I know I have asked this before, and I cannot find the question nor remember the answer.

I have an object with several methods with the same signature:

{

  foo: function(){
    return [];   // (return Array<T>)
  },

  bar: function(){
    return [];   // (return Array<T>)
  },

  baz: function(){
    return [];   // (return Array<T>)
  }

}

When I declare the interface for this object:

interface SomeObj {

   foo: Function,
   bar: Function,
   baz: Function

}

but I want to declare a type for these functions, something like this:

  type TestSuiteGetterFn <T>() => Array<T>;

  interface SomeObj {

       foo: TestSuiteGetterFn,
       bar: TestSuiteGetterFn,
       baz: TestSuiteGetterFn

    }

but this does not pile.

I have rarely found something so difficult to Google as this one.

I know I have asked this before, and I cannot find the question nor remember the answer.

I have an object with several methods with the same signature:

{

  foo: function(){
    return [];   // (return Array<T>)
  },

  bar: function(){
    return [];   // (return Array<T>)
  },

  baz: function(){
    return [];   // (return Array<T>)
  }

}

When I declare the interface for this object:

interface SomeObj {

   foo: Function,
   bar: Function,
   baz: Function

}

but I want to declare a type for these functions, something like this:

  type TestSuiteGetterFn <T>() => Array<T>;

  interface SomeObj {

       foo: TestSuiteGetterFn,
       bar: TestSuiteGetterFn,
       baz: TestSuiteGetterFn

    }

but this does not pile.

I have rarely found something so difficult to Google as this one.

• javascript
• typescript

• 1 What error do you receive? – Chris Commented Mar 5, 2017 at 8:08
• typescriptlang/play/… – Paul Samsotha Commented Mar 5, 2017 at 8:09
• @peeskillet thanks, maybe that works, but I think gyre's answer is more ideal? – Alexander Mills Commented Mar 5, 2017 at 8:15
• @Chris I got a red squiggly in my IDE (Webstorm), I didn't need to transpile to know there would be an error, but doubt it would be a meaningful error. – Alexander Mills Commented Mar 5, 2017 at 9:14

1 Answer 1

You simply forgot the equals sign when declaring your function type.

TypeScript Playground Permalink

type TestSuiteGetterFn<T> = () => Array<T>;

interface SomeObj {
     foo: TestSuiteGetterFn<string>,
     bar: TestSuiteGetterFn<number>,
     baz: TestSuiteGetterFn<string>
}