The importance of refactoring... (by Mark McDonnell ( https://gist.github.com/Integralist )

1. Refactoring - Original.js

var isProxyBased = (/S40[\w]{3,5}Browser|Opera\sMini\//i).test(navigator.userAgent);

if (('querySelector' in document && 'localStorage' in window && 'addEventListener' in window && !isProxyBased) || (isIE > 6 && document.getElementById('js-holepunched'))) {
    // do stuff
}

2. Refactoring - Refactored.js

var cutsTheMustard = 'querySelector' in document && 'localStorage' in window && 'addEventListener' in window,
    isNotProxyBased = !/S40[\w]{3,5}Browser|Opera\sMini\//i.test(navigator.userAgent),
    modernDevice = cutsTheMustard && isNotProxyBased,
    holepunched = isIE > 6 && document.getElementById('js-holepunched');
 
if (modernDevice || holepunched) {
    // do stuff
}

3. Refactoring - Notes.md

I refactored this code by making just a couple of very basic tweaks...

1. move all logic out of the conditional and into variables
2. removed unnecessary syntax cruft from isProxyBrowser
3. give the variables descriptive names
4. further break down the logic (e.g. group the checks for 'cutting the mustard' and 'being a proxy based browser')
5. reversed the isProxyBased logic so it becomes isNotProxyBased (this meant we didn't clutter modernDevice with extra syntax and made it as readable as possible)
6. use selected variables within the conditional

4. Refactoring - Extra refactoring.md

Is there any thing more we could do with this code to make it as simple as possible to understand?

Well, one small tweak we could make would be to simplify the conditional even further. We could do this by creating another variable (again with a helpfully descriptive identifier) to hold the check of modernDevice || holepunched, like so...

var cutsTheMustard = 'querySelector' in document && 'localStorage' in window && 'addEventListener' in window,
    isNotProxyBased = !/S40[\w]{3,5}Browser|Opera\sMini\//i.test(navigator.userAgent),
    modernDevice = cutsTheMustard && isNotProxyBased,
    holepunched = isIE > 6 && document.getElementById('js-holepunched'),
    enhancedExperience = modernDevice || holepunched;
 
if (enhancedExperience) {
    // do stuff
}

One thing to be aware of is: refactoring can actually be a bad thing if you don't know why you're doing it. If you're just blindly following predefined rules of refactoring then the above small tweak could end up making the code harder to read.

Imagine if we were in a situation where there wasn't a logical association between modernDevice and holepunched. We could end up creating a variable like passesOurChecks (this is a poor example I know). Which would mean this new variable would actually make the code slightly harder to read because it wouldn't be descriptive enough and would likely mean the reader would need to check what the value of that variable was to better understand the code.

Just something to keep in mind.