Typescript error: StrictResponse of generic is not assignable to AsyncResponseResolverReturnType

[gustavhagland]

Prerequisites

• I confirm my issue is not in the opened issues
• I confirm the Frequently Asked Questions didn't contain the answer to my issue

Environment check

• I'm using the latest msw version
• I'm using Node.js version 14 or higher

Browsers

No response

Reproduction repository

https://www.typescriptlang.org/play?#code/JYWwDg9gTgLgBAbzgCRjMAlApgZ0gOxywBo4ARLAMwEMBXAGxgCEIATATwBV2wS4ALNGDgBfOJSgQQcAOQgcAdxkBuAFDrKtfAGMYwCPjgQA1gHEsMZNXyt6WKAB5O2PAaJwsADxhYbOOAByBgEM9NQARnYOFDQMzGxcPFgAfMkAFL6sAAoQwPgwAFxwODBQeQDmAJSIqnBwUBa0UIZpDa6EWEXOuARE1QC8yTV1dYLoAHTlFhk2OXkwpGkDQ6joLr1Y4wBWOAatPW5YlZW1onCqIuoA9FdwAKKe1OB2cLQ41FOq2m7wXk9gdgAshBtMYrDY7FA4P0jGYLODbPYHAhqGgyuFaD4ivhaCBwvYROkAERXP7PLCkmyQeZEk5fH4CayIqD+GEAbVOZIBWGBoIRkLSKLRwAxWLgAEYRCcALpAA

Reproduction steps

I'm trying to create wrappers around the http.get method to be able to couple endpoints in our backend with their return types. Here is an example of one such wrapper:

import { HttpResponse, DefaultBodyType, http } from 'msw';

function okGetHandler<TResponse extends NonNullable<DefaultBodyType>>(endPoint: string) {
  return (response: TResponse) => {
    http.get(endPoint, () => HttpResponse.json(response))
  } 
}

Current behavior

This causes a Typescript error:
Type 'StrictResponse' is not assignable to type 'AsyncResponseResolverReturnType'.
Type 'StrictResponse' is not assignable to type 'TResponse extends undefined ? Response : StrictResponse'

Expected behavior

I expect StrictResponse to be assignable to AsyncResponseResolverReturnType.

There is a easy workaround which is to simply use the native Response like so:

export function okGetHandler<TResponse extends NonNullable<DefaultBodyType>>(endPoint: string) {
    return (response: TResponse) =>
        http.get(
            url + endPoint,
            () => new Response(JSON.stringify(response), { headers: { 'Content-Type': 'application/json' } }),
        )
}

this is not as readable however.

Is there some extends clause that I'm missing on TResponse?

[kettanaito]

Thanks for reporting this, @gustavhagland! I looked into this and discovered that this was indeed a bug in MSW. Opened a fix in #1824. Your example compiles without errors with that fix. Also take a look at how I put your example into a regression test and see if you'd get inspired from it in any way: https://github.com/mswjs/msw/pull/1824/files#diff-75a5caa27c79c5fbb266f10a9c4348c50f06e36ba284099859c04688ec183ddd

[gustavhagland]

Thank you for the quick response! That looks great!

[kettanaito]

Released: v2.0.3 🎉

This has been released in v2.0.3!

• 📄 Release notes
• 📦 npm package

Make sure to always update to the latest version (npm i msw@latest) to get the newest features and bug fixes.

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