Category: TypeScript

Sometimes, we want to use enum as index key type in TypeScript.

In this article, we’ll look at how to use enum as index key type in TypeScript.

How to use enum as index key type in TypeScript?

To use enum as index key type in TypeScript, we can use the in operator.

For instance, we write

enum DialogType {
  Options,
  Help,
}

const openDialogs: { [key in DialogType]?: Dialog } = {
  [DialogType.Options]: undefined,
};

to create the DialogType enum.

Then we use it in a type by setting openDialogs to the { [key in DialogType]?: Dialog } type.

We set the index signature’s type to key in DialogType to make sure the keys of openDialogs must be any one of the DialogType enum values.

Therefore, using [DialogType.Options] as a key value wouldn’t make the TypeScript compiler raise an error.

Conclusion

To use enum as index key type in TypeScript, we can use the in operator.

Sometimes, we want to declare a function that throws an error in TypeScript.

In this article, we’ll look at how to declare a function that throws an error in TypeScript.

How to declare a function that throws an error in TypeScript?

To declare a function that throws an error in TypeScript, we can use never as the return type.

For instance, we write

const test = (): never => {
  throw new Error();
};

to create the test function that throws an error.

We set its return type to the never type so the TypeScript compiler will know that the function always throws an error.

Conclusion

To declare a function that throws an error in TypeScript, we can use never as the return type.

Sometimes, we want to merge two interfaces with TypeScript.

In this article, we’ll look at how to merge two interfaces with TypeScript.

How to merge two interfaces with TypeScript?

To merge two interfaces with TypeScript, we can use extends to extend multiple interfaces.

For instance, we write

interface IFooBar extends IFoo, IBar {}

to create the IFooBar that extends IFoo and IBar.

This means IFooBar has all the members from both interfaces inside.

Conclusion

To merge two interfaces with TypeScript, we can use extends to extend multiple interfaces.

Sometimes, we want to add type aliases with TypeScript.

In this article, we’ll look at how to add type aliases with TypeScript.

How to add type aliases with TypeScript?

To add type aliases with TypeScript, we can use the type keyword.

For instance, we write

type PrimitiveArray = Array<string | number | boolean>;

to create the PrimitiveArray type with the type keyword which is an array with string, number, or boolean entries.

Conclusion

To add type aliases with TypeScript, we can use the type keyword.

Sometimes, we want to make a single property optional in TypeScript.

In this article, we’ll look at how to make a single property optional in TypeScript.

How to make a single property optional in TypeScript?

To make a single property optional in TypeScript, we can use the Optional type.

For instance, we write

type MakePersonInput = Optional<Person, "nickname">;

to create the MakePersonInput type that makes the nickname property in the Person interface optional.

Conclusion

To make a single property optional in TypeScript, we can use the Optional type.