Given the following object:

const parameters = {
  someNumberMember: 1 as number,
  someArrowFunctionMember: () => { return true; }
}

Is there a way to directly assign a type definition to someArrowFunctionMember? I mean a "complete" type definition of the function, not defining the parameter and return types. So something like this:

...
  someArrowFunctionMember: () => { return true; } as DesiredType,
...                                               ^^^^^^^^^^^^^^^ doesn't work

Context: I know there's 2 better ways to do this.

1. Assign a type to parameters that defines someArrowFunctionMember's type like this:

type ParametersType = {
  someNumberMember: number;
  someArrowFunctionMember: () => boolean;
}

const parameters: ParametersType = {
  someNumberMember: 1 as number,
  someArrowFunctionMember: () => { return true },
}

2. Extract the arrow function into a variable like this:

const handler: () => boolean = () => { return true };

const parameters = {
  someNumberMember: 1 as number,
  someArrowFunctionMember: handler,
}

I'm just curious if there is a syntax that allows me to do this directly.

I tried using the as keyword because it works when using a classic function:

const parameters = {
  someNumberMember: 1 as number,
  someArrowFunctionMember: function() {return true;} as () => boolean,
}                                                    ^^^^^^^^^^^^^^^^^ works like a charm

I also tried using type casting with the pointy brackets because it also works on classic functions:

const parameters = {
  someNumberMember: <number> 1,
  someArrowFunctionMember: <Function> function() {return true;},
}

Note: This initially didn't work until I realized the problem wasn't Typescript itself but rather using TS in conjunction with JSX, which makes sense. After disabling jsx in tsconfig, it works (at least in TS playground).

I found this answer but it doesn't seem to work anymore or there's something in my tsconfig preventing it from working.

CodePudding user response：

Have you tried placing the whole function within brackets:

...
  someArrowFunctionMember: (() => { return true; }) as DesiredType,
... 

Note:

Typescript will infer the type implicitly so usually you dot need a cast.