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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 make it easier to write reusable code.
Generics with functions help create more general functions that accurately represent the input and return types.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.
Generics can be used to create generalized classes, like Map.
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}: ${this._value}`;
}
}
let value = new NamedValue<number>('myNumber');
value.setValue(10);
console.log(value.toString()); // myNumber: 10
TypeScript can also infer the type of the generic parameter if it's used in a constructor parameter.Generics in type aliases allow creating types that are more reusable.
Example type Wrapped<T> = { value: T };
const wrappedValue: Wrapped<number> = { value: 10 };This also works with interfaces with the following syntax:
interface Wrapped<T> {Generics can be assigned default values which apply if no other value is specified or inferred.
class NamedValue<T = string> {
private _value: T | undefined;
constructor(private name: string) {}
public setValue(value: T) {
this._value = value;
}
public getValue(): T | undefined {
return this._value;
}
public toString(): string {
return `${this.name}: ${this._value}`;
}
}
let value = new NamedValue('myNumber');
value.setValue('myValue');
console.log(value.toString()); // myNumber: myValueConstraints can be added to generics to limit what's allowed. The constraints make it possible to rely on a more specific type when using the generic type.
Example function createLoggedPair<S extends string | number, T extends string | number>(v1: S, v2: T): [S, T] {
console.log(`creating pair: v1='${v1}', v2='${v2}'`);
return [v1, v2];
}This can be combined with a default value.