typescript utils

tsutils.ts

// Null filtering.
function isDefined<T>(value: T | undefined | null): value is T {
    return !!value;
}

// Immutability
export type Immutable<T> = T extends Function | boolean | number | string | null | undefined
    ? T
    : T extends Array<infer U>
    ? ReadonlyArray<Immutable<U>>
    : T extends Map<infer K, infer V>
    ? ReadonlyMap<Immutable<K>, Immutable<V>>
    : T extends Set<infer S>
    ? ReadonlySet<Immutable<S>>
    : {readonly [P in keyof T]: Immutable<T[P]>};


// Type narrowing.      
const isInstanceOf =
    <T>(ctor: new (...args: any) => T) =>
    (x: any): x is T =>
        x instanceof ctor;
      
type Foo = {type: "foo"; val: number};
type Bar = {type: "bar"; val: string};
type FooOrBar = Foo | Bar;

function discriminate<K extends PropertyKey, V extends string | number | boolean>(
    discriminantKey: K,
    discriminantValue: V,
) {
    return <T extends Record<K, any>>(obj: T & Record<K, V extends T[K] ? T[K] : V>): obj is Extract<T, Record<K, V>> =>
        obj[discriminantKey] === discriminantValue;
}

const fooStuff = ([] as FooOrBar[]).filter(discriminate("type", "foo"));