JavaScript Introduction

This page contains some examples of what JavaScript can do.

JavaScript Can Change HTML Content

One of many JavaScript HTML methods is getElementById().

The example below "finds" an HTML element (with id="demo"), and changes the element content (innerHTML) to "Hello JavaScript":

Example

document.getElementById("demo").innerHTML = "Hello JavaScript";

JavaScript accepts both double and single quotes:

Example

document.getElementById('demo').innerHTML = 'Hello JavaScript';

JavaScript Can Change HTML Attribute Values

In this example JavaScript changes the value of the src (source) attribute of an <img> tag:

JavaScript Can Change HTML Styles (CSS)

Changing the style of an HTML element, is a variant of changing an HTML attribute:

Example

document.getElementById("demo").style.fontSize = "35px";

JavaScript Can Hide HTML Elements

Hiding HTML elements can be done by changing the display style:

Example

document.getElementById("demo").style.display = "none";

JavaScript Can Show HTML Elements

Showing hidden HTML elements can also be done by changing the display style:

Example

document.getElementById("demo").style.display = "block";

Did You Know?

JavaScript and Java are completely different languages, both in concept and design.

JavaScript was invented by Brendan Eich in 1995, and became an ECMA standard in 1997.

ECMA-262 is the official name of the standard. ECMAScript is the official name of the language.

Comments

JavaScript Const

The const keyword was introduced in ES6 (2015) . Variables defined with const cannot be Redeclared. Variables defined with const cannot be Reassigned. Variables defined with const have Block Scope. Cannot be Reassigned A const variable cannot be reassigned : Example const PI = 3.141592653589793 ; PI = 3.14 ; // This will give an error PI = PI + 10 ; // This will also give an error Must be Assigned JavaScript const variables must be assigned a value when they are declared: Correct const PI = 3.14159265359 ; Incorrect const PI; PI = 3.1 4159 265359 ; When to use JavaScript const? As a general rule, always declare a variable with const unless you know that the value will change. Use const when you declare: A new Array A new Object A new Function A new RegExp Constant Objects and Arrays The keyword const is a little misleading. It does not define a constant value. It defines a constant reference to a value.

JavaScript Let

The let keyword was introduced in ES6 (2015) . Variables defined with let cannot be Redeclared. Variables defined with let must be Declared before use. Variables defined with let have Block Scope. Cannot be Redeclared Variables defined with let cannot be redeclared . You cannot accidentally redeclare a variable. With let you can not do this: Example let x = "John Doe" ; let x = 0 ; // SyntaxError: 'x' has already been declared With var you can: Example var x = "John Doe" ; var x = 0 ; Block Scope Before ES6 (2015), JavaScript had only Global Scope and Function Scope . ES6 introduced two important new JavaScript keywords: let and const . These two keywords provide Block Scope in JavaScript. Variables declared inside a { } block cannot be accessed from outside the block: Example { let x = 2 ; } // x can NOT be used here Variables declared with the var keyword can NOT have block sco

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