This chapter describes JavaScript's expressions and operators, including assignment, comparison, arithmetic, bitwise, logical, string, ternary and more.
A complete and detailed list of operators and expressions is also available in the reference .
JavaScript has the following types of operators. This section describes the operators and contains information about operator precedence.
JavaScript has both binary and unary operators, and one special ternary operator, the conditional operator. A binary operator requires two operands, one before the operator and one after the operator:
operand1 operator operand2
例如，
3+4
or
x*y
.
A unary operator requires a single operand, either before or after the operator:
operator operand
or
operand operator
例如，
x++
or
++x
.
An assignment operator assigns a value to its left operand based on the value of its right operand. The simple assignment operator is equal (
=
), which assigns the value of its right operand to its left operand. That is,
x = y
assigns the value of
y
to
x
.
There are also compound assignment operators that are shorthand for the operations listed in the following table:
| Name | Shorthand operator | 含义 |
|---|---|---|
| Assignment |
x = y
|
x = y
|
| Addition assignment |
x += y
|
x = x + y
|
| Subtraction assignment |
x -= y
|
x = x - y
|
| Multiplication assignment |
x *= y
|
x = x * y
|
| Division assignment |
x /= y
|
x = x / y
|
| Remainder assignment |
x %= y
|
x = x % y
|
| Exponentiation assignment |
x **= y
|
x = x ** y
|
| Left shift assignment |
x <<= y
|
x = x << y
|
| Right shift assignment |
x >>= y
|
x = x >> y
|
| Unsigned right shift assignment |
x >>>= y
|
x = x >>> y
|
| Bitwise AND assignment |
x &= y
|
x = x & y
|
| Bitwise XOR assignment |
x ^= y
|
x = x ^ y
|
| Bitwise OR assignment |
x |= y
|
x = x | y
|
| Logical AND assignment |
x &&= y
|
x && (x = y)
|
| Logical OR assignment |
x ||= y
|
x || (x = y)
|
| Logical nullish assignment |
x ??= y
|
x ?? (x = y)
|
Like most expressions, assignments like
x = y
have a return value. It can be retrieved by e.g. assigning the expression or logging it:
const z = (x = y); // Or equivalently: const z = x = y; console.log(z); // Log the return value of the assignment x = y. console.log(x = y); // Or log the return value directly.
The return value matches the expression to the right of the
=
sign in the “Meaning” column of the table above. That means that
(x = y)
返回
y
,
(x += y)
returns the resulting sum
x + y
,
(x **= y)
returns the resulting power
x ** y
, and so on.
In the case of logical assignments,
(x &&= y)
,
(x ||= y)
，和
(x ??= y)
, the return value is that of the logical operation without the assignment, so
x && y
,
x || y
，和
x ?? y
, respectively.
Note that the return values are always based on the operands’ values before the operation.
When chaining these expressions, each assignment is evaluated 从右到左 . Consider these examples:
w = z = x = y
相当于
w = (z = (x = y))
or
x = y; z = y; w = y
z += x *= y
相当于
z += (x *= y)
or
tmp = x * y; x *= y; z += tmp
(except without the
tmp
).
For more complex assignments, the 析构赋值 syntax is a JavaScript expression that makes it possible to extract data from arrays or objects using a syntax that mirrors the construction of array and object literals.
var foo = ['one', 'two', 'three']; // without destructuring var one = foo[0]; var two = foo[1]; var three = foo[2]; // with destructuring var [one, two, three] = foo;
A comparison operator compares its operands and returns a logical value based on whether the comparison is true. The operands can be numerical, string, logical, or object values. Strings are compared based on standard lexicographical ordering, using Unicode values. In most cases, if the two operands are not of the same type, JavaScript attempts to convert them to an appropriate type for the comparison. This behavior generally results in comparing the operands numerically. The sole exceptions to type conversion within comparisons involve the
===
and
!==
operators, which perform strict equality and inequality comparisons. These operators do not attempt to convert the operands to compatible types before checking equality. The following table describes the comparison operators in terms of this sample code:
var var1 = 3; var var2 = 4;
| Operator | 描述 | Examples returning true |
|---|---|---|
Equal
(
==
)
|
返回
true
if the operands are equal.
|
3 == var1
3 == '3'
|
Not equal
(
!=
)
|
返回
true
if the operands are not equal.
|
var1 != 4
|
Strict equal
(
===
)
|
返回
true
if the operands are equal and of the same type. See also
Object.is
and
sameness in JS
.
|
3 === var1
|
Strict not equal
(
!==
)
|
返回
true
if the operands are of the same type but not equal, or are of different type.
|
var1 !== "3"
|
Greater than
(
>
)
|
返回
true
if the left operand is greater than the right operand.
|
var2 > var1
|
Greater than or equal
(
>=
)
|
返回
true
if the left operand is greater than or equal to the right operand.
|
var2 >= var1
|
Less than
(
<
)
|
返回
true
if the left operand is less than the right operand.
|
var1 < var2
|
Less than or equal
(
<=
)
|
返回
true
if the left operand is less than or equal to the right operand.
|
var1 <= var2
|
注意： ( => ) is not an operator, but the notation for 箭头函数 .
An arithmetic operator takes numerical values (either literals or variables) as their operands and returns a single numerical value. The standard arithmetic operators are addition (
+
), subtraction (
-
), multiplication (
*
), and division (
/
). These operators work as they do in most other programming languages when used with floating point numbers (in particular, note that division by zero produces
Infinity
)。例如：
1 / 2; // 0.5 1 / 2 == 1.0 / 2.0; // this is true
In addition to the standard arithmetic operations (
+
,
-
,
*
,
/
), JavaScript provides the arithmetic operators listed in the following table:
| Operator | 描述 | 范例 |
|---|---|---|
Remainder
(
%
)
|
Binary operator. Returns the integer remainder of dividing the two operands. | 12 % 5 returns 2. |
Increment
(
++
)
|
Unary operator. Adds one to its operand. If used as a prefix operator (
++x
), returns the value of its operand after adding one; if used as a postfix operator (
x++
), returns the value of its operand before adding one.
|
若
x
is 3, then
++x
sets
x
to 4 and returns 4, whereas
x++
returns 3 and, only then, sets
x
to 4.
|
Decrement
(
--
)
|
Unary operator. Subtracts one from its operand. The return value is analogous to that for the increment operator. |
若
x
is 3, then
--x
sets
x
to 2 and returns 2, whereas
x--
returns 3 and, only then, sets
x
to 2.
|
Unary negation
(
-
)
|
Unary operator. Returns the negation of its operand. |
若
x
is 3, then
-x
returns -3.
|
Unary plus
(
+
)
|
Unary operator. Attempts to convert the operand to a number, if it is not already. |
+"3"
返回
3
.
+true
返回
1.
|
Exponentiation operator
(
**
)
|
Calculates the
base
到
exponent
power, that is,
base
exponent
|
2 ** 3
返回
8
.
10 ** -1
返回
0.1
.
|
A bitwise operator treats their operands as a set of 32 bits (zeros and ones), rather than as decimal, hexadecimal, or octal numbers. For example, the decimal number nine has a binary representation of 1001. Bitwise operators perform their operations on such binary representations, but they return standard JavaScript numerical values.
The following table summarizes JavaScript's bitwise operators.
| Operator | 用法 | 描述 |
|---|---|---|
| Bitwise AND |
a & b
|
Returns a one in each bit position for which the corresponding bits of both operands are ones. |
| Bitwise OR |
a | b
|
Returns a zero in each bit position for which the corresponding bits of both operands are zeros. |
| Bitwise XOR |
a ^ b
|
Returns a zero in each bit position for which the corresponding bits are the same.
[Returns a one in each bit position for which the corresponding bits are different.] |
| Bitwise NOT |
~ a
|
Inverts the bits of its operand. |
| Left shift |
a << b
|
Shifts
a
in binary representation
b
bits to the left, shifting in zeros from the right.
|
| Sign-propagating right shift |
a >> b
|
Shifts
a
in binary representation
b
bits to the right, discarding bits shifted off.
|
| Zero-fill right shift |
a >>> b
|
Shifts
a
in binary representation
b
bits to the right, discarding bits shifted off, and shifting in zeros from the left.
|
Conceptually, the bitwise logical operators work as follows:
Before: 11100110111110100000000000000110000000000001 After: 10100000000000000110000000000001
For example, the binary representation of nine is 1001, and the binary representation of fifteen is 1111. So, when the bitwise operators are applied to these values, the results are as follows:
| Expression | 结果 | Binary Description |
|---|---|---|
15 & 9
|
9
|
1111 & 1001 = 1001
|
15 | 9
|
15
|
1111 | 1001 = 1111
|
15 ^ 9
|
6
|
1111 ^ 1001 = 0110
|
~15
|
-16
|
~ 0000 0000 ... 0000 1111 = 1111 1111 ... 1111 0000
|
~9
|
-10
|
~ 0000 0000 ... 0000 1001 = 1111 1111 ... 1111 0110
|
Note that all 32 bits are inverted using the Bitwise NOT operator, and that values with the most significant (left-most) bit set to 1 represent negative numbers (two's-complement representation).
~x
evaluates to the same value that
-x - 1
evaluates to.
The bitwise shift operators take two operands: the first is a quantity to be shifted, and the second specifies the number of bit positions by which the first operand is to be shifted. The direction of the shift operation is controlled by the operator used.
Shift operators convert their operands to thirty-two-bit integers and return a result of the same type as the left operand.
The shift operators are listed in the following table.
| Operator | 描述 | 范例 |
|---|---|---|
|
Left shift
(
<<
)
|
This operator shifts the first operand the specified number of bits to the left. Excess bits shifted off to the left are discarded. Zero bits are shifted in from the right. |
9<<2
yields 36, because 1001 shifted 2 bits to the left becomes 100100, which is 36.
|
Sign-propagating right shift
(
>>
)
|
This operator shifts the first operand the specified number of bits to the right. Excess bits shifted off to the right are discarded. Copies of the leftmost bit are shifted in from the left. |
9>>2
yields 2, because 1001 shifted 2 bits to the right becomes 10, which is 2. Likewise,
-9>>2
yields -3, because the sign is preserved.
|
Zero-fill right shift
(
>>>
)
|
This operator shifts the first operand the specified number of bits to the right. Excess bits shifted off to the right are discarded. Zero bits are shifted in from the left. |
19>>>2
yields 4, because 10011 shifted 2 bits to the right becomes 100, which is 4. For non-negative numbers, zero-fill right shift and sign-propagating right shift yield the same result.
|
Logical operators are typically used with Boolean (logical) values; when they are, they return a Boolean value. However, the
&&
and
||
operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they may return a non-Boolean value. The logical operators are described in the following table.
| Operator | 用法 | 描述 |
|---|---|---|
Logical AND
&&
)
|
expr1 && expr2
|
返回
expr1
if it can be converted to
false
; otherwise, returns
expr2
. Thus, when used with Boolean values,
&&
返回
true
if both operands are true; otherwise, returns
false
.
|
Logical OR
(
||
)
|
expr1 || expr2
|
返回
expr1
if it can be converted to
true
; otherwise, returns
expr2
. Thus, when used with Boolean values,
||
返回
true
if either operand is true; if both are false, returns
false
.
|
Logical NOT
(
!
)
|
!expr
|
返回
false
if its single operand that can be converted to
true
; otherwise, returns
true
.
|
Examples of expressions that can be converted to
false
are those that evaluate to null, 0, NaN, the empty string (""), or undefined.
The following code shows examples of the
&&
(logical AND) operator.
var a1 = true && true; // t && t returns true var a2 = true && false; // t && f returns false var a3 = false && true; // f && t returns false var a4 = false && (3 == 4); // f && f returns false var a5 = 'Cat' && 'Dog'; // t && t returns Dog var a6 = false && 'Cat'; // f && t returns false var a7 = 'Cat' && false; // t && f returns false
The following code shows examples of the || (logical OR) operator.
var o1 = true || true; // t || t returns true var o2 = false || true; // f || t returns true var o3 = true || false; // t || f returns true var o4 = false || (3 == 4); // f || f returns false var o5 = 'Cat' || 'Dog'; // t || t returns Cat var o6 = false || 'Cat'; // f || t returns Cat var o7 = 'Cat' || false; // t || f returns Cat
The following code shows examples of the ! (logical NOT) operator.
var n1 = !true; // !t returns false var n2 = !false; // !f returns true var n3 = !'Cat'; // !t returns false
As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:
false
&&
anything
is short-circuit evaluated to false.
true
||
anything
is short-circuit evaluated to true.
The rules of logic guarantee that these evaluations are always correct. Note that the anything part of the above expressions is not evaluated, so any side effects of doing so do not take effect.
Note that for the second case, in modern code you can use the new
Nullish coalescing operator
(
??
) that works like
||
, but it only returns the second expression, when the first one is "
nullish
", i.e.
null
or
undefined
. It is thus the better alternative to provide defaults, when values like
''
or
0
are valid values for the first expression, too.
In addition to the comparison operators, which can be used on string values, the concatenation operator (+) concatenates two string values together, returning another string that is the union of the two operand strings.
例如，
console.log('my ' + 'string'); // console logs the string "my string".
The shorthand assignment operator += can also be used to concatenate strings.
例如，
var mystring = 'alpha'; mystring += 'bet'; // evaluates to "alphabet" and assigns this value to mystring.
conditional operator is the only JavaScript operator that takes three operands. The operator can have one of two values based on a condition. The syntax is:
condition ? val1 : val2
若
condition
is true, the operator has the value of
val1
. Otherwise it has the value of
val2
. You can use the conditional operator anywhere you would use a standard operator.
例如，
var status = (age >= 18) ? 'adult' : 'minor';
This statement assigns the value "adult" to the variable
status
if
age
is eighteen or more. Otherwise, it assigns the value "minor" to
status
.
comma operator
(
,
) simply evaluates both of its operands and returns the value of the last operand. This operator is primarily used inside a
for
loop, to allow multiple variables to be updated each time through the loop. It is regarded bad style to use it elsewhere, when it is not necessary. Often two separate statements can and should be used instead.
For example, if
a
is a 2-dimensional array with 10 elements on a side, the following code uses the comma operator to update two variables at once. The code prints the values of the diagonal elements in the array:
var x = [0,1,2,3,4,5,6,7,8,9]
var a = [x, x, x, x, x];
for (var i = 0, j = 9; i <= j; i++, j--)
// ^
console.log('a[' + i + '][' + j + ']= ' + a[i][j]);
A unary operation is an operation with only one operand.
delete
delete
operator deletes an object's property. The syntax is:
delete object.property; delete object[propertyKey]; delete objectName[index]; delete property; // legal only within a with statement
where
object
is the name of an object,
property
is an existing property, and
propertyKey
is a string or symbol referring to an existing property.
The fourth form is legal only within a
with
statement, to delete a property from an object, and also for properties of the global object.
若
delete
operator succeeds, it removes the propery from the object. Trying to access it afterwards will yield
undefined
。
delete
operator returns
true
if the operation is possible; it returns
false
if the operation is not possible.
x = 42; // implicitly creates window.x
var y = 43;
var myobj = {h: 4}; // create object with property h
delete x; // returns true (can delete if created implicitly)
delete y; // returns false (cannot delete if declared with var)
delete Math.PI; // returns false (cannot delete non-configurable properties)
delete myobj.h; // returns true (can delete user-defined properties)
Since arrays are just objects, it's technically possible to
delete
elements from them. This is however regarded as a bad practice, try to avoid it. When you delete an array property, the array length is not affected and other elements are no re-indexed. To achieve that behavior, it is much better to just overwrite the element with the value
undefined
. To actually manipulate the array, use the various array methods such as
splice
.
typeof
typeof
operator
is used in either of the following ways:
typeof operand typeof (operand)
typeof
运算符返回指示未评估操作数类型的字符串。
operand
is the string, variable, keyword, or object for which the type is to be returned. The parentheses are optional.
Suppose you define the following variables:
var myFun = new Function('5 + 2');
var shape = 'round';
var size = 1;
var foo = ['Apple', 'Mango', 'Orange'];
var today = new Date();
typeof
operator returns the following results for these variables:
typeof myFun; // returns "function" typeof shape; // returns "string" typeof size; // returns "number" typeof foo; // returns "object" typeof today; // returns "object" typeof doesntExist; // returns "undefined"
For the keywords
true
and
null
，
typeof
operator returns the following results:
typeof true; // returns "boolean" typeof null; // returns "object"
For a number or string, the
typeof
operator returns the following results:
typeof 62; // returns "number" typeof 'Hello world'; // returns "string"
For property values, the
typeof
operator returns the type of value the property contains:
typeof document.lastModified; // returns "string" typeof window.length; // returns "number" typeof Math.LN2; // returns "number"
For methods and functions, the
typeof
operator returns results as follows:
typeof blur; // returns "function" typeof eval; // returns "function" typeof parseInt; // returns "function" typeof shape.split; // returns "function"
For predefined objects, the
typeof
operator returns results as follows:
typeof Date; // returns "function" typeof Function; // returns "function" typeof Math; // returns "object" typeof Option; // returns "function" typeof String; // returns "function"
void
void
operator
is used in either of the following ways:
void (expression) void expression
void
operator specifies an expression to be evaluated without returning a value.
expression
is a JavaScript expression to evaluate. The parentheses surrounding the expression are optional, but it is good style to use them.
A relational operator compares its operands and returns a
布尔
value based on whether the comparison is true.
in
in
operator
返回
true
if the specified property is in the specified object. The syntax is:
propNameOrNumber in objectName
where
propNameOrNumber
is a string, numeric, or symbol expression representing a property name or array index, and
objectName
is the name of an object.
The following examples show some uses of the
in
operator.
// Arrays
var trees = ['redwood', 'bay', 'cedar', 'oak', 'maple'];
0 in trees; // returns true
3 in trees; // returns true
6 in trees; // returns false
'bay' in trees; // returns false (you must specify the index number,
// not the value at that index)
'length' in trees; // returns true (length is an Array property)
// built-in objects
'PI' in Math; // returns true
var myString = new String('coral');
'length' in myString; // returns true
// Custom objects
var mycar = { make: 'Honda', model: 'Accord', year: 1998 };
'make' in mycar; // returns true
'model' in mycar; // returns true
instanceof
instanceof
operator
返回
true
if the specified object is of the specified object type. The syntax is:
objectName instanceof objectType
where
objectName
is the name of the object to compare to
objectType
，和
objectType
is an object type, such as
Date
or
Array
.
使用
instanceof
when you need to confirm the type of an object at runtime. For example, when catching exceptions, you can branch to different exception-handling code depending on the type of exception thrown.
For example, the following code uses
instanceof
to determine whether
theDay
是
Date
object. Because
theDay
是
Date
object, the statements in the
if
statement execute.
var theDay = new Date(1995, 12, 17);
if (theDay instanceof Date) {
// statements to execute
}
precedence of operators determines the order they are applied when evaluating an expression. You can override operator precedence by using parentheses.
The following table describes the precedence of operators, from highest to lowest.
| 运算符类型 | Individual operators |
|---|---|
| member |
. []
|
| call / create instance |
() new
|
| negation/increment |
! ~ - + ++ -- typeof void delete
|
| multiply/divide |
* / %
|
| addition/subtraction |
+ -
|
| bitwise shift |
<< >> >>>
|
| relational |
< <= > >= in instanceof
|
| equality |
== != === !==
|
| bitwise-and |
&
|
| bitwise-xor |
^
|
| bitwise-or |
|
|
| logical-and |
&&
|
| logical-or |
||
|
| conditional |
?:
|
| assignment |
= += -= *= /= %= <<= >>= >>>= &= ^= |= &&= ||= ??=
|
| comma |
,
|
A more detailed version of this table, complete with links to additional details about each operator, may be found in JavaScript 参考 .
An expression is any valid unit of code that resolves to a value.
Every syntactically valid expression resolves to some value but conceptually, there are two types of expressions: with side effects (for example: those that assign value to a variable) and those that in some sense evaluate and therefore resolve to a value.
The expression
x = 7
is an example of the first type. This expression uses the =
operator
to assign the value seven to the variable
x
. The expression itself evaluates to seven.
代码
3 + 4
is an example of the second expression type. This expression uses the + operator to add three and four together without assigning the result, seven, to a variable.
JavaScript has the following expression categories:
JavaScript 中的基本关键字和通用表达式。
this
使用
this
keyword
to refer to the current object. In general,
this
refers to the calling object in a method. Use
this
either with the dot or the bracket notation:
this['propertyName'] this.propertyName
Suppose a function called
validate
validates an object's
value
property, given the object and the high and low values:
function validate(obj, lowval, hival) {
if ((obj.value < lowval) || (obj.value > hival))
console.log('Invalid Value!');
}
You could call
validate
in each form element's
onChange
event handler, using
this
to pass it to the form element, as in the following example:
<p>Enter a number between 18 and 99:</p> <input type="text" name="age" size=3 onChange="validate(this, 18, 99);">
The grouping operator
( )
controls the precedence of evaluation in expressions. For example, you can override multiplication and division first, then addition and subtraction to evaluate addition first.
var a = 1; var b = 2; var c = 3; // default precedence a + b * c // 7 // evaluated by default like this a + (b * c) // 7 // now overriding precedence // addition before multiplication (a + b) * c // 9 // which is equivalent to a * c + b * c // 9
Left values are the destination of an assignment.
new
可以使用
new
operator
to create an instance of a user-defined object type or of one of the built-in object types. Use
new
as follows:
var objectName = new objectType([param1, param2, ..., paramN]);
super keyword is used to call functions on an object's parent. It is useful with 类 to call the parent constructor, for example.
super([arguments]); // calls the parent constructor. super.functionOnParent([arguments]);