JavaScript Map比Object慢得多的随机查找？ [英] JavaScript Map much slower than Object for random look-ups?

问题描述

请考虑以下两个片段（来自此jsperf条目）：

  let val = 0; 
 for（let i of indices）{
 val + = map.get（i）; 
} 
 // --- 
 let val = 0; 
 for（let i of indices）{
 val + = obj [i]; 
} 
  

这里，地图是 地图 ， obj 是一个纯旧的JavaScript对象（ let obj = {} ）， code> indices 是一个随机索引数组。已经使用数据预填充了 obj 和 map ，因此查找实际返回数据。查看jsperf的完整代码。

问题：

执行 Map 5倍以上？这只是因为在写作时， Map 仍然是非常新的和未优化的？或者在 Map 查找中有一些开销，总是会阻止它与POJO一样快？

如果它还没有优化，我们期望它比POJO更快地进行随机查找？最终？为什么？为什么不呢？

解决方案

感谢@Bergi为这个答案。

与 Map 相比，简单JavaScript对象在初始jsperf中执行得很好的原因是因为在引擎盖下，JS引擎可以看到我们正在使用对象一个数组（连续的整数键），所以它假定它是一个数组，可以根据这个做一些优化。 c code code code code code code code code code $还有一个缺点，它需要一个方法调用（ map.get（p）），而不是直接属性查找（ obj [p ] ）。这没有（不能？）优化，如jsperf所示： http： //jsperf.com/map-vs-pojo-lookups

Consider the following two snippets (from this jsperf entry):

let val = 0;
for(let i of indices) {
  val += map.get(i);
}
// ---
let val = 0;
for(let i of indices) {
  val += obj[i];
}

Here, map is a Map, obj is a plain old JavaScript object (let obj = {}), and indices is an array of random indices. Both obj and map have been pre-populated with data so the lookups actually return data. Check out the jsperf for the full code.

Question:

Why does the plain old javascript object out-perform the Map by a factor of 5+? Is this simply because as of writing, Maps are still very new and un-optimised? Or is there some overhead in Map lookups that will always keep it from being as fast as a POJO?

If it's just not optimised yet, can we expect it to be faster than a POJO for random lookups eventually? Why? Why not?

解决方案

Thanks to @Bergi for this answer.

The reason the plain JavaScript object performed so well in the initial jsperf compared to the Map is because under the hood a JS engine can see that we're using the object like it's an array (consecutive integer keys), so it "assumes" that it's an array and can make a bunch of optimisations based on that. Not so with the Map.

But the Map has a further disadvantage in that it requires a method call (map.get(p)), as opposed to a direct property lookup (obj[p]). This hasn't been (can't be?) optimised away as shown by this jsperf: http://jsperf.com/map-vs-pojo-lookups

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