JavaScript loop performance – Why is to decrement the iterator toward 0 faster than incrementing

by

I’m not sure about Javascript, and under modern compilers it probably doesn’t matter, but in the “olden days” this code:

for (i = 0; i < n; i++){
  .. body..
}

would generate

move register, 0
L1:
compare register, n
jump-if-greater-or-equal L2
-- body ..
increment register
jump L1
L2:

while the backward-counting code

for (i = n; --i>=0;){
  .. body ..
}

would generate

move register, n
L1:
decrement-and-jump-if-negative register, L2
.. body ..
jump L1
L2:

so inside the loop it’s only doing two extra instructions instead of four.

More Related Contents:

  1. What’s the fastest way to loop through an array in JavaScript?
  2. Are loops really faster in reverse?
  3. How to stop intense Javascript loop from freezing the browser
  4. Is optimizing JavaScript for loops really necessary?
  5. Loop through an array in JavaScript
  6. What is the difference between call and apply?
  7. JavaScript closure inside loops – simple practical example
  8. How do I add a delay in a JavaScript loop?
  9. Access outside variable in loop from Javascript closure [duplicate]
  10. What is the best way to profile javascript execution? [closed]
  11. Iterate through object properties
  12. adding ‘click’ event listeners in loop [duplicate]
  13. Which browsers support ?
  14. How does a function in a loop (which returns another function) work? [duplicate]
  15. Unobtrusive JavaScript: at the top or the bottom of the HTML code?
  16. What’s the difference between reflow and repaint?
  17. What is the most efficient way to reverse an array in Javascript?
  18. Does javascript process using an elastic racetrack algorithm
  19. How to quickly clear a JavaScript Object?
  20. passing index from for loop to ajax callback function (JavaScript)
  21. Why is iterating through an array backwards faster than forwards
  22. Why inline JavaScript is bad?
  23. How to change the playing speed of videos in HTML5?
  24. Is ternary operator, if-else or logical OR faster in javascript?
  25. Javascript while loop return value
  26. Sum two arrays in single iteration
  27. javascript closure immediate evaluation [duplicate]
  28. Why is pop faster than shift?
  29. Sort a table fast by its first column with Javascript or jQuery
  30. Is using labels in JavaScript bad practice?

Leave a Comment