What is the return type for a function that returns another function

Question

I am working on developing Protractor tests using Typescript. It appears that the d.ts file available for protractor is very out of date. I am trying to update it to include the Expected Conditions protractor has added.

To summarize it, Expected Conditions are a set of functions within protractor that return a function that returns a promise of your value.

An example of usage:

protractor.ExpectedCondtions.visibilityOf(element(by.id('button1')))(); 

I am stumped on how to tell protractor that I am returning a function that will return a specific return type. Does anyone have any experience with this?

Answer 1

If I understood you correctly, your solution will depend on the type that the "second" function returns.

In a nutshell, there are at least 2 ways to do it:

1. Lambda syntax
2. Interfaces (normal and generic interfaces)

I've tried to explain all these in the code below, please, check it:

module main {     export class TestClass     {         // Use lamba syntax as an interface for a return function         protected returnSpecificFunctionWhichReturnsNumber(): () => number         {             return this.specificFunctionWhichReturnsNumber;         }          protected specificFunctionWhichReturnsNumber(): number         {             return 0;         }          // Use an interface to describe a return function         protected returnSpecificInterfaceFunction(): INumberFunction         {             return this.specificFunctionWhichReturnsNumber;         }          // Use a generic interface to describe a return function         protected returnSpecificGenericInterfaceFunction(): IReturnFunction<number>         {             return this.specificFunctionWhichReturnsNumber;         }     }      // An interface for a function, which returns a number     export interface INumberFunction     {         (): number;     }      // A generic interface for a function, which returns something     export interface IReturnFunction<ValueType>     {         (): ValueType;     } } 

Answer 2

Since this question popped up first in Google for how to type return function for a function that returns a function, I will add the generic solution here for declaring these types.

So if you want to add type declaration to this curried add function:

const add = (a : number) => (b: number) => a + b; 

You just duplicate what is after the = sign and make the return value the corresponding value:

export const add: (a : number) => (b: number) => number =     (a : number) => (b: number) => a + b; 

But at this point, you don't need the types for the actual function, so you can just type this, as if it was JS:

export const add: (a : number) => (b: number) => number =     a => b => a + b; 

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Writing it more extensively:

const add: (a : number) => (b: number) => number =     a => {         console.log(a);         return b => {             console.log(b);             return a + b;         }     }; 

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With generics:

export const add: <A extends number, B extends number>(a : A) => (b: B) => number =     a => b => a + b;