Weird JavaScript operators

Question

I'm having a little problem with an operator. I have a number which is either plussed or subtracted depending on key input. The weird thing is that the operators += 1 and += 11 adds the numbers literally to the static number: 60 becomes 601 and 6011 instead of 61 and 71.

Here is the code, so take into consideration that the static number is 60:

switch(e.keyCode) {
    case 37:
        boxID -= 1;
        break;
    case 38:
        boxID -= 11;
        break;
    case 39:
        boxID += 1; // Becomes 601
        break;
    case 40:
        boxID += 11; // Becomes 6011
        break;
}

how do you define boxID? the javascript runtime thinks it's a string — Jason
– Jason, Commented Apr 14, 2011 at 1:55
I define boxID like this where the ID is number 60 of the list item: var boxID = $('li.selected').attr('id'); — Jonathan
– Jonathan, Commented Apr 14, 2011 at 1:57
then follow cwolves advice var boxId = parseInt($("li.selected").attr("id")) — Jason
– Jason, Commented Apr 14, 2011 at 1:59
Nitpick: a numeric id is invalid, see w3.org/TR/html401/types.html#type-id — RobG
– RobG, Commented Apr 14, 2011 at 6:57

score 11 · Accepted Answer · 2011-04-14 02:01:26Z

11

boxId is a string in your case. Convert it to a number first using parseInt(boxId) or just boxId << 0

The reason -= works is because it only has one function (subtract using Math), so boxId is cast to a number before the operation. + is overloaded in JavaScript to mean "string concatenation OR Math addition", so if boxId is a string, it does string ops.

edited Apr 14, 2011 at 2:01

answered Apr 14, 2011 at 1:55

user578895

Sign up to request clarification or add additional context in comments.

1 Comment

hugomg Over a year ago

Remember to use the optional radix parameter and do parseInt(boxId, 10) instead. This prvents "010" from being treated as an octal number.

Collectives™ on Stack Overflow

Weird JavaScript operators

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related