validator-1.ts

// A type-first validator with full typing support.

type MakeUndefinedOptional<T> = {
    [K in keyof T as undefined extends T[K] ? never : K]: T[K]
} & {
    [K in keyof T as undefined extends T[K] ? K : never]?: T[K]
}

type UnwrapValidatorRecord<T> = {
    [K in keyof T]: T[K] extends Validator<infer U> ? U : never
}

export class Validator<T> {
    constructor(public check: (value: any) => value is T) {}

    debug(value: any) {
        console.log(value, this.check(value))
    }

    or<U>(other: Validator<U>) {
        return new Validator(
            (value): value is T | U => this.check(value) || other.check(value),
        )
    }

    and<U>(other: Validator<U>) {
        return new Validator(
            (value): value is T & U => this.check(value) && other.check(value),
        )
    }

    not(): Validator<unknown>
    not<U>(other: Validator<U>): Validator<Exclude<T, U>>
    not<U>(other?: Validator<U>) {
        if (other) {
            return new Validator(
                (value): value is Exclude<T, U> =>
                    this.check(value) && !other.check(value),
            )
        } else {
            return new Validator(
                (value): value is unknown => !this.check(value),
            )
        }
    }

    optional(): Validator<T | undefined> {
        return this.or(undefined())
    }
}

export class ArrayValidator<T> extends Validator<T[]> {
    constructor(protected itemValidator: Validator<T> = any()) {
        super(
            (value): value is T[] =>
                Array.isArray(value)
                && value.every((element) => itemValidator.check(element)),
        )
    }
}

export class ObjectValidator<
    T extends Record<string | number | symbol, Validator<unknown>>,
> extends Validator<{
    [K in keyof MakeUndefinedOptional<
        UnwrapValidatorRecord<T>
    >]: MakeUndefinedOptional<UnwrapValidatorRecord<T>>[K]
}> {
    protected object: [string | number | symbol, Validator<unknown>][]

    constructor(object: T) {
        super(
            (value): value is MakeUndefinedOptional<UnwrapValidatorRecord<T>> =>
                typeof value === "object"
                && value !== null
                && this.object.every(([k, v]) => v.check(value[k])),
        )

        this.object = Object.entries(object)
    }
}

type Primitives = {
    bigint: bigint
    boolean: boolean
    number: number
    string: string
    symbol: symbol
    undefined: undefined
}

type Primitive = bigint | boolean | null | number | string | symbol | undefined

export function primitive(type: "bigint"): Validator<bigint>
export function primitive(type: "boolean"): Validator<boolean>
export function primitive(type: "number"): Validator<number>
export function primitive(type: "string"): Validator<string>
export function primitive(type: "char"): Validator<symbol>
export function primitive(type: "undefined"): Validator<undefined>
export function primitive<T extends keyof Primitives>(type: T) {
    return new Validator(
        (value): value is Primitives[T] => typeof value === type,
    )
}

export const bigint = () => primitive("bigint")
export const boolean = () => primitive("boolean")
export const number = () => primitive("number")
export const string = () => primitive("string")
export const symbol = () => primitive("char")
export const undefined = () => primitive("undefined")

const _null = () => new Validator((value): value is null => value === null)
export { _null as null }

const _function = () =>
    new Validator(
        (
            value,
        ): value is ((...args: any[]) => any) | (new (...args: any[]) => any) =>
            typeof value === "function",
    )
export { _function as function }

export const any = () => new Validator((_value): _value is any => true)
export const never = () => new Validator((_value): _value is never => false)

export const is = <T extends Primitive>(value: T) =>
    new Validator((_value): _value is T => _value === value)

export function array<T = unknown>(item?: Validator<T>) {
    return new ArrayValidator(item)
}

export namespace array {
    export function empty() {
        return new ArrayValidator(never())
    }
}

export function object<
    T extends Record<string | number | symbol, Validator<unknown>>,
>(object: T) {
    return new ObjectValidator(object)
}

const myObj = object({
    a: is(2).optional(),
    b: string(),
    c: array(boolean()),
})

const b = null! as any
if (myObj.check(b)) {
    b.a
}