javascript - Why does my code always end with undefined? -

i wrote javascript code, ends **undefined**mycode? have done wrong/ how can prevent in future. running code through chrome javascript console.

here code

    //reverse string //-------------------------// //input string  var string = prompt("please enter string");             //console.log(string);  //find length of string  var stringlength = string.length;             //console.log(stringlength);  //creating empty string outputting answer  var reversedstring = "";  //start length of string , work backwards, inputting letter 1 @ time. (var = stringlength; >= 0; i--){     reversedstring += string[i];                 //console.log(string[i]); }  //outputting reversed string; alert(reversedstring); 

thanks answers in advance

change loop

for (var = stringlength; >= 0; i--){ 

to

for (var = stringlength-1; >= 0; i--){ 

the problem is, array indices in javascript 0 based.

lets string entered in prompt "abc", length of string 3. in loop, access string[3] undefined. hence error.

here fiddle demonstrating updated code:

• http://jsfiddle.net/rf1vmyzg/