Typescript runtime error: cannot read property of undefined (enum)

Question

I have the following interface and enum in a file RESTConfig.ts:

export const enum RESTMethod {
   POST = "POST",
   GET = "GET"
}

export interface RESTConfig {
   url: string;
   method: RESTMethod;
   data: any;
}

I want to import and use the enum in another class as such:

import { RESTConfig, RESTMethod } from './RESTConfig';

class Pipelines {
   ...
   private someMethod() {
      let rest: RESTConfig = {
         url: "",
         method: RESTMethod.POST,
         data: {}
      }
      ...

   }
   ...
}

Linting and transpiling works fine, but at runtime I get the following error:

TypeError: Cannot read property 'POST' of undefined

on the line "method: RESTMethod.POST".

Can someone tell me what I'm doing wrong?

Did find the answer?

VahidNaderi
– VahidNaderi

2018-09-15 20:23:45 +00:00
Commented Sep 15, 2018 at 20:23 — VahidNaderi
– VahidNaderi, Commented Sep 15, 2018 at 20:23
make the enum not constant - remove 'const' term

John Dow
– John Dow

2018-09-19 12:22:24 +00:00
Commented Sep 19, 2018 at 12:22 — John Dow
– John Dow, Commented Sep 19, 2018 at 12:22

Christiaan Maks · Accepted Answer · 2020-01-09 13:37:45Z

164

I just found out the hard way that this can also happen if you have circular imports.

answered Jan 9, 2020 at 13:37

Christiaan Maks

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4 Comments

Esteban Over a year ago

This is what bit me

Ahmad Over a year ago

Seems that is not so hard to happen!

glinda93 Over a year ago

This should go upward further

davetapley Over a year ago

I believe this is TypeScript/issues/3200

Gaëtan Maisse · Accepted Answer · 2018-11-09 20:27:35Z

48

In Typescript there are 2 kinds of enums:

Classic enum:

During TS to JS transpilation they are converted to real objects, so they exist at runtime.

enum Response {
    No = 0,
    Yes = 1,
}

const yes = Response.Yes; // Works at runtime

const nameOfYes = Response[yes]; // Also works at runtime because a reverse mapping is also generated during transpilation

const enum (the one you are using):

Const enums are removed during transpilation in JS so you can not use them at runtime. As per TS doc const enums exists to avoid paying the cost of extra generated code and additional indirection when accessing enum values.

const enum Response {
    No = 0,
    Yes = 1,
}

const yes = Response.Yes; // At runtime: ReferenceError: Response is not defined

const nameOfYes = Response[yes]; // During transpilation: TS2476: A const enum member can only be accessed using a string literal.

So just change your const enum to enum and your runtime error will be gone.

answered Nov 9, 2018 at 20:27

Gaëtan Maisse

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4 Comments

Michał Tatarynowicz Over a year ago

Also happens when you export declare enum.

Cody Over a year ago

@MichałTatarynowicz how do you workaround this if you need to export your enum but also need to use it at runtime?

LucaSC Jan 16 at 13:46

In my case, just removing the export was enough. Not sure what to do when it is required.

MDave Apr 14 at 22:28

export enum worked for me without declare

Tim Tyler · Accepted Answer · 2021-12-01 09:31:51Z

21

And if you want to identify your circular dependencies, run npx madge --circular --extensions ts ./

(Credit: Andrejs Abrickis's blog https://andrejsabrickis.medium.com/locate-circular-dependencies-in-typescript-modules-2b1eb03dbf2e)

answered Dec 1, 2021 at 9:31

Tim Tyler

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1 Comment

thelearner Over a year ago

This fixed my error

Yao Li · Accepted Answer · 2020-06-24 21:18:33Z

7

It works for me when put enum into an independent file

answered Jun 24, 2020 at 21:18

Yao Li

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1 Comment

Wojtek Jureczka Over a year ago

Probably you had a circular dependency

jahller · Accepted Answer · 2020-10-12 15:25:45Z

2

If you have this issue while running tests make sure that the file with the enum is included in the tsconfig.spec.json file.

e.g. when the enum is in a types.ts file

{
  [...]
  "include": ["types.ts"]
  [...]
}

answered Oct 12, 2020 at 15:25

jahller

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Comments

lynn · Accepted Answer · 2021-01-07 19:07:15Z

0

If you have --isolatedModules in tsconfig.json or transpileOnly in ts-loader, you may be hitting this issue. Basically, you cannot use these things in combination with export const enum.

In the words of a commenter there:

transpileOnly means transpile every file one at a time.. when the compiler is looking at one file, it has no way to know if the reference it is looking at is a const enum or not, since the declaration is in another file that it does not have access to..

so I do not think you can mix these two concepts, const enums (require whole program information), and transpileOnly (one file at a time).

answered Jan 7, 2021 at 19:07

lynn

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