Example of Signature in TypeScript Function Declaration

Question

Example of Signature in TypeScript Function Declaration

While going through this documentation, I found myself puzzled by the concept of having a parameter that can be both an object and a function in JavaScript.

type DescribableFunction = {
   description: string;
   (a: any): boolean;
};
function doSomething(fn: DescribableFunction) {
   console.log(fn.description + " returned " + fn(6));
};

doSomething((()=>false)); // Argument of type '() => false' is not assignable to parameter of type 'DescribableFunction'. Property 'description' is missing in type '() => false' but required in type 'DescribableFunction'.

doSomething({description: 'test'}); // fn is not a function.

The example above raises the question: how could the parameter fn serve as both an object and a function simultaneously..?

typescript

Answer 1

Answer №1

Functions are unique objects that can be called upon.

const func = () => {};
console.log(func instanceof Object);

A function's prototype chain looks like this:

func <- Function.prototype <- Object.prototype

Additionally, objects can have any key-value pairs assigned to them.

'use strict';
const func = () => {};
func.property = 'value';
console.log('property is:', func.property);

For example, to demonstrate how the function in question could operate, you might do this:

const func = Object.assign(
  () => false,
  { description: 'example' }
);

performAction(func);

Now, func can be called and contains a description property with a string value.

Answer 2

Functions are unique objects that can be called upon.

const func = () => {};
console.log(func instanceof Object);

A function's prototype chain looks like this:

func <- Function.prototype <- Object.prototype

Additionally, objects can have any key-value pairs assigned to them.

'use strict';
const func = () => {};
func.property = 'value';
console.log('property is:', func.property);

For example, to demonstrate how the function in question could operate, you might do this:

const func = Object.assign(
  () => false,
  { description: 'example' }
);

performAction(func);

Now, func can be called and contains a description property with a string value.

Example of Signature in TypeScript Function Declaration

Answer №1

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