HTML and CSS Reference
In-Depth Information
> var a = {};
> a instanceof Object
> a instanceof String
In JavaScript all objects inherit from the base Object , so the first result, comparing String , makes sense, as
does the third, comparing a , the empty object. In contrast, neither Object nor a inherits from String , so this should
return false (for details on how to structure inheritance and object-oriented (OO) code in JavaScript, see the section
“Inheritance the JavaScript Way”).
Type Coercion
JavaScript is a dynamically typed language, with automatic type conversion, meaning that types are converted as
needed, based on the operations being performed. Now, this type conversion is a little . . . misbehaved. Let's take a
look at the following example:
> x = "37" + 3
> x = 3 + "7"
> x = 10 - "3"
Wait, what?
JavaScript will automatically convert between numbers and strings, but the way it does so depends on the
operators involved. Basically, if the + operator is involved, it will convert any numbers to strings and assume that +
means “concatenate.”
However, any other operators will instead convert strings to numbers and assume that the operators involved are
What about an expression with more operators?
> x = "10" + 3 / 4 - 2
Can you tell what steps JavaScript took to get the result 98.75 ? Personally, it took me a few seconds to step
through and figure it out.
In general, you should avoid automatic coercion between types, and instead be explicit. JavaScript has a couple
of handy built-in functions to convert from strings to numbers, parseInt and parseFloat :
> parseInt("654", 10)
> parseInt("654.54", 10)
> parseInt("654", 8)
> parseFloat("654")
> parseFloat("654.54")
Search WWH ::

Custom Search