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Objects for mapping

atila profile image Atila Fassina Originally published at on ・2 min read

Mapping key to value attributes has been done the same way in JavaScript for basically its entire existence. Chances are you either wrote or read it more times that you can count:

let dictionary = {
  id: 'foo',
  url: 'bar',
  last_update: new Date()
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Caveats of Objects

Also, chances are more than once you had to transform that into an array to get around. You see, objects aren't really great for mapping variables because (1) they aren't iterables. Besides the obvious difficulty of iterating through an object (Object.entries(), Object.keys(), Object.values()), there is also an aggregated side effect of they not being iterable, you cannot check the size or length of an object.

Another important point in some use-cases is that the developer is not able to trust the order in which the object will store its keys (at least for numeric keys, where behavior can change depending on engine).

Interlude: Weakmap

A Weakmap holds a key/value reference, but holds a weak reference between them. The point of this, is to allow it to be more easily garbage collected, so if there's no other reference to the key, it can be collected. This means that there's no method to retrieve size nor iterate through a Weakmap.

Also, it carries interesting requirements: the key cannot be of a primitive type, as a matter of fact, it needs to be an object. Though it holds a possible advantage against using an Object as a map: it holds the order of its keys.

⚠️ if you plan on using a Weakmap, be aware that the value of length will always be 0.

Browser compatibility


Differently from object, a map object preserves the insertion order of its keys. And as a counter from both Weakmap which only accepts object as key, and object which only accepts string or number, any primitive type can be a key in a map.

Maps are not only iterables, but enumerables.

let myMap = new Map([['foo', 'bar'], [0, 22]])

let iterator = myMap.entries()

console.log( // ['foo', 'bar']
console.log( // [0, 22]

console.log(myMap.size()) // 2
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Browser compatibility


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