Measuring JavaScript performance

Posted on May 10, 2009 by Frederik Vig in Code Snippet, JavaScript

According to Response Time overview, the response time for JavaScript code to execeute should be no more than 0.1 seconds (100 milliseconds). Especially on JavaScript/Ajax heavy sites this can be a problem. That’s why it is always good thing to check the performance of your code.
To measure your JavaScript you can use the code below:

function somefunction() {
    var start = new Date().getMilliseconds();
    // code here
    var stop = new Date().getMilliseconds();
    var executionTime = stop - start;
    console.log("Execution time " + executionTime +
        " milliseconds"); // or alert("Execution time " + executionTime + " milliseconds");
}

Note that Firebug for Firefox also has a code profiler which does the same thing. To use it install Firebug, enable firebug (f12), activate the console window, press the profile button, refresh the page, click the profile button again (more information: Firebug JavaScript Debugger and Profiler).