Did you forget to include 'void' in your type argument to 'Promise'

# Did you forget to include 'void' in your type argument to 'Promise'

The error "Expected 1 argument, but got 0. Did you forget to include 'void' in your type argument to Promise" occurs when we use the Promise constructor without passing it an argument.

To solve the error, use a generic to type the value of the Promise to void.

Here is an example of how the error occurs.

index.ts

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function getPromise() {
  return new Promise((resolve, reject) => {
    // ⛔️ Error: Expected 1 arguments, but got 0.
    // Did you forget to include 'void' in your type argument to 'Promise'?ts(2794)
    resolve();
  });
}

expected arguments but got did you forget to include void

We used the Promise() constructor without resolving it with a value.

By default the Promise constructor takes 1 argument of type unknown, so we should either pass it an argument or update the type of its parameters.

# Type the value of the `Promise` to `void` to solve the error

To solve the error in this specific situation, pass void to the generic of the Promise constructor.

index.ts

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function getPromise() {
  // 👇️ type Promise value as void
  return new Promise<void>((resolve, reject) => {
    resolve();
  });
}

type value of promise to void

The void type represents the return value of a function that doesn't return anything.

When a function definition has a void return type, it means the function must not return anything.

If you need to resolve the Promise with a value of another type, pass the specific type to the generic.

index.ts

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function getPromise() {
  return new Promise<number>((resolve, reject) => {
    resolve(1000);
  });
}

The Promise above can only be resolved with a number.

I've also written a detailed article on how to declare a function with a Promise return type.

# Use a union type if the Promise may get resolved with multiple types

If your Promise may get resolved with multiple types, use a union type.

index.ts

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function getPromise() {
  return new Promise<number | string>((resolve, reject) => {
    if (Math.random() > 0.5) {
      resolve(1000);
    }

    resolve('bobbyhadz.com');
  });
}

use union type if the promise may get resolved with multiple types

The promise in the example can be resolved with a value of type number or string.

Note that you should also use the Promise generic if you have to specify the return type of a function that returns a Promise.

index.ts

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async function getPromise(): Promise<string> {
  return 'bobbyhadz.com';
}

We declared an async function in the example.

Regardless of what you return from an async function, it gets wrapped in a Promise. In other words, an async function is always going to return a Promise.

If you need to explicitly type the function's return type, you have to use the Promise generic just like we did in the previous examples.

If you'd like to read more on how to type an async function in TypeScript, check out the following article.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

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Did you forget to include 'void' in your type argument to 'Promise'