CODEWITHSUNDEEP
typescripttypescript-typings
Rolly
Rolly

Reputation: 3375

ts a type that is a function and a property at the same time

I am reading ts handbook and I found something really confusing I am not sure how to use it. Basically is a param that is a function and a property at the same time.

// the definition
type DescribableFunction = {
  description: string;
  (someArg: number): boolean;
};
// the function that acceps the params
function doSomething(fn: DescribableFunction) {
  console.log(fn.description + " returned " + fn(6));
}

// but how can I use it?
// case 1 fail

doSomething({ description: 'xxx'})

/* Argument of type '{ description: string; }' is not assignable to parameter of type 'DescribableFunction'.
  Type '{ description: string; }' provides no match for the signature '(someArg: number): boolean'.(2345)
*/


//case 2 fail

doSomething((x) => x > 0})

/*
Argument of type '(x: number) => boolean' is not assignable to parameter of type 'DescribableFunction'.
  Property 'description' is missing in type '(x: number) => boolean' but required in type 'DescribableFunction'.(2345)
input.tsx(2, 3): 'description' is declared here.
*/

document https://www.typescriptlang.org/docs/handbook/2/functions.html#call-signatures

example https://www.typescriptlang.org/play?#code/C4TwDgpgBAIhDOBjATgSwEYEN0BsIDEBXAO0WFQHtioBeKAbwCgooATBFVMcqgLinjA0xAOYBuZlAAU8CgFsIAQWQj+xQnPQRkASn7oKFPJmISAvhIBmJMpWqsKAZXkRgAC1Sipl4vzhI0LFwCGx5iHQZJRCpZPAA6HAoRb2I49gCuMKgAaigAIihkV0JkYghWfJyoHykANh0dc0ZGB2cFd09k+jYONG47fgByAA9RwbMdFqcXDq8pYYiaAD4oYagVgAYJoA

Upvotes: 2

Views: 112

Answers (2)

Dimitar Getsov
Dimitar Getsov

Reputation: 1

the main point is : 1.first to declare Your function ,like: const fn: DescribableFunction = (someArg: number) => true

2.then to add value for property "description",like: fn.description = 'hello'; If You do 2. before 1.,wouldn't work.

Upvotes: 0

captain-yossarian from Ukraine
captain-yossarian from Ukraine

Reputation: 33051

This code

type DescribableFunction = {
  description: string;
  (someArg: number): boolean;
};

means that there is some function (someArg: number): boolean with static property description.

Just like any function has static property name or static methods call,apply,bind ...

type DescribableFunction = {
  description: string;
  (someArg: number): boolean;
};

function doSomething(fn: DescribableFunction) {
  console.log(fn.description + " returned " + fn(6));
}

const fn: DescribableFunction = (someArg: number) => true
fn.description = 'hello';

doSomething(fn) // ok

Upvotes: 1

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