Iterating over Typescript Map
TypescriptIteratorMapsTypescript Problem Overview
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?
Typescript Solutions
Solution 1 - Typescript
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);
});
Solution 2 - Typescript
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];
}
Solution 3 - Typescript
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);
}
Solution 4 - Typescript
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]));
Solution 5 - Typescript
This worked for me. TypeScript Version: 2.8.3
for (const [key, value] of Object.entries(myMap)) {
console.log(key, value);
}
Solution 6 - Typescript
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).
Solution 7 - Typescript
This worked for me.
Object.keys(myMap).map( key => {
console.log("key: " + key);
console.log("value: " + myMap[key]);
});
Solution 8 - Typescript
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);
}
Solution 9 - Typescript
You can also apply the array map method to the Map.entries() iterable:
[...myMap.entries()].map(
([key, value]: [string, number]) => console.log(key, value)
);
Also, as noted in other answers, you may have to enable down level iteration in your tsconfig.json (under compiler options):
"downlevelIteration": true,
Solution 10 - Typescript
On Typescript 3.5 and Angular 8 LTS, it was required to cast the type as follows:
for (let [k, v] of Object.entries(someMap)) {
console.log(k, v)
}
Solution 11 - Typescript
Just a simple explanation to use it in an HTML document.
If you have a Map of types (key, array) then you initialise the array this way:
public cityShop: Map<string, Shop[]> = new Map();
And to iterate over it, you create an array from key values.
Just use it as an array as in:
keys = Array.from(this.cityShop.keys());
Then, in HTML, you can use:
*ngFor="let key of keys"
Inside this loop, you just get the array value with:
this.cityShop.get(key)
Done!
Solution 12 - Typescript
I tried using Array.from( myMap.keys() )
to get an array of keys in my VSCode extension that's implemented using node.js, but it did not work, even after adding "downlevelIteration": true
to my tsconfig.json with the target set to es2017
as suggested by Ahmed Fasih and others.
What ultimately worked was Jason Slobotski's suggestion to use Object.keys
:
let keys = Object.keys(myMap);
Solution 13 - Typescript
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.