What is the best way to define an object with properties that can be called in TypeScript? And, can you provide an example

Question

What is the best way to define an object with properties that can be called in TypeScript? And, can you provide an example

type DescribableFunction = {
  description: string;
  (someArg: number): boolean;
};
function doSomething(fn: DescribableFunction) {
  console.log(fn.description + " returned " + fn(6));
}
console.log(doSomething({description="how are you", (9)=>return true;})) //error

I attempted to invoke the aforementioned function with arguments, but encountered an error:

"This expression is not callable. Type '{ description: string; }' has no call signatures.(2349) (property) description: any"

How can I properly execute this function?

typescript

Answer 1

Answer №1

In order to reference your function field later on, it is essential to give it a name. Additionally, ensure there are no syntax errors when passing the object as a parameter:

type DescribableFunction = {
  description: string;
  call: (someArg: number) => boolean;
};
function doSomething(fn: DescribableFunction) {
  console.log(fn.description + " returned " + fn.call(6));
}
console.log(doSomething({description: "how are you", call: (x: number) => true }))

https://www.typescriptlang.org/play?#code/C4TwDgpgBAIhDOBjATgSwEYEN0Bs...

Answer 2

In order to reference your function field later on, it is essential to give it a name. Additionally, ensure there are no syntax errors when passing the object as a parameter:

type DescribableFunction = {
  description: string;
  call: (someArg: number) => boolean;
};
function doSomething(fn: DescribableFunction) {
  console.log(fn.description + " returned " + fn.call(6));
}
console.log(doSomething({description: "how are you", call: (x: number) => true }))

https://www.typescriptlang.org/play?#code/C4TwDgpgBAIhDOBjATgSwEYEN0Bs...

Answer 3

Answer №2

Don't worry! You have the ability to assign properties to any object, even functions, by using Object.assign.

Your code is starting off correctly, and it will function properly if you structure it in the following way:

let described = Object.assign((x: number) => true, { description: "how are you" });
console.log(doSomething(described));

If you're curious, here's a method to create a helper function that adds just one property:

/** Adds a property to any object, and returns the same object with a type that includes 
    the new property. */
export function attachProp<T extends {}, PName extends string, PType>(
    obj: T, prop: PName, value: PType
) {
    (obj as any)[prop] = value;
    return obj as T & { [p in PName]: PType };
}

Here's how you can use it:

let described = attachProp((x: number) => true, 'description', "how are you");
console.log(doSomething(described));

Answer 4

Don't worry! You have the ability to assign properties to any object, even functions, by using Object.assign.

Your code is starting off correctly, and it will function properly if you structure it in the following way:

let described = Object.assign((x: number) => true, { description: "how are you" });
console.log(doSomething(described));

If you're curious, here's a method to create a helper function that adds just one property:

/** Adds a property to any object, and returns the same object with a type that includes 
    the new property. */
export function attachProp<T extends {}, PName extends string, PType>(
    obj: T, prop: PName, value: PType
) {
    (obj as any)[prop] = value;
    return obj as T & { [p in PName]: PType };
}

Here's how you can use it:

let described = attachProp((x: number) => true, 'description', "how are you");
console.log(doSomething(described));

What is the best way to define an object with properties that can be called in TypeScript? And, can you provide an example

Answer №1

Answer №2

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