JavaScript's Statement Performance Questions

Question

Hi, Can you guys help me determine the performance difference of each of these statements? Which one would you use?

1. Making a new Array using

    - var new_list = new Array();  or
    - var new_list = []
   

2. Appending element using

   • push('a')
   • new_list[i]; (if i know the length)

3. Ternary operator or if() {} else (){}

4. Trying to make isodd function, which is faster
   (! (is_even)) or (x%2!=0)

5. foreach or normal iteration

one more

1. a= b = 3; or b=3; a=b;

[edit: Im making a Math Library. So any performance hacks discussions are also welcome :) ]

Thanks for your help.

Answer 1

I'd suggest you code a simple script like:

for(var i = 0; i < 1000; i++){
  // Test your code here.
}

You can benchmark whatever you want that way, possibly adding timing functions before and after the for statement to be more accurate.

Of course you'll need to tweak the upper limit (1000 in this example) depending on the nature of your operations - some will require more iterations, others less.

Answer 2

1. Both are native constructors probably no difference.
2. push is faster, it maps directly to native, where as [] is evaluative
3. Probably not much of a difference, but technically, they don't do the same thing, so it's not apples to apples
4. x%2, skips the function call which is relatively slow
5. I've heard, though can't find the link at the moment, that iteration is faster than the foreach, which was surprising to me.

Edit: On #5, I believe the reason is related to this, in that foreach is ordered forward, which requires the incrementor to count forward, whereas for loops are ever so infinitesimally faster when they are run backward:

for(var i=a.length;i>-1;i--) {
    // do whatever
}

the above is slightly faster than:

for(var i=0;i<a.length;i++) {
    // do whatever
}

Answer 3

As other posters suggest, I think doing some rough benchmarking is your best bet... however, I'd also note that you'll probably get very different results from different browsers, since I'm sure most of the questions you're asking come down to specific internal implementation of the language constructs rather than the language itself.

Answer 4

I've always assumed that since (x&1) is a bitwise operation, it would be the fastest way to check for even/odd numbers, rather than checking for the remainder of the number.

Answer 5

You may be interested in this question and the answers to it

Answer 6

This page says push is slower. http://dev.opera.com/articles/view/efficient-javascript/?page=2