JavaScript的队列优化-shift和pop

最近在看webpack的源码, 发现webpack封装的ArrayQueue类中, 实现的出队列方法dequeue在数组长度大于16时, 采用reverse+pop来代替shift.

dequeue() {
    if (this._listReversed.length === 0) {
        if (this._list.length === 0) return undefined;
        if (this._list.length === 1) return this._list.pop();
        if (this._list.length < 16) return this._list.shift();
        const temp = this._listReversed;
        this._listReversed = this._list;
        this._listReversed.reverse();
        this._list = temp;
    }
    return this._listReversed.pop();
}

benchmark测试

采用benchmark进行两种方式的性能测试.

const Benchmark = require("benchmark");
const suite = new Benchmark.Suite();
suite
  .add("shift", function () {
    let arr = [];
    for (let i = 0; i < 100000; i++) {
      arr[i] = i;
    }
    let length = arr.length;
    for (let i = 0; i < length; i++) {
      arr.shift();
    }
  })
  .add("reverse-pop", function () {
    let arr = [];
    for (let i = 0; i < 100000; i++) {
      arr[i] = i;
    }
    let length = arr.length;
    arr.reverse();
    for (let i = 0; i < length; i++) {
      arr.pop();
    }
  })
  .on("cycle", function (event) {
    console.log(String(event.target));
  })
  .on("complete", function () {
    console.log("Fastest is " + this.filter("fastest").map("name"));
  })
  .run({ async: true });

当数组长度是10时, 测试结果:

shift x 12,899,872 ops/sec ±1.55% (88 runs sampled)
reverse-pop x 14,808,207 ops/sec ±1.31% (92 runs sampled)
Fastest is reverse-pop

当数组长度是1000时, 测试结果:

shift x 13,518 ops/sec ±1.42% (88 runs sampled)
reverse-pop x 117,351 ops/sec ±1.03% (85 runs sampled)
Fastest is reverse-pop

当数组长度是100000时, 测试结果:

shift x 1.02 ops/sec ±5.80% (7 runs sampled)
reverse-pop x 523 ops/sec ±3.62% (84 runs sampled)
Fastest is reverse-pop

当数组长度越大, 两种方式的性能差距越大.

原因查找

shift方法每次调用时, 都需要遍历一次数组, 将数组进行一次平移, 时间复杂度是O(n). 而pop方法每次调用时, 只需进行最后一个元素的处理, 时间复杂度是O(1).

具体可参考ECMAScript language specification中关于Array.prototype.shift()和Array.prototype.pop()介绍.