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What is the proper way to loop through literals of an enum in TypeScript?

(I am currently using TypeScript 1.8.1.)

I've got the following enum:

export enum MotifIntervention { Intrusion, Identification, AbsenceTest, Autre } export class InterventionDetails implements OnInit { constructor(private interService: InterventionService) { let i:number = 0; for (let motif in MotifIntervention) { console.log(motif); } }

The result displayed is a list

0 1 2 3 Intrusion, Identification, AbsenceTest, Autre

I do want only four iterations in the loop as there are only four elements in the enum. I don't want to have 0 1 2 and 3 that seem to be index numbers of the enum.

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  • 1
    checkout this enum-for package Commented Jun 3, 2020 at 16:33
  • If you're not locked into using enum, it looks like this answer gives a better alternative. Commented Feb 20 at 1:41

4 Answers 4

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705

Two options:

for (let item in MotifIntervention) { if (isNaN(Number(item))) { console.log(item); } }

Or

Object.keys(MotifIntervention).filter(key => !isNaN(Number(MotifIntervention[key])));

(code in playground)

Edit

String enums look different than regular ones, for example:

enum MyEnum { A = "a", B = "b", C = "c" }

Compiles into:

var MyEnum; (function (MyEnum) { MyEnum["A"] = "a"; MyEnum["B"] = "b"; MyEnum["C"] = "c"; })(MyEnum || (MyEnum = {}));

Which just gives you this object:

{ A: "a", B: "b", C: "c" }

You can get all the keys (["A", "B", "C"]) like this:

Object.keys(MyEnum);

And the values (["a", "b", "c"]):

Object.keys(MyEnum).map(key => MyEnum[key])

Or using Object.values():

Object.values(MyEnum)
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11 Comments

Thanks for this approach. For the Object.keys method, you'll want to use Object.keys(MotifIntervention).filter(key => isNaN(Number(key))).
This doesn’t work if you're expecting to loop through values of the enum type. So for (let item in MotifIntervention) { doThing(item) } will fail if typeof doThing function is (arg: MotifIntervention) => void. That's because the for..in loop will treat item as type string, not type MotifIntervention.
@NitzanTomer - that sounds like a good workaround at runtime, but i'm talking about compile time. When you attempt to call doThing(item) and doThing is of type (arg: MotifIntervention) => void, you'll get a compile time error "Argument of type 'string' is not assignable to parameter of type 'MotifIntervention'."
I also include (typeof enumType[key] !== "function") in the filter to ignore methods I may have added to the enum type.
this will not work for enum X { "012" = 1 }
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3

Enum Keys

Here is a solution which works for both string and numeric enums:

function enumKeys<T extends object>(e: T) { const keys = Object.keys(e) const isStringEnum = isNaN(Number(keys[0])) return isStringEnum ? keys : keys.slice(keys.length / 2) }

To use simply do as following:

enum En { A, B, C, } console.log(enumKeys(En)); // ["A", "B", "C"] enum Es { A = "a", B = "b", C = "c", } console.log(enumKeys(Es)); // ["A", "B", "C"]

Here is TS playground

Enum values

Also, to get enum values one can use:

function enumValues<T extends object>(e: T) { const values = Object.values(e) const isNumEnum = e[e[values[0]]] === values[0] return isNumEnum ? values.slice(values.length / 2) : values }

To use simply do as following:

enum En { A, B, C, } console.log(enumValues(En)); // [0, 1, 2] enum Es { A = "a", B = "b", C = "c", } console.log(enumValues(Es)); // ["a", "b", "c"]

Here is TS Playground

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1 Comment

Very elegant solution.
1 
export enum MotifIntervention { Intrusion, Identification, AbsenceTest, Autre } const values = Object.values(MotifIntervention) values.forEach(val => { // -------> here is the value }) const keys = Object.keys(MotifIntervention) keys.forEach(key => { // -------> here is the key })

for string keys:

export enum LOCAL_STORAGE_KEYS { AUTH = 'auth', PERMISSIONS = 'permissions' } const storageKeys = Object.values(LOCAL_STORAGE_KEYS) storageKeys.forEach(val => { // -------> here the val is type if LOCAL_STORAGE_KEYS })
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Comments

0

You can also use Enum.enums.

example Enum:

const eventType = new Enum({ value0: 0, value1: 1, value2: 2 });

loop through like this:

for (let enumValues of eventType.enums) { let key = enumValues.key; let value = enumValues.value; }

You can then display any key or value.

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