indexOfSubstring.js

function indexOf (string, sub) {  
  if(string === sub || !sub) return 0;
  if(!string ) return -1;
  
  let res = -1;
  let k = 0;
  
  for(let i = 0 ; i < string.length; i++){
    if(string.charAt(i) === sub.charAt(k)){
    	k++;      
      if(k === sub.length){
      	res = i + 1 - sub.length;
        break;
      }			
    }else{
    	i = i - k;
    	k = 0;
    }
  }
  return res;
}