SOURCE

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))