phone.js

// Given a phone number (array of 10 digits), find all alphabetical permutations using the following digit mapping.
// Dont look up the answer, that ruins the fun!

var digitMap = {
    0: ['0'],
    1: ['1'],
    2: ['a','b','c'],
    3: ['d','e','f'],
    4: ['g','h','i'],
    5: ['j','k','l'],
    6: ['m','n','o'],
    7: ['p','q','r','s'],
    8: ['t','u','v'],
    9: ['w','x','y','z']
};

function objectToFunction (obj) {
    return function (key) {
        return obj[key];
    };
}

function flatten (arrayOfArrays) {
    return arrayOfArrays.reduce(function (res, array) {
        return res.concat(array);
    }, []);
}

function builder(phone){
    // your code here
    return phone.map(objectToFunction(digitMap))
    .reduce(function (permutations, digitChars) {
        return flatten(permutations.map(function (permutation) {
            return digitChars.map(function (digitChar) {
                return permutation + digitChar;
            });
        }));
    });           
}

var start = Date.now();
(builder([5,5,5,0,2,9,6,8,1,3]));
console.log(Date.now() - start);