Type { [key: string]: string } is not assignable to type { [key: string]: string } | 'undefined'

Question

I'm trying to create a function that wraps fetch from node-fetch by just adding a cookie:

import fetch from 'node-fetch';

const api = (path: string, params: RequestInit) => fetch(
  path, {
    ...(params || {}),
    headers: {
      ...(params?.headers || {}),
      cookie: 'mycookie'
    },
  }
)

I'm getting this error on headers:

(property) RequestInit.headers?: string[][] | Headers | {
    [key: string]: string;
} | undefined
Type '{ cookie: string; append(name: string, value: string): void; delete(name: string): void; get(name: string): string | null; has(name: string): boolean; set(name: string, value: string): void; forEach(callbackfn: (value: string, key: string, parent: Headers) => void, thisArg?: any): void; } | { ...; } | { ...; }' is not assignable to type 'string[][] | Headers | { [key: string]: string; } | undefined'.
  Type '{ cookie: string; append(name: string, value: string): void; delete(name: string): void; get(name: string): string | null; has(name: string): boolean; set(name: string, value: string): void; forEach(callbackfn: (value: string, key: string, parent: Headers) => void, thisArg?: any): void; }' is not assignable to type 'string[][] | Headers | { [key: string]: string; } | undefined'.
    Type '{ cookie: string; append(name: string, value: string): void; delete(name: string): void; get(name: string): string | null; has(name: string): boolean; set(name: string, value: string): void; forEach(callbackfn: (value: string, key: string, parent: Headers) => void, thisArg?: any): void; }' is not assignable to type 'undefined'.ts(2322)

At the end it says it is not assignable to type 'undefined'. I thought if a value for a type is guaranteed to be one of it's union types, it shouldn't complain.

It seems like the methods are tripping typescript up.

Aplet123
– Aplet123

2021-01-04 16:13:01 +00:00
Commented Jan 4, 2021 at 16:13 — Aplet123
– Aplet123, Commented Jan 4, 2021 at 16:13

Explosion Pills · Accepted Answer · 2021-01-04 16:47:35Z

1

node-fetch defines its own types that are incompatible with the DefinitelyTyped type definitions (node-fetch's are presumably more up-to-date). You should generally use a library's own types instead of DefinitelyTyped if both are available.

https://github.com/node-fetch/node-fetch/blob/master/@types/index.d.ts#L60 https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/node-fetch/index.d.ts#L48

The library's type has RequestInit's body property as BodyInit | null, so the ability to sign it as null makes it incompatible with the other typing. Use the library's typing simply by importing it:

import fetch, { RequestInit } from 'node-fetch';

answered Jan 4, 2021 at 16:47

Explosion Pills

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