find-prefixes.js

function findPrefixes(words) {
    let sample = words[0]
    let currentSet = []
    let lastCommonSet = []
    let currentPrefix = "";
    let lastValidPrefix = "";
    for(let i = sample.length; i > 0; i--) {
        currentPrefix = sample.slice(0, i)
        currentSet = words.filter( w => w.indexOf(currentPrefix) == 0)
        if(currentSet.length > 0 && currentSet.length > lastCommonSet.length) {
            lastCommonSet = currentSet 
            lastValidPrefix = currentPrefix
        }
    }
    let toReturn = [lastValidPrefix, 
                    ...(lastCommonSet.map(w => w.slice(lastValidPrefix.length))
                                                .filter(w => w.length > 0))]
    return {
        group: toReturn,
        usedWords: lastCommonSet
    }
}