Expressions
JavaScript expressions are very similar to those in PHP. As you
learned in Chapter 4, an
expression is a combination of values, variables, operators, and
functions that results in a value; the result can be a number, a string,
or a Boolean value (which evaluates to either true
or false
).
Example 15-1 shows some
simple expressions. For each line, it prints out a letter between
a
and d
, followed by a colon and the result of the
expressions (the <br />
tag is
there to create a line break and separate the output into four
lines).
<script> document.write("a: " + (42 > 3) + "<br />") document.write("b: " + (91 < 4) + "<br />") document.write("c: " + (8 == 2) + "<br />") document.write("d: " + (4 < 17) + "<br />") </script>
The output from this code is as follows:
a: true b: false c: false d: true
Notice that both expressions a:
and d:
evaluate to true
. But b:
and c:
evaluate to false
. Unlike PHP (which
would print the number 1 and nothing, respectively), actual strings of
“true” and “false” are displayed.
In JavaScript, when checking whether a value is true
or false
, all values evaluate to true
with the exception of the following,
which evaluate to false
: the string
false
itself, 0, −0, the empty
string, null
, undefined, and NaN (Not
a Number, a computer engineering concept for an illegal floating-point
operation such as division by zero).
Note how I am referring to true
and false
in lowercase. This is because, unlike in PHP, these values ...
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