为什么使用循环从数组的开始迭代到结束比迭代开始到结束和结束到开始更快?

Why is using a loop to iterate from start of array to end faster than iterating both start to end and end to start?

const arr = Array.from({length: 100}, (_, i) => i);
const n = 51;
const len = arr.length;

console.time("iterate from start");
for (let i = 0; i < len; i++) {
  if (arr[i] === n) break;
}
console.timeEnd("iterate from start");

const arr = Array.from({length: 100}, (_, i) => i);
const n = 51;
const len = arr.length;

console.time("iterate from start and end");
for (let i = 0, k = len - 1; i < len && k >= 0; i++, k--) {
  if (arr[i] === n || arr[k] === n) break;
}
console.timeEnd("iterate from start and end");

jsperf https://jsperf.com/iterate-from-start-iterate-from-start-and-end/1

推荐答案

答案很明显:

判断代码的速度时，看它会执行多少操作.只需一步一步计算它们.每条指令都将占用一个或多个 CPU 周期，并且运行时间越长.不同的指令占用不同数量的周期大多无关紧要 - 虽然数组查找可能比整数算术更昂贵，但它们基本上都需要恒定时间，如果有太多，它会主导我们算法的成本.

               |  forwards  | forwards and backwards
---------------+------------+------------------------
>=/===/<       |       100  |                   200
++/--          |        50  |                   100
a[b]           |        50  |                   100
&&/||/if/for   |       100  |                   200