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Tech Tips, Tricks & Trivia

by 'Anil' Radhakrishna
An architect's notes, experiments, discoveries and annotated bookmarks.

Search from over a hundred HOW TO articles, Tips and Tricks

JavaScript Performance tips

Excerpted from "Chapter 7: Writing Efficient JavaScript" of the book "Even Faster Websites":
  • Performance .. is not just about how long it takes for the page to load, but also about how it responds as it’s being used.
  • Out-of-scope variables take longer to access than local variables.
  • A very common mistake that leads to performance issues is to omit the var keyword when assigning a variable’s value for the first time. 
  • If an array item or object property is used more than once, store it in a local variable to speed up access to the value.
  • Generally speaking, interacting with DOM objects is always more expensive than interacting with non-DOM objects.
  • The if statement is best used with a small number of discrete values or a range of values; the switch statement is best used when there are between 3 and 10 discrete values to test for; array lookup is most efficient for a larger number of discrete values.
  • To make a loop the most efficient, reverse the order in which you process the items so that the control condition compares the iterator to zero.
  • Trimming strings may be expensive, depending on the size of the string. 
  • Steven Levithan's optimized string trimming function:
    function trim(text){
    text = text.replace(/^\s+/, "");
    for (var i = text.length - 1; i >= 0; i--) {
    if (/\S/.test(text.charAt(i))) {
    text = text.substring(0, i + 1);
    return text;
  • Array processing is one of the most frequent causes of long-running scripts.
  • Generally speaking, no single continuous script execution should take longer than 100 milliseconds...
  • Because JavaScript is a single-threaded language, only one script can be run at a time per window or tab.
  • Exactly what causes the browser to display the long-running script dialog varies depending on the vendor:
    • Internet Explorer displays it when 5 million (by default) statements have been executed. 
    • Firefox shows it when a script takes longer than 10 seconds (default).
    • Safari displays it when the execution time exceeds default timeout of five seconds
    • Chrome (as of version 1.0) has no set limit on how long JavaScript is allowed to run. The process will crash when it has run out of memory.
    • Opera is the only browser that doesn’t protect against long-running scripts. Scripts are allowed to continue until execution is complete


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