JavaScript is one of the 3 languages all web developers must learn:

   1. HTML to define the content of web pages

   2. CSS to specify the layout of web pages

   3. JavaScript to program the behavior of web pages

One of many JavaScript HTML methods is getElementById().

This example uses the method to "find" 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'; 

 

This example changes an HTML image by changing the src (source) attribute of an <img> tag:

Example

Turn on the light Turn off the light

Source:

<button onclick="document.getElementById('RevplitLight').src='img/pic_bulbon.gif'">Turn on the light</button>

<img id="RevplitLight" src="img/pic_bulboff.gif" style="width:100px">

<button onclick="document.getElementById('RevplitLight').src='img/pic_bulboff.gif'">Turn off the light</button>

 

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

Example:

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

 

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

Example:

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

 

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

Example:

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