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);
  break;
  }
  }
  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