Not all operators are
symbols. Some are written as words. One example is the typeof
operator, which produces a string value naming the type of the value
you give it.
Having such numbers is
useful for storing strings inside a computer because it makes it
possible to represent them as a sequence of numbers. When comparing
strings, JavaScript goes over them from left to right, comparing the
numeric codes of the characters one by one.
There is only one value in JavaScript
that is not equal to itself, and that is NaN, which stands for “not
a number”.
In practice, you can usually get by with knowing that of the
operators we have seen so far, || has the lowest precedence, then
comes &&, then the comparison operators (>, ==, and so on), and
then the rest. This order has been chosen such that, in typical
expressions like the following one, as few parentheses as possible are
necessary:
The difference in meaning between undefined and null is an accident
of JavaScript’s design, and it doesn’t matter most of the time. In the cases
where you actually have to concern yourself with these values, I
recommend treating them as interchangeable (more on that in a moment).
. Yet in the third expression, + tries string
concatenation before numeric addition
When something that
doesn’t map to a number in an obvious way (such as "five" or
undefined) is converted to a number, the value NaN is produced.
Further arithmetic operations on NaN keep producing NaN, so if you
find yourself getting one of those in an unexpected place, look for
accidental type conversions.
g ==, the
outcome is easy to predict: you should get true when both values are
the same, except in the case of NaN.
But when the types differ,
JavaScript uses a complicated and confusing set of rules to determine
what to do. In most cases, it just tries to convert one of the values
to the other value’s type. However, when null or undefined occurs
on either side of the operator, it produces true only if both sides
are one of null or undefined.
That last piece of behavior is often useful. When you want to test
whether a value has a real value instead of null or undefined, you
can simply compare it to null with the == (or !=) operator.
The rules for
converting strings and numbers to Boolean values state that 0,
NaN, and the empty string ("") count as false, while all the
other values count as true.
where you
do not want any automatic type conversions to happen, there are two
extra operators: === and !==. The first tests whether a value is
precisely equal to the other, and the second tests whether it is not
precisely equal. So "" === false is false as expected.
The
logical operators && and || handle values of different types in a
peculiar way. They will convert the value on their left side to
Boolean type in order to decide what to do, but depending on the
operator and the result of that conversion, they return either the
original left-hand value or the right-hand value.
The || operator, for example, will return the value
to its left when that can be converted to true and will return the
value on its right otherwise. This conversion works as you’d expect
for Boolean values and should do something analogous for values of
other types.
This functionality allows the || operator to be
used as a way to fall back on a default value. If you give it an
expression that might produce an empty value on the left, the value on
the right will be used as a replacement in that case.
The && operator works similarly, but the other way
around. When the value to its left is something that converts to
false, it returns that value, and otherwise it returns the value on
its right.
Another important property of these two
operators is that the expression to their right is evaluated only when
necessary. In the case of true || X, no matter what X is—even if
it’s an expression that does something terrible—the result will be
true, and X is never evaluated. The same goes for false && X,
which is false and will ignore X. This is called short-circuit
evaluation.
" " " " and "\u00A0" have
nothing, NOTHING to do with UTF-8.
There is a character -- an abstract unit in a "script" (a writing
system;
we are using Latin right now) -- called NO-BREAK SPACE by the Unicode
Standard and ISO/IEC 10646. Unicode and ISO/IEC 10646 assign this
character an integer number, 160, which is A0 in hex.
UTF-8 is an encoding scheme that provides a way of representing any
of the approximately 1.1 million possible abstract characters in Unicode
as a sequence of 1 to 4 bytes.
« [...] " " " " and "\u00A0" have nothing, NOTHING to do with UTF-8 [...] Unicode and ISO/IEC 10646 assign this character an integer number, 160, which is A0 in hex [...] UTF-8 is an encoding scheme [...] The UTF-8 representation of the Unicode character 160 (no-break space), is the pair of bytes C2 A0. »
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