Micro Api for building type predicates in TypeScript.

micro_type_predicate.ts

/** Micro Api for building type predicates */
module TypePredicateApi {
    export type TypePredicate<refined> = (val: any) => val is refined;
    export type TypeOfTypePredicate<refiner> = refiner extends TypePredicate<infer refined> ? refined : never;
    export type TypePredicateMap = { [propertyName: string]: TypePredicate<any> }
    export type ObjectFromTypePredicateMap<map extends { [propertyName: string]: TypePredicate<any> }> = {
        [propertyName in keyof map]: TypeOfTypePredicate<map[propertyName]>;
    }

    /** Create a predicate which tests for an object. */
    export const createObjectPredicate = <typePredicateMap extends TypePredicateMap>(typePredicateMap: typePredicateMap) =>
        (obj: any): obj is ObjectFromTypePredicateMap<typePredicateMap> =>
            (obj === null) ? false
                : (typeof obj !== "object") ? false
                    : Object.keys(obj).every(propertyName => typePredicateMap[propertyName](obj[propertyName]));
    export const isString: TypePredicate<string> = (val: any): val is string => typeof val === "string";
    export const isNumber: TypePredicate<number> = (val: any): val is number => typeof val === "number";
    export const isBoolean: TypePredicate<boolean> = (val: any): val is boolean => typeof val === "boolean";
    export const isArray: TypePredicate<any[]> = (val: any): val is any[] => !!val && typeof val.push === "function";
    export const createIsArray = <Item extends unknown>(pred: TypePredicate<Item>): TypePredicate<Item[]> =>
        (val: any): val is Item[] =>
            isArray(val) && val.every(pred);
    export const isValue = <value extends unknown>(value: value) => (val: any): val is value => (val === value);
    export const isNull: TypePredicate<null> = (val: any): val is null => val === null;
    export const isValue2 = <value1 extends unknown, value2 extends unknown>(value1: value1, value2: value2) => 
        (val: any): val is (value1 | value2) => 
            (val === value1 || val === value2);
    export const isValue3 = <value1 extends unknown, value2 extends unknown, value3 extends unknown>(value1: value1, value2: value2, value3: value3) => 
        (val: any): val is (value1 | value2 | value3) => 
            (val === value1 || val === value2 || val === value3);
    export const isValue4 = <value1 extends unknown, value2 extends unknown, value3 extends unknown, value4 extends unknown>(value1: value1, value2: value2, value3: value3, value4: value4) => 
        isValue2(isValue3(value1, value2, value3), value4);
    export const or = <left, right>(left: TypePredicate<left>, right: TypePredicate<right>) => 
        (val: any): val is (left | right) => 
            left(val) || right(val);
    export const or3 = <one, two, three>(one: TypePredicate<one>, two: TypePredicate<two>, three: TypePredicate<three>) => 
        (val: any): val is (one | two | three) => 
            one(val) || two(val) || three(val);
}

module Example {
    const isGender = TypePredicateApi.isValue3("male" as const, "female" as const, "unknown" as const);
    
    const isPerson = TypePredicateApi.createObjectPredicate({
        firstName: TypePredicateApi.isString,
        gender: isGender,
        lastName: TypePredicateApi.isString,
        ageInYears: TypePredicateApi.isNumber,
        isCool: TypePredicateApi.isBoolean
    })
    // Observe the use of TypeOfTypePredicate to get ahold of the computed type.
    type Person = TypePredicateApi.TypeOfTypePredicate<typeof isPerson>;
    const bob: Person = {
        firstName: "Bob",
        lastName: "Smith",
        ageInYears: 30,
        gender: "male",
        isCool: true
    }

    const isCompany = TypePredicateApi.createObjectPredicate({
        ceo: isPerson,
        coo: isPerson,
        cto: isPerson,
        workers: TypePredicateApi.createIsArray(isPerson)
    })

    export function doSomething(valueFromOutsideWord: any) {
        if (isCompany(valueFromOutsideWord)) {
            console.log(`CEO Age: ${valueFromOutsideWord.ceo.ageInYears}`);
            console.log(`Company has ${valueFromOutsideWord.workers.length}`);
        }
    }
}