String.prototype.mysubstring=function(beginIndex,endIndex){
  var str=this,
    newArr=[];
  if(!endIndex){
    endIndex=str.length;
  }
  for(var i=beginIndex;i<endIndex;i++){
    newArr.push(str.charAt(i));
  }
  return newArr.join("");
}

//test
"Hello world!".mysubstring(3);//"lo world!"
"Hello world!".mysubstring(3,7);//"lo w"

String.prototype.mysubstring=function(beginIndex,endIndex){
  var str=this,
    strArr=str.split("");
  if(!endIndex){
    endIndex=str.length;
  }
  return strArr.slice(beginIndex,endIndex).join("");
}

//test
console.log("Hello world!".mysubstring(3));//"lo world!"
console.log("Hello world!".mysubstring(3,7));//"lo w"

String.prototype.mysubstring=function(beginIndex,endIndex){
  var str=this,
    beginArr=[],
    endArr=[];
  if(!endIndex){
    endIndex=str.length;
  }
  for(var i=0;i<beginIndex;i++){
    beginArr.push(str.charAt(i));
  }
  for(var i=endIndex;i<str.length;i++){
    endArr.push(str.charAt(i));
  }
  return str.replace(beginArr.join(""),"").replace(endArr.join(""),"");
}

//test
console.log("Hello world!".mysubstring(3));//"lo world!"
console.log("Hello world!".mysubstring(3,7));//"lo w"