How to set CSS value with JavaScript?

Question

I want to change the background color of an HTML element whose ID is foo. I have this code at present:

var hexcode = new Array('0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f');
// this chooses a random value from the array hexcode
var ranval = function() {
 return hexcode[Math.floor(Math.random()*hexcode.length)];
}
// six ranval() are put together to get color
var colorname = "#" + ranval() + ranval() + ranval() + ranval() + ranval() + ranval();
// trying to put the value on the background of element with "foo" ID.
document.getElementById("foo").style.color = colorname;

This code is throwing this error:

Uncaught TypeError: Cannot read property 'style' of null 

I'm sure that ID foo exists.

• Blender: Are you positive? Where is your <script> tag (before or after the body)?
• VLS: Not related to the error, but if you need to change the background color you should use style.backgroundColor instead of style.color.

Answer 1

Your error occurs because you're trying to access your element before the DOM is ready. Wait for the window to load before accessing it:

// Credit: http://paulirish.com/2009/random-hex-color-code-snippets/
function random_color() {
 return '#' + ('00000' + (Math.random() * 16777216 << 0).toString(16)).substr(-6);
}
window.onload = function() {
 document.getElementById("foo").style.backgroundColor = random_color();
};

Demo: http://jsfiddle.net/Blender/xQure/1/

• Gabriel Gartz: Your script will have errors @Blender, because if you get a number less then 6 characters the color wont change or changes wrong.
• Blender: @SantoshKumar: Sorry, load should be onload.
• Gabriel Gartz: @Blender still with the same problem, will not aways have 6 characters. You can complete it using RegExp, or use another like solution like what I did.
• Santosh Kumar: What if I want to change background color of body?
• Gabriel Gartz: @SantoshKumar Use document.body.style.backgroundColor = random_color() on the onload callback

Answer 2

The simple way to fix your code is:

var random_color = function() {
 function randomHex() {
 return Math.floor(Math.random() * 15).toString(16);
 }
 var str = '#';
 for(var i = 6; i--;) {
 str += randomHex();
 }
 return str;
}
window.onload = function() {
 // For your body background color
 document.body.style.backgroundColor = random_color();
 // For your foo element TEXT color
 document.getElementById("foo").style.color = random_color();
 // For your foo element BACKGROUND color
 document.getElementById("foo").style.backgroundColor = random_color();
};