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JavaScript: When parseFloat is really needed

September 22nd, 2011 Leave a comment Go to comments

     parseFloat function parses a passed string and returns a number with floating point. The given function determines whether the first character in the specified string is a number, then if it’s true, the function parses the string until it reaches the end of the number, and returns the number as a number, not as a string. In other words, parseFloat makes an explicit type conversion from a string to a proper number with floating point.

Today I’ve used this JavaScript function for the first time. On my page I extract some values from query string (window.location.search), then these values take part in some calculations. I’ve met with the fact that the calculation was absolutely wrong. Let’s see the next simplest formula:

var res = 90 + someVal;

someVal is a value extracted from query string. if someVal will be, for example, “30.555” (exactly with quotes), res will be “9030.555”. It’s kind of slightly unexpected result, though it’s absolutely explainable. JavaScript makes automatic type conversion when dealing with the operator ‘+’. If one operand is a string and the other is not, the other is converted to a string and the result is concatenated. Obviously, we have to turn string someVal into an appropriate number. parseFloat helps us out:

var someNumber = parseFloat(someVal);
var res = 90 + someNumber;

OK, now res is expectedly 120.555.

There is an alternative – Number():

var someNumber = Number(someVal);

I use parseFloat instead Number(), because in my case the value extracted from query string may contain some additional alphabetical characters. For example, I can get from query string something like “30.555oz.”. I’m too lazy to cut out the excess symbols, I prefer using parseFloat. Repeating what I said in the beginning of this article, parseFloat successfully parses a string, which starts with a string representation of a number. All symbols after last digit symbol will be ignored. That means that the string “30.555oz.” will be transformed to the number 30.555. Another string “3YQJGP7CCACF” will be transformed to 3 and so on. In contrast, Number() accepts only a valid string number without any additional symbols.

I’d like also to mention about another useful function – parseInt, which is logically associated with parseFloat. Good examples of its usage you can find here.

In addition I’d like to note that some people complain about a parseFloat rounding problem (e.g. here or here): when, for example, string “3.05” turns into number 3.04999999999…. The solution is to use the toFixed function, which formats a number to use a specified number of trailing decimals. toFixed accepts the number of digits after the decimal point.

var someNumber = parseFloat('3.05');
someNumber = someNumber.toFixed(2);

toFixed helps to overcome this rounding problem, however, for justice’ sake, I need to note that we may not always know how many digits after point our initial string number had. Therefore, the usage of toFixed function with an inappropriate number of digits may affect accuracy of calculations.

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