What is the best way to do loops in JavaScript

Question

I have stumbled into several methods of looping in JavaScript, what I like the most is:

for(var i = 0; i < a.length; i++){
    var element = a[i];
}
         


        

Answer 1

I don't see what the problem with using a standard for(;;) loop is. A little test

var x;
var a = [];
// filling array
var t0 = new Date().getTime();
for( var i = 0; i < 100000; i++ ) {
    a[i] = Math.floor( Math.random()*100000 );
}

// normal loop
var t1 = new Date().getTime();
for( var i = 0; i < 100000; i++ ) {
    x = a[i];
}

// using length
var t2 = new Date().getTime();
for( var i = 0; i < a.length; i++ ) {
    x = a[i];
}

// storing length (pollution - we now have a global l as well as an i )
var t3 = new Date().getTime();
for( var i = 0, l = a.length; i < l; i++ ) {
    x = a[i];
}

// for in
var t4 = new Date().getTime();
for( var i in a ) {
    x = a[i];
}

// checked for in
var t5 = new Date().getTime();
for( var i in a ) {
    if (a.hasOwnProperty(i)) {
        x = a[i];
    }
}

var t6 = new Date().getTime();
var msg = 'filling array: '+(t1-t0)+'ms\n'+
          'normal loop: '+(t2-t1)+'ms\n'+
          'using length: '+(t3-t2)+'ms\n'+
          'storing length: '+(t4-t3)+'ms\n'+
          'for in: '+(t5-t4)+'ms\n'+
          'checked for in: '+(t6-t5)+'ms';
console.log( msg );

results in:

filling array: 227ms
normal loop: 21ms
using length: 26ms
storing length: 24ms 
for in: 154ms
checked for in: 176ms

So:- for in's take the longest, using the length property (which is a property and doesn't need to be calculated) is nearly as fast as storing it first - which is only a whisker slower than using an integer.
AND a for() is the usual way to loop over an array, which everyone expects and understands.

All of them add a variable to the scope they run in - i - which is a common name for this use and so shouldn't be used for other things. Storing the length first adds another var - l - to the scope, which is unnecesary