I need javascript to add 5 to an integer variable, but instead it treats the variable as a string, so it write out the variable, then add 5 onto the end of the "string". How can I force it to do math instead?
var dots = document.getElementById("txt").value; // 5
function increase(){
dots = dots + 5;
}
Output: 55
How can I force it to output 10?
You have the line
dots = document.getElementById("txt").value;
in your file, this will set dots to be a string because the contents of txt is not restricted to a number.
to convert it to an int change the line to:
dots = parseInt(document.getElementById("txt").value, 10);
Note: The 10 here specifies decimal (base-10). Without this some browsers may not interpret the string correctly. See MDN: parseInt.
a and b are strings you need them to be numbers so you can do math on them. This is done by var c = parseInt(a,10)+parseInt(b,10)the simplest:
dots = dots*1+5;
the dots will be converted to number.
dots = +dots+5.DON'T FORGET - Use parseFloat(); if your dealing with decimals.
Number.I'm adding this answer because I don't see it here.
One way is to put a '+' character in front of the value
example:
var x = +'11.5' + +'3.5'
x === 15
I have found this to be the simplest way
In this case, the line:
dots = document.getElementById("txt").value;
could be changed to
dots = +(document.getElementById("txt").value);
to force it to a number
NOTE:
+'' === 0
+[] === 0
+[5] === 5
+['5'] === 5
parseInt() should do the trick
var number = "25";
var sum = parseInt(number, 10) + 10;
var pin = number + 10;
Gives you
sum == 35
pin == "2510"
http://www.w3schools.com/jsref/jsref_parseint.asp
Note: The 10 in parseInt(number, 10) specifies decimal (base-10). Without this some browsers may not interpret the string correctly. See MDN: parseInt.
You can add + behind the variable and it will force it to be an integer
var dots = 5
function increase(){
dots = +dots + 5;
}
This also works for you:
dots -= -5;
dots = document.getElementById("txt").value;
dots = Number(dots) + 5;
// from MDN
Number('123') // 123
Number('123') === 123 /// true
Number('12.3') // 12.3
Number('12.00') // 12
Number('123e-1') // 12.3
Number('') // 0
Number(null) // 0
Number('0x11') // 17
Number('0b11') // 3
Number('0o11') // 9
Number('foo') // NaN
Number('100a') // NaN
Number('-Infinity') //-Infinity
its really simple just
var total = (1 * yourFirstVariablehere) + (1 * yourSecondVariablehere)
this forces javascript to multiply because there is no confusion for * sign in javascript.
After trying most of the answers here without success for my particular case, I came up with this:
dots = -(-dots - 5);
The + signs are what confuse js, and this eliminates them entirely. Simple to implement, if potentially confusing to understand.
UPDATED since this was last downvoted....
I only saw the portion
var dots = 5
function increase(){
dots = dots+5;
}
before, but it was later shown to me that the txt box feeds the variable dots. Because of this, you will need to be sure to "cleanse" the input, to be sure it only has integers, and not malicious code.
One easy way to do this is to parse the textbox with an onkeyup() event to ensure it has numeric characters:
<input size="40" id="txt" value="Write a character here!" onkeyup="GetChar (event);"/>
where the event would give an error and clear the last character if the value is not a number:
<script type="text/javascript">
function GetChar (event){
var keyCode = ('which' in event) ? event.which : event.keyCode;
var yourChar = String.fromCharCode();
if (yourChar != "0" &&
yourChar != "1" &&
yourChar != "2" &&
yourChar != "3" &&
yourChar != "4" &&
yourChar != "5" &&
yourChar != "6" &&
yourChar != "7" &&
yourChar != "8" &&
yourChar != "9")
{
alert ('The character was not a number');
var source = event.target || event.srcElement;
source.value = source.value.substring(0,source.value-2);
}
}
</script>
Obviously you could do that with regex, too, but I took the lazy way out.
Since then you would know that only numbers could be in the box, you should be able to just use eval():
dots = eval(dots) + 5;
eval() is dangerous if dots is from user input, particularly if it's from stored input. More information
dots was a defined variable that was being incremented and just being mistaken for a string. So I'll qualify my answer that it shouldn't be used lightly, as you said, from user/stored input that could have the potential for evil scripting injection. But also in this case, such injection would cause a math error/exception, anyway, so it really wouldn't do anything, either way.eval() is so dangerous, you need to look at the code in the context the OP was using. It is not, in this case, because he's using input that is not coming from a textbox or any other form of user input where there could be any form of injection of a non-numeric value! I wish I could downvote your downvotes!eval() is just fine, and you need to look at the context in which it is being used, instead of just blindly believing everything you read without actually understanding it!var dots = 5 function increase(){ dots = dots+5; } does not use a text field, and assigns using direct variables, not from user input. So that's what I went off of. But I saw you're right that he's assigning that text field txt to dots, and dots is a global variable, and so gets updated by that input. You're right. Thanks.