GetUp Test JA.js

var numPad = {
    0: null,
    1: null,
    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"],
}

const telephoneWords = ( numsArr ) => {

    let results = [];

    numsArr.forEach((pressedNum, pressedNumIndex) => {

        if (pressedNum < 2 || pressedNum > 9) {
            throw "telephoneWords() - only accepts integers from 2-9"
        }

        let currentNumResult = [];

        numPad[pressedNum].forEach((letter) => {
            // go through all the next number
            numsArr.forEach((pressedNumCheck, pressedNumCheckIndex) => {
                if (pressedNumCheck !== pressedNum && pressedNumCheckIndex === pressedNumIndex + 1) {
                    numPad[pressedNumCheck].forEach((letterCheck) => {
                        currentNumResult.push(letter + letterCheck);
                    })
                }
            })
        })

        // APPEND NEXT MATCHES TO ROOT LETTERS

        if ( results.length < 1 ) {
            results = currentNumResult;
        } else {
            currentNumResult.forEach((letters) => {
                results.forEach((rootLetters, index) => {
                    if(letters[0] === rootLetters[rootLetters.length - 1]) {
                        results[index] = rootLetters + letters[1];
                    }
                })
            })
        }

    })

    return results.toString();
}

console.log(telephoneWords([2, 5]));
console.log(telephoneWords([2, 5, 8, 9]));