Table 5-8. typeof return values
Expression
Return value
Notes
typeof undefined
"undefined"
typeof null
"object"
Unfortunate, but true
typeof {}
"object"
typeof true
"boolean"
typeof 1
"number"
typeof ""
"string"
typeof Symbol()
"symbol"
New in ES6
typeof function() {} "function"
Because typeof is an operator, parentheses are not required. That
is, if you have a variable x, you can use typeof x instead of
typeof(x)
. The latter is valid syntax—the parentheses just form an
unnecessary expression group.
void Operator
The
void
operator has only one job: to evaluate its operand and then return
undefined
. Sound useless? It is. It can be used to force expression evaluation where
you want a return value of
undefined
, but I have never run across such a situation in
the wild. The only reason I include it in this book is that you will occasionally see it
used as the URI for an HTML
<a>
tag, which will prevent the browser from navigat‐
ing to a new page:
<
a
href
=
"javascript:void 0"
>Do nothing.</
a
>
This is not a recommended approach, but you do see it from time to time.
Assignment Operators
The assignment operator is straightforward: it assigns a value to a variable. What’s on
the lefthand side of the equals sign (sometimes called the lvalue) must be a variable,
property, or array element. That is, it must be something that can hold a value
(assigning a value to a constant is technically part of the declaration, not an assign‐
ment operator).
96 | Chapter 5: Expressions and Operators
