let arr = ["radog","racecar","racar"]
const commonStr =(arr)=>{
let str = arr[0]
let comStr =''
for(let i = 0;i<str.length;i++){
let j = 0
for(;j<str.length&&j<arr.length;j++){
if(str[i] != arr[j][i]) {
comStr = str.slice(0,i)
return comStr
}
}
}
}
console.log(commonStr(arr))