为什么在JavaScript中要实现哈希表，ES6映射通常比普通对象快？

Question

I've been grinding LeetCode these days using JavaScript. I found that if I need to implement a hash table, for example this famous question, https://leetcode.com/problems/two-sum/ , using a ES6 map generally results in better speed compared to plain old javascript object, like 20ms faster.

var twoSum = function(nums, target) {
  const map = new Map();

  for (let [index, value] of nums.entries()) {
    if (map.has(target - value)) return [map.get(target - value), index];
    map.set(value, index);
  }

};


var twoSum = function(nums, target) {
  const map = {};

  for (let [index, value] of nums.entries()) {
    if (map[target - value] !== undefined) return [map[target - value], index];
    map[value] = index;
  }
};

To me that the biggest use case for ES6 Map over plain old objects is when we want the keys not only to be strings. Can someone please explain to me why Map is superior in terms of speed and what are some other use cases where Map is better than plain old objects in JavaScript

Answer 1

In your example, the Map version uses numbers for keys, while the object version uses strings.

Converting all those numbers (like target-value ) into strings probably accounts for most of the extra cost in the object version.