JavaScript Optimization Tests

The following time tests are all performed by running 1,000,000 iterations and reporting results in milliseconds, averaged after 5 times. Some results are obvious, while others are surprising. These tests can be used as guidelines for optimizing code.

Absolute Value (Integer)

y = Math.abs(x);
y = (x^(x>>31))-(x>>31);
y = x<0?-x:x;

	FF2.0	IE7.0	Op9.25
1	1538	1928	1443
2	1475	1072	1456
3	1131	956	1106

Truncation

y = Math.floor(x); (Note: This will round down negative values.)
y = x|0;
y = x>>0;

	FF2.0	IE7.0	Op9.25
1	1544	2000	1543
2	1212	875	922
3	1191	850	922

For Loops

for(var iter=1; iter<times; iter++){ }
for(var iter=times-1; iter>0; iter--){ }
for(var iter=times; --iter>=0;){ }

	FF2.0	IE7.0	Op9.25
1	857	631	784
2	547	369	459
3	384	222	334

Function Calls

x = function(){ ... }
x = setx(); function setx(){ ... }

	FF2.0	IE7.0	Op9.25
1	750	2403	1522
2	865	2653	1141

Swapping Values

var hold=x; x=y; y=hold;
x^=y; y^=x; x^=y;

	FF2.0	IE7.0	Op9.25
1	1546	1206	1309
2	2079	1290	1900