ilyo ilyo - 21 days ago 6
Javascript Question

Javascript (+) sign concatenates instead of giving sum of variables

Why when i use this: (assuming i=1)

divID = "question-" + i+1;


I get question-11 and not question-2?

Answer

Use this instead:

var divID = "question-" + (i+1)

It's a fairly common problem and doesn't just happen in JavaScript. The idea is that + can represent both concatenation and addition.

Since the + operator will be handled left-to-right the decisions in your code look like this:

  • "question-" + i: since "question-" is a string, we'll do concatenation, resulting in "question-1"
  • "question-1" + 1: since "queston-1" is a string, we'll do concatenation, resulting in "question-11".

With "queston-" + (i+1) it's different:

  • since the (i+1) is in parenthesis, its value must be calculated before the first + can be applied:
    • i is numeric, 1 is numeric, so we'll do addition, resulting in 2
  • "question-" + 2: since "question-" is a string, we'll do concatenation, resulting in "question-2".