Javascript: Can't get variable sum to work with addition operator

Question

I can't get the var total to resolve by using addition. It will work with multiplication operator: var total = subtotal * sales_tax but not with the + sign addition operator: var total = subtotal + sales_tax. Any help would be most appreciated.

var calculate_click = function () { 
    var subtotal = parseFloat(document.getElementById("subtotal").value).toFixed(3); 
    var taxRate  = parseFloat(document.getElementById("tax_rate").value).toFixed(3);  

    if (isNaN(subtotal) || isNaN(taxRate)) {    
    } 
        else {                                  
        var sales_tax = (subtotal * taxRate / 100); 
        parseFloat(document.getElementById("sales_tax").value = sales_tax.toFixed(3));  

        var total = subtotal + sales_tax;   
        parseFloat(document.getElementById("total").value = total.toFixed(3));  
    }
}

When you say that it doesn't work, what's the output? Can you link to a running example (JSFiddle or StackOverflow's new code editor)? — Eric Nguyen
– Eric Nguyen, Commented Oct 2, 2014 at 19:08
Also the calls to parseFloat() around the code where you set the field values are pointless. — Pointy
– Pointy, Commented Oct 2, 2014 at 19:08
Your indentation is a little misleading. It makes it look like the else is inside the if block. — techfoobar
– techfoobar, Commented Oct 2, 2014 at 19:09
@EricNguyen Well, it works in my browser, but apparently not in JSFiddle. The result is simply a blank space when trying to get a total. jsfiddle.net/Ronando/m9jLt0L2 — Padawan
– Padawan, Commented Oct 2, 2014 at 19:15

techfoobar · Accepted Answer · 2014-10-02 19:50:06Z

5

toFixed() formats the number into a string. So arithmetic operations afterwards will not work as expected.

Note:

+ (concatenation) is a valid operation for strings as well, so it'll return "string1string2" - For all other arithmetic operations it auto converts the strings to numbers and performs the operation. If the data within the strings cannot be converted, it returns NaN.
"12" + "2" => "122" whereas "12" * "2" => 24 (number) and "hello" * "3" => NaN

edited Oct 2, 2014 at 19:50

answered Oct 2, 2014 at 19:08

techfoobar

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5 Comments

Padawan Over a year ago

but the string will work with a multiplication operator? That's what confused me, the the formula worked with multiplication but not addition.

techfoobar Over a year ago

Thats because + (concatenation) is a valid operation for strings as well, so it'll return "string1string2" - For all other arithmetic operations it will auto convert the strings to numbers and perform the operation (when possible, otherwise it returns NaN).

techfoobar Over a year ago

"12" + "2" gives "122" whereas "12" * "2" gives 24 and "hello" * "3" gives NaN

Padawan Over a year ago

I see! That makes sense. Thanks. Previously I thought it might have to do with being a string, so I tried parsing it, but I still couldn't get it to work. Can you help in getting it to work? Also, I seem to have trouble in understanding parsing since others say I don't need it on the subtotal and taxRate variables. When I remove the parse, it doesn't work due to the parentheses (which I can't find where to put). var subtotal = parseFloat(document.getElementById("subtotal").value).toFixed(3);

Padawan Over a year ago

Yes! That was perfect! I should have known it was something that simple. Thank you. This is very helpful and allowed me to complete my homework after 6 hours of slamming my head against the keyboard.

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