TypeScript Generics

Generics provide a way to create reusable components that can work over a variety of types rather than a single one, while maintaining precise type information.

Type Variables

A type variable (e.g., <Type>) captures the type provided by the user so that it can be used to denote other types (like the return type).

function identity<Type>(arg: Type): Type {
  return arg;
}

• Inference: TypeScript can usually automatically set the value of Type based on the argument passed.

Generic Types and Interfaces

You can describe the type of a generic function or create entirely generic interfaces and classes.

interface GenericIdentityFn<Type> {
  (arg: Type): Type;
}

Generic Constraints

Limit the kinds of types a generic parameter can accept using the extends keyword.

interface Lengthwise {
  length: number;
}

function loggingIdentity<Type extends Lengthwise>(arg: Type): Type {
  console.log(arg.length);
  return arg;
}

Using Type Parameters in Constraints

You can constrain one type parameter by another (e.g., to ensure a key exists on an object).

function getProperty<Type, Key extends keyof Type>(obj: Type, key: Key) {
  return obj[key];
}

Generic Parameter Defaults

Make type arguments optional by providing a default.

interface Container<T = string> {
  value: T;
}

References

• Source: 00_Raw/typescript-handbook.md
• typescript-moc
• typescript-functions
• typescript-objects