Iterating over Typescript Map

问题:

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:

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

And I get the Error:

Type 'IterableIteratorShim<[string, boolean]>' is not an array type or a string type.

Full Stack Trace:

Error: Typescript found the following errors: /home/project/tmp/broccoli_type_script_compiler-input_base_path-q4GtzHgb.tmp/0/src/app/project/project-data.service.ts (21, 20): Type 'IterableIteratorShim<[string, boolean]>' is not an array type or a string type. at BroccoliTypeScriptCompiler._doIncrementalBuild (/home/project/node_modules/angular-cli/lib/broccoli/broccoli-typescript.js:115:19) at BroccoliTypeScriptCompiler.build (/home/project/node_modules/angular-cli/lib/broccoli/broccoli-typescript.js:43:10) at /home/project/node_modules/broccoli-caching-writer/index.js:152:21 at lib$rsvp$$internal$$tryCatch (/home/project/node_modules/rsvp/dist/rsvp.js:1036:16) at lib$rsvp$$internal$$invokeCallback (/home/project/node_modules/rsvp/dist/rsvp.js:1048:17) at lib$rsvp$$internal$$publish (/home/project/node_modules/rsvp/dist/rsvp.js:1019:11) at lib$rsvp$asap$$flush (/home/project/node_modules/rsvp/dist/rsvp.js:1198:9) at _combinedTickCallback (internal/process/next_tick.js:67:7) at process._tickCallback (internal/process/next_tick.js:98:9)

I'm using angular-cli beta5 and typescript 1.8.10 and my target is es5. Has anyone had this Problem?

回答1:

You could use Map.prototype.forEach((value, key, map) => void, thisArg?) : void instead

Use it like this:

myMap.forEach((value: boolean, key: string) => {
    console.log(key, value);
});

回答2:

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);
}

回答3:

This worked for me. TypeScript Version: 2.8.3

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

回答4:

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

回答5:

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).

回答6:

Using Array.from, Array.prototype.forEach(), and arrow functions:

Iterate over the keys:

Array.from(myMap.keys()).forEach(key => console.log(key));

Iterate over the values:

Array.from(myMap.values()).forEach(value => console.log(value));

Iterate over the entries:

Array.from(myMap.entries()).forEach(entry => console.log('Key: ' + entry[0] + ' Value: ' + entry[1]));

回答7:

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);
}

回答8:

This worked for me.

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

回答9:

Just simple explanation to do it from HTML if you have a Map of types (key, array):

I initialize the array this way:

public cityShop: Map<string, Shop[]> = new Map();

And for iterate over it, I create an array from key values: - just use it as an array with : keys = Array.from(this.cityShop.keys());

Then, in HTML, I can use:

*ngFor="let key of keys"

Inside this bucle, I just get the array value with this.cityShop.get(key)

And... done!

回答10:

If you don't really like nested functions, you can also iterate over the keys:

myMap : Map<string, boolean>;
for(let key of myMap) {
   if (myMap.hasOwnProperty(key)) {
       console.log(JSON.stringify({key: key, value: myMap[key]}));
   }
}

Note, you have to filter out the non-key iterations with the hasOwnProperty, if you don't do this, you get a warning or an error.