longestCommonPrefix.js

var longestCommonPrefix = function(strs) {
  const firstStr = strs[0]; // (1)
  let condition = true;
  let count = -1;

  while(condition) { // (2)
    count += 1;
    condition = strs.every(str => (
      firstStr[count] === str[count] 
    ));

    if (count === firstStr.length) { // (3)
      condition = false;
    }
  }
  return firstStr.slice(0, count); // (4)
};