For example, consider Unix-style file permissions: read, write, and execute. A given
user can have any combination of these three settings, making them ideal for flags.
Because we have three flags, we need three bits to store the information:
const
FLAG_READ
1
// 0b001
const
FLAG_WRITE
2
// 0b010
const
FLAG_EXECUTE
4
// 0b100
With bitwise operators, we can combine, toggle, and detect individual flags in a single
numeric value:
let
p
=
FLAG_READ
|
FLAG_WRITE
;
// 0b011
let
hasWrite
=
p
&
FLAG_WRITE
;
// 0b010 - truthy
let
hasExecute
=
p
&
FLAG_EXECUTE
;
// 0b000 - falsy
p
=
p
^
FLAG_WRITE
;
// 0b001 -- toggle write flag (now off)
p
=
p
^
FLAG_WRITE
;
// 0b011 -- toggle write flag (now on)
// we can even determine multiple flags in one expression:
const
hasReadAndExecute
=
p
&
(
FLAG_READ
|
FLAG_EXECUTE
);
Note that for
hasReadAndExecute
we had to use the grouping operator; AND has a
higher precedence than OR, so we had to force the OR to evaluate before the AND.
typeof Operator
The
typeof
operator returns a string representing the type of its operand. Unfortu‐
nately, this operator doesn’t exactly map to the seven data types in JavaScript (
unde
fined
,
null
, boolean, number, string, symbol, and object), which has caused no end
of criticism and confusion.
The
typeof
operator has one quirk that’s usually referred to as a bug:
typeof null
returns
"object"
.
null
is not, of course, an object (it is a primitive). The reasons are
historical and not particularly interesting, and it has been suggested many times that
it be fixed, but too much existing code is already built on this behavior, so it’s now
immortalized in the language specification.
typeof
is also often criticized for not being able to distinguish arrays and nonarray
objects. It correctly identifies functions (which are also special types of objects), but
typeof []
results in
"object"
.
lists the possible return values of
typeof
.
Grouping Operator | 95
