Solutions

/*
* #1
*/
let findLongestPrefix = (list) => {
    let prefix = '';
    let charIndex = 0;
    let allMatch = true;
    while(allMatch) {
        let currentChar = list[0][charIndex];
        for(let i = 1; i < list.length; i++) {
            if(list[i][charIndex] !== currentChar) {
                allMatch = false;
            }
        }
        charIndex++;
        if(allMatch) {
            prefix += currentChar;
        }
    }
    
    return prefix;
};

console.log(`findLongestPrefix(['abcdefabc', 'abcdfff', 'abcodo'])=='abc' = ${findLongestPrefix(['abcdefabc', 'abcdfff', 'abcodo'])=='abc'}`);
console.log(`findLongestPrefix(['aab', 'adc', 'aef'])=='abc' = ${findLongestPrefix(['aab', 'adc', 'aef'])=='a'}`);
console.log(`findLongestPrefix(['abc', 'def', 'cdb'])=='abc' = ${findLongestPrefix(['abc', 'def', 'cdb'])==''}`);