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3.8.3 Object to Primitive Conversions

Object-to-boolean conversions are trivial: all objects (including arrays and functions)
convert to true. This is so even for wrapper objects: new Boolean(false) is an object
rather than a primitive value, and so it converts to true.
Object-to-string and object-to-number conversions are performed by invoking a meth-
od of the object to be converted. This is complicated by the fact that JavaScript objects
have two different methods that perform conversions, and it is also complicated by
some special cases described below. Note that the string and number conversion rules
described here apply only to native objects. Host objects (defined by web browsers, for
example) can convert to numbers and strings according to their own algorithms.
All objects inherit two conversion methods. The first is called toString(), and its job
is to return a string representation of the object. The default toString() method does
not return a very interesting value (though we’ll find it useful in Example 6-4):
({x:1, y:2}).toString()    // => "[object Object]"
2. In ECMAScript 3, parseInt() may parse a string that begins with “0” (but not “0x” or “0X”) as an octal
number or as a decimal number. Because the behavior is unspecified, you should never use parseInt()
to parse numbers with leading zeros, unless you explicitly specify the radix to be used! In ECMAScript 5,

parseInt() only parses octal numbers if you explicitly pass 8 as the second argument.

Many classes define more specific versions of the toString() method. The toString()
method of the Array class, for example, converts each array element to a string and
joins the resulting strings together with commas in between. The toString() method
of the Function class returns an implementation-defined representation of a function.
In practice, implementations usually convert user-defined functions to strings of Java-
Script source code. The Date class defines a toString() method that returns a human-
readable (and JavaScript-parsable) date and time string. The RegExp class defines a
toString() method that converts RegExp objects to a string that looks like a RegExp
literal:
[1,2,3].toString()                  // => "1,2,3"
(function(x) { f(x); }).toString()  // => "function(x) {\n    f(x);\n}"
/\d+/g.toString()                   // => "/\\d+/g"
new Date(2010,0,1).toString()  // => "Fri Jan 01 2010 00:00:00 GMT-0800 (PST)"
The other object conversion function is called valueOf(). The job of this method is less
well-defined: it is supposed to convert an object to a primitive value that represents the
object, if any such primitive value exists. Objects are compound values, and most ob-
jects cannot really be represented by a single primitive value, so the default valueOf()
method simply returns the object itself rather than returning a primitive. Wrapper
classes define  valueOf() methods that return the wrapped primitive value. Arrays,
functions, and regular expressions simply inherit the default method. Calling
valueOf() for instances of these types simply returns the object itself. The Date class
defines a  valueOf() method that returns the date in its internal representation: the
number of milliseconds since January 1, 1970:
var d = new Date(2010, 0, 1);   // January 1st, 2010, (Pacific time)
d.valueOf()                     // => 1262332800000
With the toString() and valueOf() methods explained, we can now cover object-to-
string and object-to-number conversions. Do note, however, that there are some special
cases in which JavaScript performs a different object-to-primitive conversion. These
special cases are covered at the end of this section.
To convert an object to a string, JavaScript takes these steps:
• If the object has a toString() method, JavaScript calls it. If it returns a primitive
value, JavaScript converts that value to a string (if it is not already a string) and
returns the result of that conversion. Note that primitive-to-string conversions are
all well-defined in Table 3-2.
• If the object has no toString() method, or if that method does not return a primitive
value, then JavaScript looks for a valueOf() method. If the method exists, Java-
Script calls it. If the return value is a primitive, JavaScript converts that value to a
string (if it is not already) and returns the converted value.
• Otherwise, JavaScript cannot obtain a primitive value from either toString() or
valueOf(), so it throws a TypeError.

To convert an object to a number, JavaScript does the same thing, but it tries the
valueOf() method first:
• If the object has a valueOf() method that returns a primitive value, JavaScript con-
verts (if necessary) that primitive value to a number and returns the result.
• Otherwise, if the object has a toString() method that returns a primitive value,
JavaScript converts and returns the value.
• Otherwise, JavaScript throws a TypeError.
The details of this object-to-number conversion explain why an empty array converts
to the number 0 and why an array with a single element may also convert to a number.
Arrays inherit the default valueOf() method that returns an object rather than a prim-
itive value, so array-to-number conversion relies on the toString() method. Empty
arrays convert to the empty string. And the empty string converts to the number 0. An
array with a single element converts to the same string that that one element does. If
an array contains a single number, that number is converted to a string, and then back
to a number.
The + operator in JavaScript performs numeric addition and string concatenation. If
either of its operands is an object, JavaScript converts the object using a special object-
to-primitive conversion rather than the object-to-number conversion used by the other
arithmetic operators. The == equality operator is similar. If asked to compare an object
with a primitive value, it converts the object using the object-to-primitive conversion.
The object-to-primitive conversion used by + and == includes a special case for Date
objects. The Date class is the only predefined core JavaScript type that defines mean-
ingful conversions to both strings and numbers. The object-to-primitive conversion is
basically an object-to-number conversion (valueof() first) for all objects that are not
dates, and an object-to-string conversion (toString() first) for Date objects. The con-
version is not exactly the same as those explained above, however: the primitive value
returned by valueOf() or toString() is used directly without being forced to a number
or string.
The < operator and the other relational operators perform object-to-primitive conver-
sions like == does, but without the special case for Date objects: any object is converted
by trying valueOf() first and then toString(). Whatever primitive value is obtained is
used directly, without being further converted to a number or string.
+, ==, != and the relational operators are the only ones that perform this special kind of
string-to-primitive conversions. Other operators convert more explicitly to a specified
type and do not have any special case for Date objects. The - operator, for example,
converts its operands to numbers. The following code demonstrates the behavior of
+, -, ==, and > with Date objects:
var now = new Date();     // Create a Date object
typeof (now + 1)          // => "string": + converts dates to strings
typeof (now - 1)          // => "number": - uses object-to-number conversion

now == now.toString()     // => true: implicit and explicit string conversions
now > (now -1)            // => true: > converts a Date to a number

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