javascript - Loop counter resetting to -1 -

this course project simple search counter word "wrox" in string.

my code:

var mystring = "welcome wrox books. "; mystring = mystring + "the wrox website www.wrox.com. "; mystring = mystring + "visit wrox website today. buying wrox. ";  var = 0; var wroxcount = 0;  while (i <= mystring.length) {     = mystring.indexof("wrox",i);      wroxcount++;       i++; } 

it works fine until i decides reset -1 reason. works until doesn't. don't know i'm doing wrong.

string.indexof returns -1 when substring searching not found in string.

so should check against in while condition, instead of i <= mystring.length because if i positive, substring found inside string, index lower length.

i = mystring.indexof('wrox'); while (i > 0) {     wroxcount++;     = mystring.indexof('wrox', + 1); }