Iterating over Typescript Map

Question

I\'m trying to iterate over a typescript map but I keep getting errors and I could not find any solution yet for such a trivial problem.

My code is:

Answer 1

I'm using latest TS and node (v2.6 and v8.9 respectively) and I can do:

let myMap = new Map<string, boolean>();
myMap.set("a", true);
for (let [k, v] of myMap) {
    console.log(k + "=" + v);
}

Answer 2

This worked for me.

Object.keys(myMap).map( key => {
    console.log("key: " + key);
    console.log("value: " + myMap[key]);
});

Answer 3

Per the TypeScript 2.3 release notes on "New --downlevelIteration":

for..of statements, Array Destructuring, and Spread elements in Array, Call, and New expressions support Symbol.iterator in ES5/E3 if available when using --downlevelIteration

This is not enabled by default! Add "downlevelIteration": true to your tsconfig.json, or pass --downlevelIteration flag to tsc, to get full iterator support.

With this in place, you can write for (let keyval of myMap) {...} and keyval's type will be automatically inferred.

---

Why is this turned off by default? According to TypeScript contributor @aluanhaddad,

It is optional because it has a very significant impact on the size of generated code, and potentially on performance, for all uses of iterables (including arrays).

If you can target ES2015 ("target": "es2015" in tsconfig.json or tsc --target ES2015) or later, enabling downlevelIteration is a no-brainer, but if you're targeting ES5/ES3, you might benchmark to ensure iterator support doesn't impact performance (if it does, you might be better off with Array.from conversion or forEach or some other workaround).

Answer 4

This worked for me. TypeScript Version: 2.8.3

for (const [key, value] of Object.entries(myMap)) { 
    console.log(key, value);
}

Answer 5

Just use Array.from() method to convert it to an Array:

myMap : Map<string, boolean>;
for(let key of Array.from( myMap.keys()) ) {
   console.log(key);
}

Answer 6

es6

for (let [key, value] of map) {
    console.log(key, value);
}

es5

for (let entry of Array.from(map.entries())) {
    let key = entry[0];
    let value = entry[1];
}