Guide to utilizing a TypeScript declaration file for a different function

Question

Guide to utilizing a TypeScript declaration file for a different function

I'm a beginner in TS and I've created a definition file to specify the argument types for a function in another file.

customFunctions.ts

export default ({}) => {
  const myCustomFunction = ({ param1 }: { param1: string }) => {
    return param1;
  };

  return {
    myCustomFunction
  };
};

customFunctions.d.ts

declare var _default: ({}: {}) => {
    myCustomfunction: ({ param1 }: {
        param1: string;
    }) => string;
};
export default _default;

otherfile.ts

export default ({ customFunctions }) => {
  customFunctions().myCustomFunction({ param1: 1 });
};

The customFunctions parameter is defined in customFunctions.ts, but otherfile.ts doesn't know its structure.

How can I utilize customFunctions.d.ts here? Do I really need it?

javascript typescript

Answer 1

Answer №1

If you're facing an issue, you can resolve it by creating a new type specifically for your requirement.

// customFunctions.ts
export type customFunctions = ({}: {}) => {
  myCustomFunction: ({ argument }: {
      argument: string;
  }) => string;
};

You can then implement it as follows:

//anotherfile.ts
import { customFunctions } from './customFunctions';

export default (functions: customFunctions) => {
  functions({}).myCustomFunction({ argument: "example" });
};

Check out this StackBlitz example to see how to handle errors related to incorrect argument types.

Answer 2

If you're facing an issue, you can resolve it by creating a new type specifically for your requirement.

// customFunctions.ts
export type customFunctions = ({}: {}) => {
  myCustomFunction: ({ argument }: {
      argument: string;
  }) => string;
};

You can then implement it as follows:

//anotherfile.ts
import { customFunctions } from './customFunctions';

export default (functions: customFunctions) => {
  functions({}).myCustomFunction({ argument: "example" });
};

Check out this StackBlitz example to see how to handle errors related to incorrect argument types.

Answer 3

Answer №2

It doesn't function in that manner.

To assign types to untyped JavaScript code that you import, you utilize definitions. For example, if you imported library X into your project and there was no @types/X package available, you could create a .d.ts file to incorporate it into your project. However, this step is entirely optional as you can also work with JS code without types.

If you wish for a function's return to be based on its arguments, then you are likely interested in generics, which can be quite complex. For instance:

const myFunction = <Input extends { param1: any }>(inp: Input) => {
    return inp.param1
};

In the above snippet, we specify that myFunction accepts any object with at least a property named param1. The return value will be inferred based on the type of param1. This differs from:

const myFunction = (inp: { param1: any, [k: string]: any }) => {
    return inp.param1
};

In this latter example, similar constraints are applied to the function inputs, but the return type will always default to any.

The distinction lies in the fact that while in the 2nd scenario we are explicitly defining the input type, in the 1st case, we are generically working with whatever type is provided while enforcing a restriction.

Answer 4

It doesn't function in that manner.

To assign types to untyped JavaScript code that you import, you utilize definitions. For example, if you imported library X into your project and there was no @types/X package available, you could create a .d.ts file to incorporate it into your project. However, this step is entirely optional as you can also work with JS code without types.

If you wish for a function's return to be based on its arguments, then you are likely interested in generics, which can be quite complex. For instance:

const myFunction = <Input extends { param1: any }>(inp: Input) => {
    return inp.param1
};

In the above snippet, we specify that myFunction accepts any object with at least a property named param1. The return value will be inferred based on the type of param1. This differs from:

const myFunction = (inp: { param1: any, [k: string]: any }) => {
    return inp.param1
};

In this latter example, similar constraints are applied to the function inputs, but the return type will always default to any.

The distinction lies in the fact that while in the 2nd scenario we are explicitly defining the input type, in the 1st case, we are generically working with whatever type is provided while enforcing a restriction.

Guide to utilizing a TypeScript declaration file for a different function

Answer №1

Answer №2

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