Question

Would like to know when to use, This applies for what browser ?

if (document.all&&document.getElementById) {
// Some code block
}

Answer 1

I don’t think there’s any significant browser left that doesn’t support document.getElementById. Even IE6 supports it (although it gets it wrong — details — but it does the same thing with document.all). I’d just use document.getElementById rather than document.all.

If you’re using this just to detect the browser, rather than specifically because you want to use document.all or document.getElementById, browser detection is generally a bad idea. Instead, look at doing feature detection, testing for the actual features you want. There are some great examples of feature detection here and here.

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Off-topic: If you’re looking to do any significant cross-browser work, you might consider using a library like jQuery, Prototype, YUI, Closure, or any of several others. They’ll smooth over browser differences for you, provide some very useful utility functions, and even (in some cases) fix browser bugs like IE’s broken handling of document.getElementById (jQuery does that, for instance, some others may as well).