javascript

C H A P T E R 2
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Type Conversion
Iron Man, Superman, Batman, Spider-Man, X-Men, and countless other superheroes have many traits in
common. Most have a distinctive costume, unwavering morals, underlying motif, secret identity,
supervillains to fight, and, of course, extraordinary powers. Those powers may or may not be innate,
though. For example, Clark Kent doesn’t need his Superman costume to fly, but Tony Stark would drop
like a stone without his Iron Man armor.
JavaScript values of object type or array and function subtypes are like Superman or Spider-Man.
They innately have extraordinary powers, referred to as members or methods. On the other hand, values
of the string, number, or boolean type are like Iron Man or Batman in that they need to put on their
costume, referred to as a wrapper, to have extraordinary powers.
So, just as seeing the bat signal appear in the night sky over Gotham City tells Bruce Wayne to put on
the bat suit in order to become Batman, seeing the . operator appear to their right tells a string, number,
or boolean to put on a wrapper in order to become an object.
Conversely, just as Batman returns to being Bruce Wayne after defeating the Joker, Penguin, or
Catwoman, a wrapper object returns to being a string, number, or boolean after invoking a method. To
convert a string, number, or boolean to a wrapper object, JavaScript invokes String(), Number(), or
Boolean(). Those are referred to as constructor functions. To reverse the conversion, that is, to convert a
wrapper object back to a string, number, or boolean, JavaScript invokes valueOf() on the wrapper.
Insofar as JavaScript converts string, number, and boolean values to and from wrapper objects
behind the scenes, we just need to explore their features. Moreover, string wrappers are useful, but those
for numbers and booleans are not. So, we won’t waste time on those.
String Members
Open firebug.html in Firefox, and then press F12 to enable Firebug. If you’re just joining us, flip back to
the preface for details on how to do this. In Chapter 1, you learned how to glue one string to another with
the + operator. concat() does the same thing. So in Firebug, let’s glue "man" to "Bat" by way of the +
operator and concat() method, verifying our work with Figure 2–1:
"Bat" + "man";
// "Batman"
"Bat".concat("man");
// "Batman"
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