谁能解释一下这两个循环之间的区别？

Question

http://jsperf.com/jquery-each-vs-for-loop/108

for (var b = a[0], len = a.length; len; b = a[--len]) {
  newArray.push(
    b
  );
}

and

for (var i = 0, len = a.length; i < len; i++) {
  newArray.push(
    a[i]
  );
}

1. According to jsref, it says the first one is faster. why?
2. Can anyone explain me the for loop on whats its doing compared to traditional way?

Answer 1

Your first example just does something very different. Check it out:

var a = [1,2,3,4],
    newArray = [];
for (var b = a[0], len = a.length; len; b = a[--len]) {
    newArray.push(b);
}

> newArray
[1, 4, 3, 2]

Your second example results in the expected [1, 2, 3, 4] .

---

If you need to understand the algorithms, it might be easier when converting the for -notation to while-loops and to expand the decrement and increment operators:

/* first example */
var b = a[0],
    len = a.length;
while (len) {
    newArray.push(b);
    len = len-1;
    b = a[len];
}

/* second example */
var i = 0,
    len = a.length;
while (i < len) {
    newArray.push( a[i] );
    i = i+1;
}

Answer 2

I think the major gain of the first loop is that a is being cached in len and then used as a single boolean value : there's a little time to be gained by simplifying the loop condition. However! It will not produce the same array as output.

Assume you have ten elements in a : [0,1,2,3,4,5,6,7,8,9] -- the loop will process a[0] first, then a[--len], which is a[9] in our example. Your end result will be newArray = [0, 9, 8, 7, 6, 5, 4, 3, 2, 1].

So it doesn't matter why it's faster, because it's doing the wrong thing.