function name(parameter1, parameter2, parameter3) {
    code to be executed
}

Example

In this example, we call a function which performs an addition and returns the result

The code inside the function will execute when "something" invokes (calls) the function:

When an event occurs
When it is called from JavaScript code
Automatically

Function Return

When JavaScript reaches a return statement, the function will stop executing.

If the function was called from a statement, JavaScript will "return" to execute the code after the invoking statement.

Functions often compute a return value. The return value is "returned" back to the "caller":

Example

Calculate the product of two numbers, and return the result:

Why Functions?

You can reuse code: Define the code once, and use it many times. You can use the same code many times with different arguments, to produce different results.

Example: Convert Fahrenheit to Celsius:

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JavaScript Functions Tutorial