Not adding 2 numbers correctly in javascript

Question

i have a drop down list in my html with values show here,

<select id="thedropdown">
<option value="1">test1</option>
<option value="2" selected="selected">test2</option>
<option value="3">test3</option>
</select>

And my javascript:

function myFunction()
{
var a = document.getElementById("thedropdown");
var b = a.options[a.selectedIndex].value;

var y=b;
var x=y+2;
var demoP=document.getElementById("demo")
demoP.innerHTML= x;
}

However the answer when i click the button makes it 22 when it should be 4. you can see my problem. Cheers for help in advance

Probably because b is a string, and thence y, and thence you are converting the number 2 to a string and tacking it on the end of the first "2" string — Xitalogy, Feb 09 '13 at 16:51

Matt Ball · Answer 1 · 2013-02-10T00:14:44.913

6

Right now your code concatenates strings. You need to parseInt() or parseFloat() (or Number() or +) the values before adding.

When using parseInt() make sure to always specify the second argument (the radix):

b = parseInt(a.options[a.selectedIndex].value, 10);

edited Feb 10 '13 at 00:14

answered Feb 09 '13 at 16:50

Matt Ball

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so do i have to create a new var for that like var c = parseInt(b)? – user2057440 Feb 09 '13 at 16:51
That would work, as would `b = parseInt(b)`. – Matt Ball Feb 09 '13 at 16:52

score 3 · Answer 2 · answered Feb 09 '13 at 16:52

3

You can use unary + to convert numeric string to a number:

var y=+b;

or

var x=+y+b;

answered Feb 09 '13 at 16:52

Plynx

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score 2 · Answer 3 · answered Feb 09 '13 at 16:50

2

You are concatenating a string instead of adding numbers:

var x=y+2;

Try that:

var x=parseInt(y)+2;

answered Feb 09 '13 at 16:50

mika

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score 0 · Answer 4 · answered Feb 09 '13 at 16:50

0

You have to convert your values with parseInt function:

    var y = parseInt(b, 10) + 2;

answered Feb 09 '13 at 16:50

Mickael

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score 0 · Answer 5 · answered Feb 09 '13 at 16:58

If one of the operands is a string , + operator performs concatenation.

this will work

function myFunction()
{
  var a = document.getElementById("thedropdown");
  var b = a.options[a.selectedIndex].value;

  var y=b;
  var x= parseInt(y) + 2;
  var demoP=document.getElementById("demo")
  demoP.innerHTML= x;
}

Note var x= parseInt(y) + 2;

Not adding 2 numbers correctly in javascript

5 Answers5

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