A pattern I see quite often in JavaScript is of people using a regular expression in a loop, e.g.
for (var i = 0; i < array.length; i++) {
array[i].match(/something/);
}
The naive assumption is that a regex is some special native thing, like a boolean. However regular expressions have a cost to construct, which you can see in this performance test – http://jsperf.com/creating-a-regex-in-and-outside-a-loop
So, if you’re using a regular expression in JavaScript more than once, declare it first and reuse it to see a dramatic performance gain
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