I was creating a custom pipe when I came across this. Why is the example below valid?

async transform(value: any): Promise<string> {
    let fullNameBasedOnPreference: string;

    fullNameBasedOnPreference = value.property;

    return fullNameBasedOnPreference;
  }

Link for reference: https://stackblitz.com/edit/angular-ivy-iphhv3?file=src/app/my-pipe.pipe.ts

CodePudding user response：

I am not sure if you are referring to the fact you can return string as any, or why you can return a string when the return type is Promise<any>. I.E. Your question would be the same if the return type was Promise.

Assuming the later: the async modifier will cause the return value to be wrapped by the Typescript compiler as a promise*.

*It's probably more complicated than this under the hood, but that's the gist of it.

https://devblogs.microsoft.com/typescript/what-about-async-await/

CodePudding user response：

You are giving the any type to your value variable. In fact your telling typescript that all types apply to your value. So typescript will assume that value.property is a string value as it could be for example:

const value = {
    property: 'string'
}

That's why it works fine. I would suggest not using any if you know the structure of your variable. Changes are that a custom type has already been defined that you can reuse. If not, create an Interface describing the type of your data and import it in your component/service.