Question 173719 in Javascript Question posted in
A very good W3school tutorial can be found here.

extend keys() and other generator method of Map Class in javascript

I need to use an object as keys for my map, therefore I extended the map class that stringifies the object passed, like so

class CoordMapper extends Map {
    set = (k: ISquareCoordinate, v: Array<ISquareCoordinate>) => {
        const stringifiedKey = JSON.stringify(k)
        return super.set(stringifiedKey,v)
    }

    get = (k: ISquareCoordinate) => {
        const stringifiedKey = JSON.stringify(k)
        return super.get(stringifiedKey)
    }
}

As far as I understand keys(), values() and entries() are generator methods, therefore I can do something like

* keys() {
   const keysArr = [...super.keys()]
   for (const key of keysArr){
      yield JSON.parse(key)
   }
}

But this causes me to load all the keys which I wish to avoid, is there a better way?

EDIT:
While Map does take in objects as keys but it only checks objects by reference. As an example

let newMap = Map()
const obj1 = {'a': 1, 'b' :2}
newMap.set(obj1, 123)
const copyObj1 = {...obj1}
console.log(newMap.get(obj1)) //returns 123
console.log(newMap.get(copyObj1)) //returns undefined

And i need the second console.log to return 123 aswell

Tags:

2

Answers


  1. 0

    Rather than collecting all the parent values into an array, just iterate over them directly:

    * keys() {
   const parentKeyIterator = super.keys();
   for (const key of parentKeyIterator){
      yield JSON.parse(key)
   }
}

    That way, the laziness of the iterator is retained: each time next() is called on your extended iterator, it will call next() on parentKeyIterator once, then reach your yield statement, and pause.

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  2. 0

    Just in case someone stumble here in the future, there’s a stage-3 proposal which, if approved, would add syntactic sugar for iterators so that you can do something like this:

    class CoordMapper extends Map {
  *keys() {
    yield* super.keys().map(key => JSON.parse(key));
  }
}

    Try it (this doesn’t work yet):

    console.config({ maximize: true });

class CoordMapper extends Map {
  set(k, v) {
    return super.set(JSON.stringify(k), v)
  }
  get(k) {
    return super.get(JSON.stringify(k));
  }
  *keys() {
    console.log(super.keys())
    yield* super.keys().map(JSON.parse);
  }
}

const c = new CoordMapper();

c.set({ foo: 'bar' }, 0);
c.set({ baz: 42 }, 1);
c.set({ qux: { lorem: 'ipsum' } }, [null, undefined]);

for (const key of c.keys()) {
  console.log(key);
}
    <script src="https://gh-canon.github.io/stack-snippet-console/console.min.js"></script>
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