PART IV
Appendix A: JavaScript Quick Reference 557
Type Conversion
Type conversion is automatically carried out in JavaScript. Table A-6 shows the common
conversion rules when data is automatically converted to one type or another. Automatic
conversion happens very often when using relational operators discussed later in the
section. It is also possible to force type conversion using a variety of built-in methods
summarized in Table A-7.
Value
Convert to
Boolean
Convert to
Number Convert to String Convert to Object
true 1 "true" Boolean object with
value of true
false 0 "false" Boolean object with
value of false
0 false "0" Number object with
value of 0
Any nonzero
number including
negative
numbers
true The number as
a string, so 40
becomes “40”
while −1.13
becomes "−1.13"
Number object of
the value indicated
Empty string "" false 0 String object with
no value
A nonempty
string
true
This means
that strings like
“0” and “false”
convert to true
as well.
If the string
contains solely
a number like
"4" it will be
converted into
the number. All
other strings will
be converted to
NaN. Note that
strings must
strictly contain
a number for
conversion for
example, "4no"
converts to NaN
and not 4.
String object
containing the string
primitive value
Any existing
object
true NaN Value of
toString()
method of the
object
null false 0 "null" TypeError Exception
thrown
undefined false NaN "undefined" TypeError Exception
thrown
TABLE A-6 Primitive JavaScript Data Types