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

Question

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

divID = "question-" + i+1; 

I get question-11 and not question-2?

Answer 1

You may also use this

divID = "question-" + (i*1+1);  

to be sure that i is converted to integer.

Answer 2

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 "question-" + (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".