var.replace is not a function

probable issues:

• variable is NUMBER (instead of string);
  num=35; num.replace(3,'three'); =====> ERROR
num=35; num.toString().replace(3,'three'); =====> CORRECT !!!!!!
num='35'; num.replace(3,'three'); =====> CORRECT !!!!!!
• variable is object (instead of string);
• variable is not defined;

Replace wouldn't replace numbers. It replaces strings only.

This should work.

function trim(str) {
    return str.toString().replace(/^\s+|\s+$/g,'');
}

If you only want to trim the string. You can simply use "str.trim()"

My guess is that the code that's calling your trim function is not actually passing a string to it.

To fix this, you can make str a string, like this: str.toString().replace(...)
_{...as alper pointed out below.}