Question

i’m doing some javascript coding and I was wondering if the length method is “precomputed”, or remembered by the JS engine.

So, the question is:

If I’m checking really often for an array length, and supposing that i’m not changing it (making it immutable through a closure), should I precompute the length method and store it in some variable?

Thanks!

Answer 1

All major interpreters provide efficient accessors for the lengths of native arrays, but not for array-like objects like NodeLists.

"Efficient looping in Javascript"

Test / Browser                Firefox 2.0 Opera 9.1   Internet Explorer 6
Native For-Loop               155 (ms)    121 (ms)    160 (ms)
...
Improved Native While-Loop    120 (ms)    100 (ms)    110 (ms)

"Efficient JavaScript code" suggests

for( var i = 0; i < document.getElementsByTagName('tr').length; i++ ) {
  document.getElementsByTagName('tr')[i].className = 'newclass';
  document.getElementsByTagName('tr')[i].style.color = 'red';
  ...
}

---

var rows = document.getElementsByTagName('tr');
for( var i = 0; i < rows.length; i++ ) {
  rows[i].className = 'newclass';
  rows[i].style.color = 'red';
  ...
}

Neither of these are efficient. getElementsByTagName returns a dynamic object, not a static array. Every time the loop condition is checked, Opera has to reassess the object, and work out how many elements it references, in order to work out the length property. This takes a little more time than checking against a static number.