Published by powerflyang on Jul 18, 2022

Useful TypeScript features:

• infer
• never
• keyof + in

The infer keyword allows you to deduce a type from another type within a conditional type.

Here’s an example:

typescript
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const User = {
  id: 123,
  username: 'John',
  email: 'john@gmail.com',
  addons: [
    { name: 'WriteAddon', id: 1 },
    { name: 'TodoAddon', id: 2 }
  ]
};


type UnpackArray<T> = T extends (infer R)[] ? R: T;

type AddonType = UnpackArray<typeof User.addons>; // { name: string, id: number }

The never type represents a value that is never observed.

In a return type, this means that the function throws an exception or terminates the execution of the program.

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type DeepReadonly<T> = {
  readonly [P in keyof T]: DeepReadonly<T[P]>;
};