I want to build an hashmap class, or use one that es with the language.

The index are integers. If I try to do this with an array, I see on the debugger than the size of the array is equals to the higher integer key.

Ie, if my hashmap has two elements, Map[0]= 'word1' and Map[1023]= 'word2' I can see that the array has size 1024. I'd prefer not to waste so much space.

I can't make any assumption on how the keys are distributed.

I want to build an hashmap class, or use one that es with the language.

The index are integers. If I try to do this with an array, I see on the debugger than the size of the array is equals to the higher integer key.

Ie, if my hashmap has two elements, Map[0]= 'word1' and Map[1023]= 'word2' I can see that the array has size 1024. I'd prefer not to waste so much space.

I can't make any assumption on how the keys are distributed.

• javascript
• ecmascript-6

• 2 var blah = {} (don't use var Map ... Map is a thing in browsers except early internet exploders < 11) – Jaromanda X Commented Dec 24, 2015 at 1:21
• No, the size is still 1023. – Snick Commented Dec 24, 2015 at 1:34
• You're doing it wrong. A js object doesn't have a length – Jaromanda X Commented Dec 24, 2015 at 1:35
• "I'd prefer not to waste so much space." Arrays in JavaScript work differently than in other languages. They are basically just objects. An array that has length 1000 doesn't necessarily take up that much space. – Felix Kling Commented Dec 24, 2015 at 8:47
• Correct, but you are not taking advantage of new ES6 browser implementations. An array is probably implemented with a linked list and a hashmap, but we don't have control on the pre allocated size. – Snick Commented Dec 26, 2015 at 19:08

1 Answer 1

ECMAScript 6 introduces a true Map type which can be used as follows:

const m = new Map();
m.set(0, "word1");
m.set(1023, "word2");
for(const [key, value] of m) {
    console.log(key, value, typeof key);
}

Note how key is still a number - using an object literal, keys are always strings. It also provides a size property for counting key/value pairs:

const m = new Map();
m.set(0, "word1");
m.set(1023, "word2");
console.log(m.size); // 2

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