Is there ArgumentsType<T> like ReturnType<T> in Typescript?

2 answers

• answered 2018-07-12 07:45 Cerberus

  There is no way for now to extract both the types and amount of arguments for any function possible. But you can try something like this:

  type ArgumentTypes<T> = T extends () => any ? never[] :
                          T extends (a1: infer T1) => any ? [T1] :
                          T extends (a1: infer T1, a2: infer T2) => any ? [T1, T2] :
                          // continue here for any reasonable number of args
                          never;
  

  Check it with the following:

  const args0: ArgumentTypes<() => boolean> = []; // correct
  const args1: ArgumentTypes<(a: number) => boolean> = [1]; // correct
  const args2: ArgumentTypes<(a: number, b: string) => boolean> = [1, 'str']; // correct
  
  const oops0null: ArgumentTypes<() => boolean> = null; // error, arguments are array (but empty one)
  const oops01: ArgumentTypes<() => boolean> = [1]; // error, arguments must be empty
  
  const oops10: ArgumentTypes<(a: number) => boolean> = []; // error, we need one argument
  const oops12: ArgumentTypes<(a: number) => boolean> = [1, 2]; // error, we need only one argument
  const oops1wrong: ArgumentTypes<(a: number) => boolean> = ['str']; // error, argument must be number
  
  const oops21: ArgumentTypes<(a: number, b: string) => boolean> = [1]; // error, we need two arguments
  const oops23: ArgumentTypes<(a: number, b: string) => boolean> = [1, 'str', undefined]; // error, we need only two arguments
  const oops2wrong: ArgumentTypes<(a: number, b: string) => boolean> = ['str', 1]; // error, arguments are reversed
  

  Note that this don't have any use of optional arguments - they are just omitted from the output. I wasn't able to find a way to catch them for now.

• answered 2018-07-12 07:48 Titian Cernicova-Dragomir

  While this is not possible in the current version of typescript (2.9) without spelling out all parameters. It will become possible in the next version of typescript (3.0) which will be released in the next few days:

  type ArgumentsType<T> = T extends  (...args: infer U) => any ? U: never;
  
  type argsEmpty = ArgumentsType<() => void> // []
  type args = ArgumentsType<(x: number, y: string, z: boolean) => void> // [number, string, boolean]
  type argsOpt = ArgumentsType<(x: number, y?: string, z?: boolean) => void> // [number, (string | undefined)?, (boolean | undefined)?]
  

  If you install npm install typescript@next you can already play with this, it should be available sometime this month.

  Note

  We can also spread a tuple into arguments with this new feature:

  type Spread<T extends any[]> = (...args: T)=> void;
  type Func = Spread<args> //(x: number, y: string, z: boolean) => void
  

  You can read more about this feature here