Which is faster?
var str = ''; for(var i=0; i<1000; i++) { str += i; } or
var arr = []; for(var i=0; i<1000; i++) { arr.push(i); } var str = arr.join(''); I ask because I have written a CSS parser which works really well, but is very slow for larger stylesheets (understandably). I am trying to find ways of making it faster, I wondered if this would make a difference. Thanks in advance!
It is faster to do string concatenation (this thread explains why) which can be proved by this simple test
function f1() { var str = ''; for (var i = 0; i < 1000; i++) { str += i; } } function f2() { var arr = []; for (var i = 0; i < 1000; i++) { arr.push(i); } var str = arr.join(''); } var startTime = new Date(); for (var counter = 0; counter < 10000; counter++) { f1() } var endTime = new Date(); alert("String-concate Time Diff = " + (endTime - startTime)); var startTime = new Date(); for (var counter = 0; counter < 10000; counter++) { f2() } var endTime = new Date(); alert("Array join Time Diff = " + (endTime - startTime));As you can see string-concatenation takes 1/3rd of the time taken by Array-join.
console.time('a') var str = ''; for(var i=0; i<10000; i++) { str += i; } console.timeEnd('a') VM474:6 a: 1.940ms
console.time('a') var arr = []; for(var i=0; i<10000; i++) { arr.push(i); } var str = arr.join(''); console.timeEnd('a') VM476:7 a: 2.542ms
I did some benchmarking as follows. It seems the length of the string is important here especially for the Array.prototype.join() operation. Unless essential shouldn't be used for string concatenation.
String concat:
var t = 0, p = 0, s = "", i = 0; while (t<=1000) { p = performance.now(); s +=i; t += performance.now()-p; i++; } document.write("<pre> Done " + i + " string concat op/s</pre>");Array.push() + Array.join()
var t = 0, p = 0, a = [], s = "", i = 0; while (t<=1000) { p = performance.now(); a.push(i) s = a.join("") t += performance.now()-p; i++; } document.write("<pre> Done " + i + " Array.push() + Array.join() op/s</pre>");
i < 100000, but actual repeat function), mark times.