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/**
 * @param {Function[]} functions
 * @return {Function}
 */
var compose = function(functions) {
    
    return function(x) {
        functions.reverse();
        functions.forEach((func) => {
            x = func(x);
        });
        return x;
    }
};

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