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Generics allow creating 'type variables' which can be used to create classes, functions & type aliases that don't need to explicitly define the types that they use.

Generics makes it easier to write reusable code.

Functions

Generics with functions help make more generalized methods which more accurately represent the types used and returned.

Example

function createPair<S, T>(v1: S, v2: T): [S, T] {
  return [v1, v2];
}
console.log(createPair<string, number>('hello', 42)); // ['hello', 42]

TypeScript can also infer the type of the generic parameter from the function parameters.

Classes

Generics can be used to create generalized classes, like .

Example

class NamedValue<T> {
  private _value: T | undefined;

  constructor(private name: string) {}

  public setValue(value: T) {
    this._value = value;
  }

  public getValue(): T | undefined {
    return this._value;
  }

  public toString(): string {
    return `${this.name}: ${this._value}`;
  }
}

let value = new NamedValue<number>('myNumber');
value.setValue(10);
console.log(value.toString()); // myNumber: 10

TypeScript can also infer the type of the generic parameter if it's used in a constructor parameter.

Type Aliases

Generics in type aliases allow creating types that are more reusable.

Example

type Wrapped<T> = { value: T };

const wrappedValue: Wrapped<number> = { value: 10 };

This also works with interfaces with the following syntax: interface Wrapped<T> {

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Default Value

Generics can be assigned default values which apply if no other value is specified or inferred.

Example

class NamedValue<T = string> {
  private _value: T | undefined;

  constructor(private name: string) {}

  public setValue(value: T) {
    this._value = value;
  }

  public getValue(): T | undefined {
    return this._value;
  }

  public toString(): string {
    return `${this.name}: ${this._value}`;
  }
}

let value = new NamedValue('myNumber');
value.setValue('myValue');
console.log(value.toString()); // myNumber: myValue

Extends

Constraints can be added to generics to limit what's allowed. The constraints make it possible to rely on a more specific type when using the generic type.

Example

function createLoggedPair<S extends string | number, T extends string | number>(v1: S, v2: T): [S, T] {
  console.log(`creating pair: v1='${v1}', v2='${v2}'`);
  return [v1, v2];
}

This can be combined with a default value.

TypeScript Exercises

Test Yourself With Exercises

Exercise:

Complete the Generic:

function createPair, (x: typeX, y: typeY): [typeX, typeY] {
 return [x, y];
}
console.log(createPair<string, number>('Meaning', 42));

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