为什么在 Firefox 中 for (var i = 100; i--;) {} 比 for (var i = 100; i-->0;) {} 慢得多（70%）？

Question

Here's the test: http://jsperf.com/forloopspeed

As you can see, the difference is huge in Firefox, present to a much lesser extent in Safari, and absent in Chrome and Opera.

The analogous thing happens with while loops too: http://jsperf.com/whileloopspeed

Answer 1

My guess is that checking whether i (a Number) is a falsy value is more computationally expensive than checking true / false (the result of the comparison).

Answer 2

I suppose the internal ToBoolean() that is performed on the result of the expression is a bit slower when being given a number as compared to being given a boolean .

In this test I get a difference in performance when converting to boolean from a boolean vs a number using !! .

Answer 3

This looks like some issue specific to Jaegermonkey. If I run the test under Tracemonkey, the effect disappears.

Filed https://bugzilla.mozilla.org/show_bug.cgi?id=670493