如何在打字稿中的对象作为地图中获取数字键？

Question

I am using objects as maps in typescript:

const myMap: { [myKey: number]: string } = {};
myMap[42] = 'some string';

It works fine...

console.log(myMap[42]); // => 'some string'

...until I try to get the keys in the map

for (let key of Object.keys(myMap)) {
  // key is a string, not a number
}

I guess this is because Javascript actually stores the keys as strings internally, but is there a way to get the keys as numbers?

I guess I could use a Map but I'm a little wary of its performances. Also, I would lose some compatibility...

Answer 1

Well, iterating over the keys of an object is exactly what the for-in loop does. You just have to use parseInt on it:

for (let key in myMap) {
    if(myMap.hasOwnProperty(key)) {
        console.log(parseInt(key)); // 42
        console.log(myMap[key]); // some string
    }
}

Answer 2

Actually, Javascript convert any Index Signature to string by implicitly call toString().

You can check it by simply call:

console.log(myMap['42']); // => 'some string'

and it will still work. so whatever key you get from Object.keys() will be string.
If you still want to use it as number, you can do:

for (let key of Object.keys(myMap)) {
  console.log(typeof +key,'would be a number')
  console.log(typeof key, 'would be a string')
}

Answer 3

You could use an array, but then it puts undefined into every index not used. That makes iteration more challenging, and means it's impossible to purposefully add undefined to the "map" without losing it.

 const myMap = []; myMap[42] = "some string"; // 42 commas, then "some string" console.log(myMap.join(',')); // just "some string", but you lose the 42. console.log(myMap.filter(e => typeof e !== "undefined").join(',')); // Something similar to Map.prototype.entries for (let a of myMap.reduce((arr, cur, idx) => typeof cur !== "undefined" ? [...arr, [idx, cur]] : arr, [])) { console.log(a); // [ 42, "some string" ] } // Of course, a Map does this natively const myTrueMap = new Map(); myTrueMap.set(42, "some string"); for (let e of myTrueMap.entries()) { console.log(e); // [ 42, "some string" ] } 

There are some polyfills for Map (eg, https://www.npmjs.com/package/es6-map , https://github.com/paulmillr/es6-shim ), but YMMV.