get possible letter combinations from a string of digits 2-9 inclusive

letterCombos.js

/**
 * @param {string} digits
 * @return {string[]}
 */
// 2 = abc
// 3 = def
// 4 = ghi
// 5 = jkl
// 6 = mno
// 7 = pqrs
// 8 = tuv
// 9 = wxyz

var letterCombinations = function(digits) {
    // given a string of unknown length
    // containing digits 2-9
    
    //assuming all chars are definitely digits and not 0 or 1
    
    let letterCombos = []
    let newLetterCombos = []
    
    //probably use a hashmap
    // map digits to chars
    const digitMap = {
        2: 'abc',
        3: 'def',
        4: 'ghi',
        5: 'jkl',
        6: 'mno',
        7: 'pqrs',
        8: 'tuv',
        9: 'wxyz',
    }
    
    //for each digit in input string
    for (let i = 0; i < digits.length; i++) {
    // get possible chars
        const possibleChars = digitMap[digits[i]]
    // for each possible char
        for (let j = 0; j < possibleChars.length; j++) {
            const possibleChar = possibleChars[j]
            // if no combos yet
            if (!letterCombos.length) {
            // add first digit's possible chars to start combo
                newLetterCombos.push(possibleChar)
            } else {
                // for each existing letter combo
                for (let k = 0; k < letterCombos.length; k++) {
                // add this char to end of combo
                    newLetterCombos.push(letterCombos[k] + possibleChar)
                }
            }
        }
        // copy combos over to letter combos by ref
        letterCombos = newLetterCombos
        // reset newLetterCombos
        newLetterCombos = []
    }
    
    // return phone number letter combos
    return letterCombos
};