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up vote 18 down vote favorite

Why is the value of data-value="2.0" cast to a String and the value of data-value="2.5" cast to a Number? I can handle this fine within my function. I'm just trying to understand a little more about how Javascript handles Numbers and Strings. This kind of caught me off guard.

<a data-value="2.0">2.0</a>
<a data-value="2.5">2.5</a>

$("a").click(function() {
    alert(typeof $(this).data( "value"));   
});

[ Fiddle With It ]

share|improve this question
    
I don't know WHY this is happening but you can see that floats ending with a 0 are always pretended to be strings. Maybe because the last 0 is irrelevant and so interpreted as text. For example: 2.5 is number and 2.50 is string. –  Doodlebunch 19 hours ago
9  
jQuery's .data method will attempt to convert whatever is in the data-* attribute - if you want it as the raw string value, use .attr("data-*") –  tymeJV 19 hours ago
    
@tymeJV thanks for the heads up! –  Matt 18 hours ago

6 Answers 6

up vote 17 down vote accepted

Those values are simply strings to vanilla javascript. Javascript isn't trying to do any conversion on its own.

[].forEach.call(document.querySelectorAll("a"), function(a){
    console.log("type: %s, value: %o", 
                typeof a.dataset.value, 
                a.dataset.value);
});
// type: string, value: "2.0"
// type: string, value: "2.5" 
<a data-value="2.0">2.0</a>
<a data-value="2.5">2.5</a>

jQuery, on the other hand, tries to determine and convert to the appropriate type when accessing data attributes via data(). It's (arguably) an issue with their implementation. Their documentation (emphasis mine) actually addresses this:

Every attempt is made to convert the string to a JavaScript value (this includes booleans, numbers, objects, arrays, and null). A value is only converted to a number if doing so doesn't change the value's representation. For example, "1E02" and "100.000" are equivalent as numbers (numeric value 100) but converting them would alter their representation so they are left as strings. The string value "100" is converted to the number 100.

See also HTMLElement.dataset

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1  
Thank you! I did not realise JQuery did this. I spent hours combining through a 600 line script looking for a bug and now feel very foolish! +1 For the example. –  Matt 18 hours ago
    
Good to know about this change. Turns out it's been this way since 1.8 rc 1. –  zzzzBov 8 hours ago
up vote 17 down vote

Looking through jQuery's source code, i have identified the reason.

It will parse it as a number, if, and only if, doing so does not change the string. Source: https://github.com/jquery/jquery/blob/master/src/data.js#L36

(+"2.0") + "" === "2" // !== the original string
(+"2.1") + "" === "2.1" // == the original string
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up vote 2 down vote

It appears to be treating that ".0" extension of the number like a string.

If all of your data values are going to be coming in in that format, just whip a parseFloat() on those suckers like this:

$("a").click(function() {
    alert(typeof parseFloat($(this).data( "value")));   
});

That way it won't matter if they are strings or numbers, they will always be treated like numbers(decimals intact).

share|improve this answer
2  
Wouldn't parseFloat() be more appropriate given the 2.5? –  canon 19 hours ago
    
@canon is absolutely correct –  Dave Lunny 19 hours ago
    
You can edit your answer to include that. –  Scimonster 19 hours ago
up vote 1 down vote

As tymeJV mentioned, this looks like an issue with how jquery handles autoconversion. If you use "2", it gives a number as well, so I'm guessing its just a weird edge case in how they handle things. I would encourage just using .attr('xxx') and parsing out your data to its known type.

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up vote 1 down vote

By default anything parsed from an attribute will be a string, you will need to convert it to a number if you need to use it as a number. The best way I've been able to handle this is by wrapping it with the Number function (native javascript)

var number = Number($(this).data("value"));

parseInt will return an integer (whole number), so if you want to keep your decimal values you'll need to use Number.

Word of caution with parseInt, it is always better to specify the base if you want to parse an int, 

parseInt($(this).data("value"), 10);
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up vote 0 down vote

I think it's more a question about how jquery identifies numbers. If you use alert(typeof(this.getAttribute("data-value")));, it spits out that it's a string both times (which is expected). So far as I know, everything in an html attribute is considered a string, just different libraries may have default behavior that interprets them differently.

Also, a shorthand way to get a number would be something along the lines of...

var someNumber = +$(someElt).data("value");

the + will cause type coercion if the returned value is a string, or leave it alone if it's already a number.

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