1.4 Expressions and Operators
JavaScript expressions are formed by combining values (which may be
literals, variables, object properties, array elements, or function
invocations) using JavaScript operators. Parentheses can be used in
an expression to group subexpressions and alter the default order of
evaluation of the expression. Some examples:
1+2
total/n
sum(o.x, a[3])++
JavaScript defines a complete set of operators, most of which should
be familiar to C, C++, and Java programmers. They are listed in the
table below, and a brief explanation of the non-standard operators
follows. The P column specifies operator precedence and the A column
specifies operator associativity: L means left-to-right
associativity, and R means right-to-left associativity.
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15
|
L
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.
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Access an object property
|
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L
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[ ]
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Access an array element
|
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L
|
( )
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Invoke a function
|
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R
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new
|
Create new object
|
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14
|
R
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++
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Pre-or-post increment (unary)
|
| |
R
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--
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Pre-or-post decrement (unary)
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R
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-
|
Unary minus (negation)
|
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R
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+
|
Unary plus (no-op)
|
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R
|
~
|
Bitwise complement (unary)
|
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R
|
!
|
Logical complement (unary)
|
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R
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delete
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Undefine a property (unary) (JS 1.2)
|
| |
R
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typeof
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Return data type (unary) (JS 1.1)
|
| |
R
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void
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Return undefined value (unary) (JS 1.1)
|
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13
|
L
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*, /, %
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Multiplication, division, remainder
|
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12
|
L
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+, -
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Add, subtract
|
| |
L
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+
|
Concatenate strings
|
|
11
|
L
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<<
|
Integer shift left
|
| |
L
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>>
|
Shift right, sign-extension
|
| |
L
|
>>>
|
Shift right, zero extension
|
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10
|
L
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<, <=
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Less than, less than or equal
|
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L
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>, >=
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Greater than, greater than or equal
|
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L
|
instanceof
|
Check object type (JS 1.5)
|
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L
|
in
|
Check whether property exists (JS 1.5)
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9
|
L
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==
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Test for equality
|
| |
L
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!=
|
Test for inequality
|
| |
L
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===
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Test for identity (JS 1.3)
|
| |
L
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!==
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Test for non-identity (JS 1.3)
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|
8
|
L
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&
|
Integer bitwise AND
|
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7
|
L
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^
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Integer bitwise XOR
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6
|
L
|
|
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Integer bitwise OR
|
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5
|
L
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&&
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Logical AND
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4
|
L
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||
|
Logical OR
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3
|
R
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?:
|
Conditional operator (3 operands)
|
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2
|
R
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=
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Assignment
|
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R
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*=, +=, -=,
etc.
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Assignment with operation
|
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1
|
L
|
,
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Multiple evaluation
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JavaScript operators that are not familiar from C, C++, and Java are
the following:
- === and !==
-
The JavaScript equality operator, ==, defines
equality loosely and allows type conversions. For example, it
considers the number 3 and the string
"3" to be equal,
it considers false to be equal to
0, and it considers null and
undefined to be equal. The identity operator,
===, written with three equals signs, is stricter:
it only evaluates to true if its operands are
identical: i.e. if they have the same type and are equal. Similarly,
the JavaScript non-identity operator !== is
stricter than the non-equality != operator.
- String operators
-
In JavaScript, the + operator concatenates string
arguments in addition to adding numeric arguments. The
== and === operators compare
strings by value by testing to see whether they contain exactly the
same characters. The relational operators <,
<=, >, and
>= compare strings based on alphabetical order.
- typeof
-
Return the type of the operand as a string. Evaluates to
"number",
"string",
"boolean",
"object",
"function", or
"undefined". Evaluates to
"object" if the operand is
null.
- instanceof
-
Evaluates to true if the object on the left was
created with the constructor function (such as
Date or RegExp) on the right.
- in
-
Evaluates to true if the object on the right has
(or inherits) a property with the name on the left.
- delete
-
Deletes an object property. Note that this is not the same as simply
setting the property to null. Evaluates to
false if the property could not be deleted, or
true otherwise.
- void
-
Ignores the operand and evaluates to undefined.
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