How do you performance test JavaScript code?

Question

CPU Cycles, Memory Usage, Execution Time, etc.?

Added: Is there a quantitative way of testing performance in JavaScript besides just perception of how fast the code

Answer 1

I do agree that perceived performance is really all that matters. But sometimes I just want to find out which method of doing something is faster. Sometimes the difference is HUGE and worth knowing.

You could just use javascript timers. But I typically get much more consistent results using the native Chrome (now also in Firefox and Safari) devTool methods console.time() & console.timeEnd()

Example of how I use it:

var iterations = 1000000;
console.time('Function #1');
for(var i = 0; i < iterations; i++ ){
    functionOne();
};
console.timeEnd('Function #1')

console.time('Function #2');
for(var i = 0; i < iterations; i++ ){
    functionTwo();
};
console.timeEnd('Function #2')

Update (4/4/2016):

Chrome canary recently added Line Level Profiling the dev tools sources tab which let's you see exactly how long each line took to execute!