User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:48.0) Gecko/20100101 Firefox/48.0 Build ID: 20160419030312 Steps to reproduce: Chrome, Edge, IE11 and IE10 perform the following benchmarks in approx the same time (or faster for Object.keys versions), while Firefox is 60% slower: // single-hash var hash = {}, key; for (var i = 0, len = 1e6; i < len; i++) { key = Math.random().toString(36).substring(4); hash[key] = true; } var x; console.time("for-in"); for (var i in hash) x = hash[i]; console.timeEnd("for-in"); console.time("for/keys"); for (var i = 0, keys = Object.keys(hash), len = keys.length; i < len; i++) x = hash[keys[i]]; console.timeEnd("for/keys"); // multi-hash var len = 1e3, hashes = Array(len), key; for (var j = 0; j < len; j++) { var hash = {}; for (var i = 0; i < len; i++) { key = Math.random().toString(36).substring(4); hash[key] = true; } hashes[j] = hash; } console.time("for-in (multi)"); for (var j = 0; j < len; j++) { hash = hashes[j]; for (var i in hash) x = hash[i]; } console.timeEnd("for-in (multi)"); console.time("for/keys (multi)"); for (var j = 0; j < len; j++) { hash = hashes[j]; for (var i = 0, keys = Object.keys(hash), len = keys.length; i < len; i++) x = hash[keys[i]]; } console.timeEnd("for/keys (multi)"); Actual results: 50% perf drop. Expected results: Faster or same perf.

I came across this bug as I was working on a small JS script to create fractals. I may be wrong but my suspicion is that the performance has less to do with Object.keys and more to do with how for loops are treated by Spidermonkey. Consider the following script: --------------------------------------------------------- var a,b; randomArray = (length, max) => [...new Array(length)] .map(() => Math.round(Math.random() * max)); var test = randomArray(1000000, 10000); console.time('oldfor'); for (var i = 0 ; i < test.length; i++) { b = test[i]; } console.timeEnd('oldfor'); console.time('newfor'); for (var t in test) { a = test[t];} console.timeEnd('newfor'); --------------------------------------------------------- On Edge, this produces: oldfor: 463.17ms newfor: 494.23ms On Firefox, this produces: oldfor: 826.65ms newfor: 652.41ms It's entirely possible that Object.keys accentuates this difference, but I couldn't be sure. I'll play around a little more tomorrow.