This content originally appeared on Lea Verou’s blog and was authored by Lea Verou
When using CSS, we can just include both declarations, one using rgba, and one without it, as mentioned in my post on cross-browser RGBA backgrounds. When writing JavaScript however, it’s a waste of resources to do that (and requires more verbose code), since we can easily check whether the browser is RGBA-capable, almost as easily as we can check whether it suppports a given property. We can even follow the same technique to detect the support of other CSS3 values (for instance, multiple backgrounds support, HSLA support, etc).
The technique I’m going to present is based on the fact that when we assign a non-supported CSS value on any supported CSS property, the browser either throws an error AND ignores it (IE-style), or simply ignores it (Firefox-style). Concequently, to check whether RGBA is supported, the algorithm would be:
- Get the
color(orbackgroundColor, orborderColoror any property that is widely supported and accepts color values) value of thestyleobject of any element that exists in the page for sure (for instance, the first script tag) and store it in a variable. - Assign an RGBA color to the
colorproperty of that element and catch any errors produced. - Assign to a variable whether the
colorof that element did change (booleantrueorfalse). - Restore the previous color to the
colorproperty, so that our script doesn’t interfere with the designer’s decisions. - Return the stored result.
and it would result in the following code:
function supportsRGBA()
{
var scriptElement = document.getElementsByTagName('script')[0];
var prevColor = scriptElement.style.color;
try {
scriptElement.style.color = 'rgba(1,5,13,0.44)';
} catch(e) {}
var result = scriptElement.style.color != prevColor;
scriptElement.style.color = prevColor;
return result;
}
Performance improvements
The code above works, but it wastes resources for no reason. Every time the function is called, it tests RGBA support again, even though the result will never change. So, we need a way to cache the result, and return the cached result after the first time the function is called.
This can be achieved in many ways. My personal preference is to store the result as a property of the function called, named 'result':
function supportsRGBA()
{
if(!('result' in arguments.callee))
{
var scriptElement = document.getElementsByTagName('script')[0];
var prevColor = scriptElement.style.color;
try {
scriptElement.style.color = 'rgba(0, 0, 0, 0.5)';
} catch(e) {}
arguments.callee.result = scriptElement.style.color != prevColor;
scriptElement.style.color = prevColor;
}
return arguments.callee.result;
}
Making it bulletproof
There is a rare case where the script element might already have rgba(0,0,0,0.5) set as it’s color value (don’t ask me why would someone want to do that :P ), in which case our function will return false even if the browser actually supports RGBA. To prevent this, you might want to check whether the color property is already set to rgba(0,0,0,0.5) and return true if it is (because if the browser doesn’t support rgba, it will be blank):
function supportsRGBA()
{
if(!('result' in arguments.callee))
{
var scriptElement = document.getElementsByTagName('script')[0];
var prevColor = scriptElement.style.color;
var testColor = 'rgba(0, 0, 0, 0.5)';
if(prevColor == testColor)
{
arguments.callee.result = true;
}
else
{
try {
scriptElement.style.color = testColor;
} catch(e) {}
arguments.callee.result = scriptElement.style.color != prevColor;
scriptElement.style.color = prevColor;
}
}
return arguments.callee.result;
}
Done!
This content originally appeared on Lea Verou’s blog and was authored by Lea Verou
Lea Verou | Sciencx (2009-03-01T00:00:00+00:00) Check whether the browser supports RGBA (and other CSS3 values). Retrieved from https://www.scien.cx/2009/03/01/check-whether-the-browser-supports-rgba-and-other-css3-values/
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