Rather than calling functions where the arguments are passed individually, I prefer to pass them as an object so that the order is not important.

e.g.

const sum = ({ arg1, arg2, arg3 }) => arg1 + arg2 + arg3;

Now I'm migrating some code that I have across, to Typescript and I am unsure how I am supposed to define the interface for such a function.

I've tried something like this and it doesn't work:

Any clues?

Rather than calling functions where the arguments are passed individually, I prefer to pass them as an object so that the order is not important.

e.g.

const sum = ({ arg1, arg2, arg3 }) => arg1 + arg2 + arg3;

Now I'm migrating some code that I have across, to Typescript and I am unsure how I am supposed to define the interface for such a function.

I've tried something like this and it doesn't work:

Any clues?

• javascript
• reactjs
• typescript

4 Answers 4

How about:

interface InputObj {
    arg1: number;
    arg2: number;
    arg3: number;
}

interface ExampleProps {
    sum: (input: InputObj) => number
}

Or inline:

interface ExampleProps {
  sum: (
    input: {
      arg1: number;
      arg2: number;
      arg3: number;
    }
  ) => number;
}

But depending on your use case you may not need to define ExampleProps. Here is your sum function without the arbitrary input object name:

const sum = ({
  arg1,
  arg2,
  arg3
}: {
  arg1: number;
  arg2: number;
  arg3: number;
}) => arg1 + arg2 + arg3;

If you need to explicitly annotate a function type you can go with:

type Foo = (object: { arg1: number, arg2: number; arg3: number }) => number;

const bar: Foo = ({arg1, arg2, arg3}) =>  arg1 + arg2 + arg3 

To play around with type annotations and share results I advise TypeScript Playground <= check there above :)

Here is a fully annotated example as function expression:

const sum: ({ 
  arg1, 
  arg2,
  arg3 
}: {
  arg1: number;
  arg2: number;
  arg3: number;
}) => number = ({ arg1, arg2, arg3 }) => arg1 + arg2 + arg3;

Here is another and better alternative for arrow functions. Only arguments are annotated and compiler can infer return type correctly. Function has less clutter and works just as before.

const sum = ({ 
  arg1,
  arg2,
  arg3
}: {
  arg1: number;
  arg2: number;
  arg3: number;
}) => arg1 + arg2 + arg3;

If you are going to annotate your function in a seperate file:

interface Args {
  arg1: number;
  arg2: number;
  arg3: number;
}

type Sum = (input: Args) => number;

const sum: Sum = ({ arg1, arg2, arg3 }) => arg1 + arg2 + arg3;

You can use any as argument type if argument types are not known. Return type will be inferred as any:

const sum = ({ 
  arg1,
  arg2,
  arg3
}: any) => arg1 + arg2 + arg3;

So this one is equivalent to previous example:

const sum: ({ arg1, arg2, arg3 }: any) => any

This may not make that much sense for arrow functions but you can set types for known arguments and use key-value pairs for annotating addititional argumens:

const sum = ({ 
  arg1,
  arg2,
  arg3
}: {
  arg1: number;
  arg2: number;
  arg3: number;
  [key: string]: number;
}) => arg1 + arg2 + arg3;

You can also use generics:

interface Args {
  arg1: number;
  arg2: number;
  arg3: number;
}

const sum = <T extends Args>({ 
  arg1,
  arg2,
  arg3
}: T) => arg1 + arg2 + arg3;

Here is same examples, sum as function statement.

function sum({
  arg1,
  arg2,
  arg3
}: {
  arg1: number;
  arg2: number;
  arg3: number;
}): number {
  return arg1 + arg2 + arg3;
}

If you have complicated implementation detail in your function's body, function statement can be better choice for its ergonomics. Plus generics looks less clumsy on function statements.

Have not tried TypeScript. In JavaScript would also consider including default values for the properties of the object to avoid the error

Uncaught TypeError: Cannot destructure property `arg1` of 'undefined' or 'null'.

for

sum()

const sum = ({ arg1, arg2, arg3 }) => 
  arg1 + arg2 + arg3;

try {
  console.log(sum())
} catch (e) {
  console.error(e)
}

which can be avoided by setting each value to 0 where the expected parameters and return value is a JavaScript integer

const sum = ({ arg1 = 0, arg2 = 0, arg3 = 0 } = {}) => 
  arg1 + arg2 + arg3;

try {
  console.log(sum()) // 0
} catch (e) {
  console.error(e)
}