flow.ts

type Many<T> = T | ReadonlyArray<T>;

interface Flow {
    flow<A extends any[], R1, R2, R3, R4, R5, R6, R7>(
        f1: (...args: A) => R1,
        f2: (a: R1) => R2,
        f3: (a: R2) => R3,
        f4: (a: R3) => R4,
        f5: (a: R4) => R5,
        f6: (a: R5) => R6,
        f7: (a: R6) => R7,
    ): (...args: A) => R7;
    flow<A extends any[], R1, R2, R3, R4, R5, R6, R7>(
        f1: (...args: A) => R1,
        f2: (a: R1) => R2,
        f3: (a: R2) => R3,
        f4: (a: R3) => R4,
        f5: (a: R4) => R5,
        f6: (a: R5) => R6,
        f7: (a: R6) => R7,
        ...func: Array<Many<(a: any) => any>>
    ): (...args: A) => any;
    flow<A extends any[], R1, R2, R3, R4, R5, R6>(
        f1: (...args: A) => R1,
        f2: (a: R1) => R2,
        f3: (a: R2) => R3,
        f4: (a: R3) => R4,
        f5: (a: R4) => R5,
        f6: (a: R5) => R6,
    ): (...args: A) => R6;
    flow<A extends any[], R1, R2, R3, R4, R5>(
        f1: (...args: A) => R1,
        f2: (a: R1) => R2,
        f3: (a: R2) => R3,
        f4: (a: R3) => R4,
        f5: (a: R4) => R5,
    ): (...args: A) => R5;
    flow<A extends any[], R1, R2, R3, R4>(
        f1: (...args: A) => R1,
        f2: (a: R1) => R2,
        f3: (a: R2) => R3,
        f4: (a: R3) => R4,
    ): (...args: A) => R4;
    flow<A extends any[], R1, R2, R3>(f1: (...args: A) => R1, f2: (a: R1) => R2, f3: (a: R2) => R3): (...args: A) => R3;
    flow<A extends any[], R1, R2>(f1: (...args: A) => R1, f2: (a: R1) => R2): (...args: A) => R2;
    flow(...func: Array<Many<(...args: any[]) => any>>): (...args: any[]) => any;
}

const flow: Flow['flow'] = (...funcs: any[]) => {
    const length = funcs.length;
    let index = length;
    while (index--) {
        if (typeof funcs[index] !== 'function') {
            throw new TypeError('Expected a function');
        }
    }
    return (...args: any[]) => {
        let index = 0;
        let result = length ? funcs[index].apply(this, args) : args[0];
        while (++index < length) {
            result = funcs[index].call(this, result);
        }
        return result;
    };
};