TypeScript Basic Generics
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]
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TypeScript can also infer the type of the generic parameter from the function parameters.
Classes
Generics can be used to create generalized classes, like Map.
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}: ${this._value}`;
}
}
let value = new NamedValue<number>('myNumber');
value.setValue(10);
console.log(value.toString()); // myNumber: 10
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TypeScript can also infer the type of the generic parameter if it's used in a constructor parameter.
Type Aliases
Generics in type aliases allow creating types that are more reusable.
Example
type Wrapped<T> = { value: T };
const wrappedValue: Wrapped<number> = { value: 10 };
This also works with interfaces with the following syntax: interface Wrapped<T> {
Default Value
Generics can be assigned default values which apply if no other value is specified or inferred.
Example
class NamedValue<T = string> {
private _value: T | undefined;
constructor(private name: string) {}
public setValue(value: T) {
this._value = value;
}
public getValue(): T | undefined {
return this._value;
}
public toString(): string {
return `${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.