CODEWITHSUNDEEP
typescript
O'dallagher
O'dallagher

Reputation: 67

Call Signatures example in TS Handbook

I am reading the Typescript Handbook and, right now, I am currently stuck at Call Signatures subsection. In the example given:

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

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

I cannot figure out in TS Playground how to invoke the doSomething function. I tried the below but it is not working.

doSomething({ description: "The code", (5): false})

Upvotes: 6

Views: 731

Answers (2)

brunnerh
brunnerh

Reputation: 184526

Functions with additional properties seem unergonomic; at least you cannot declare them using a single plain object.

This is one way:

const fn = ((x: number) => false) as DescribableFunction;
fn.description = 'description';
doSomething(fn);

Upvotes: 1

jcalz
jcalz

Reputation: 328272

A DescribableFunction is first and foremost a function that takes a single number input and returns a boolean. It also has a string-valued description property.

Since TypeScript 3.1, and as implemented in microsoft/TypeScript#26368, you've been allowed to add properties to functions after their declarations without running into compiler warnings. So you can make a DescribableFunction relatively straightfowardly.

Here's how you could do it with a function statement:

function greaterThanTen(someArg: number) {
    return someArg > 10;
}
greaterThanTen.description = "greaterThanTen";

doSomething(greaterThanTen); // "greaterThanTen returned false"

And here's how you could do it with a function expression:

const isEven: DescribableFunction = someArg => someArg % 2 === 0;
isEven.description = "isEven"

doSomething(isEven); // "isEven returned true"

If you want a one-liner, you could use Object.assign() to add properties to a target and return the augmented target, which TypeScript represents as the intersection of the function type and the added property objects. (This worked even before TypeScript 3.1.) Observe:

const isNegative = Object.assign(
    (someArg: number) => someArg < 0,
    { description: "isNegative" }
);
doSomething(isNegative); // "isNegative returned false"

Playground link to code

Upvotes: 6

Related Questions