Function Composition Example

Given an array of functions, return a function that is the function composition of the array of functions.

The function composition of [f(x), g(x), h(x)] is fn(x) = f(g(h(x))).

The function of an empty list of functions is the identity function: f(x) = x.

To solve this, the functions need to be evaluated from right to left.

Therefore, we use the functions array length, subtracting 1 and have i to run the loop until zero. A totalCount variable keeps track of the initial x and then the subsequent count before returning the total.

var compose = function(functions) {
    return function(x) {
        if (functions.length === 0) {
            return x
        }

        let totalCount = x

        for (let i=functions.length - 1; i >= 0; i--) {
            let fn = functions[i]
            totalCount = fn(totalCount);
        }

        return totalCount
    }
};

/**
 * const fn = compose([x => x + 1, x => 2 * x])
 * fn(4) // 9
 */


/**
 * const fn = compose([x => 10 * x, x => 10 * x, x => 10 * x])
 * fn(1) // 1000
 */

 /**
 * const fn = compose([])
 * fn(42) // 42
 */