Notes of Maks Nemisj

Experiments with JavaScript

Why isNaN and Number is not enough to cast string to a number

Recently I was doing string casting to a number and while you think it’s simple as it is, it took me a bit to make it fully functional.

My first approach was to use a Number constructor for casting, which would return number or NaN if value was not parsable to the number. But for empty strings, boolean values and null values, this approach gave unsatisfying result.

   Number("123zork") // NaN
   Number("")) // 0
   Number(" "))  // 0
   Number(null) // 0
   Number(false) // 0
   Number(true) // 1

Then, I decided to use isNaN to check the string before applying Number constructor. But isNaN was for no help either. It looks like underneath isNaN uses Number implementation and gives the same result as above 🙁

   isNaN("123zork")) // true
   isNaN("")) // false
   isNaN(" "))  // false
   isNaN(0))  // false
   isNaN(null) // false
   isNaN(false) // false

After my isNaN attempt faild my next idea was to use parseFloat. But problem with parseFloat is that strings like “123aa” are parsed into 123.

   parseFloat('123zork') // 123
   parseFloat("")) // NaN
   parseFloat(" "))  // NaN
   parseFloat(null) // NaN
   parseFloat(false) // NaN
   parseFloat(true) // NaN
   parseFloat('11111') // 11111
   parseFloat('1.2') // 1.2

The one solution which is left, was to cast value to the String by using String constructor and then pass it to the Number constructor, which would make boolean and null in a string representation and would result in NaN value for bad strings. Great.

   Number(String('123zork') // NaN
   Number(String("")) // 0
   Number(String(null)); // NaN
   Number(String(false)); // NaN
   Number(String('1.2')) // 1.2

Of course empty string is a breaker here, so time test string before using for emptiness:

    Number(String.trim(value) === '' ? NaN : String(value))

Note: dont’ forget that trim ( https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/String/Trim ) works only in IE9+ and normal modern browsers

JavaScript keeps surprising me over and over again. I Love this language, but some of the aspects of it, like this one, are just nuts 🙂

Related StackOverflow question:

http://stackoverflow.com/questions/825402/why-does-isnan-equal-false

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