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?

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?

• javascript
• ecmascript-6

• 1 I suspect it's because of the method call overhead. Also, you are abusing the collection(s) as an array, storing only integer indices. For which property access is super-optimised. Try with some type of key. – Bergi Commented Jun 2, 2017 at 5:23
• @Bergi fixed the array (though to be clear, it's not relevant to this question) cheers