function longest_common_starting_substring(arr1){  
var arr= arr1.concat().sort(),  
a1= arr[0], a2= arr[arr.length-1], L= a1.length, i= 0;  
while(i< L && a1.charAt(i)=== a2.charAt(i)) i++;  
return a1.substring(0, i);  
}  
  
console.log(longest_common_starting_substring(['go', 'google']));   
  
console.log(longest_common_starting_substring(['SQLInjection', 'SQLTutorial']));   
  
console.log(longest_common_starting_substring(['abcd', '1234']));