JS replace not working on string [duplicate]

.replace() returns a new string (it does not modify the existing string) so you would need:

answer_form = answer_form.replace(/#/g, question_num); 

You probably should also make question_num a string though auto type conversions probably handle that for you.

Working example: http://jsfiddle.net/jfriend00/4cAz5/

FYI, in Javascript, strings are immutable - an existing string is never modified. So any method which makes a modification to the string (like concat, replace, slice, substr, substring, toLowerCase, toUpperCase, etc...) ALWAYS returns a new string.

Your code is correct. Just add the value to the variable like this:

answer_form = '<textarea name="answer_#" rows="5"></textarea>'+
              '<input type="file" name="img_#" />';

question_num = 5;

answer_form = answer_form.replace(/#/g, question_num);