Expressions and operators
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.
Operators
JavaScript has the following types of operators. This section describes the operators and contains information about operator precedence.
 Assignment operators
 Comparison operators
 Arithmetic operators
 Bitwise operators
 Logical operators
 String operators
 Conditional (ternary) operator
 Comma operator
 Unary operators
 Relational operators
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
For example, 3+4
or x*y
.
A unary operator requires a single operand, either before or after the operator:
operator operand
or
operand operator
For example, x++
or ++x
.
Assignment operators
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  Meaning 

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) 
Return value and chaining
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)
returns
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)
, and (x ??= y)
, the return value is that of the
logical operation without the assignment, so x && y
,
x  y
, and 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 righttoleft. Consider these examples:
w = z = x = y
is equivalent tow = (z = (x = y))
orx = y; z = y; w = y
z += x *= y
is equivalent toz += (x *= y)
ortmp = x * y; x *= y; z += tmp
(except without thetmp
).
Destructuring
For more complex assignments, the destructuring assignment 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;
Comparison operators
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  Description  Examples returning true 

Equal
(== ) 
Returns true if the operands are equal. 
3 == var1
3 == '3'

Not equal (!= )

Returns true if the operands are not equal. 
var1 != 4 
Strict
equal (=== ) 
Returns true if the operands are equal and of the same type. See
also Object.is and sameness in
JS. 
3 === var1 
Strict
not equal (!== ) 
Returns true if the operands are of the same type but not equal, or
are of different type. 
var1 !== "3" 
Greater
than (> ) 
Returns true if the left operand is greater than the right operand.

var2 > var1 
Greater
than or equal (>= ) 
Returns true if the left operand is greater than or equal to the
right operand. 
var2 >= var1 
Less
than (< ) 
Returns true if the left operand is less than the right operand.

var1 < var2 
Less
than or equal (<= ) 
Returns true if the left operand is less than or equal to the right
operand. 
var1 <= var2 
Note: =>
is not an operator, but the notation
for Arrow
functions.
Arithmetic operators
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
). For example:
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  Description  Example 

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. 
If 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.  If 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.  If x is 3, then x returns 3. 
Unary
plus (+ ) 
Unary operator. Attempts to convert the operand to a number, if it is not already. 

Exponentiation
operator (** ) 
Calculates the base to the exponent power, that is,
base^exponent 
2 ** 3 returns 8 .10 ** 1 returns 0.1 .

Bitwise operators
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  Usage  Description 

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. 
Signpropagating right shift  a >> b 
Shifts a in binary representation b bits to the right,
discarding bits shifted off. 
Zerofill 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. 
Bitwise logical operators
Conceptually, the bitwise logical operators work as follows:
 The operands are converted to thirtytwobit integers and expressed by a series of
bits (zeros and ones). Numbers with more than 32 bits get their most significant bits
discarded. For example, the following integer with more than 32 bits will be converted
to a 32 bit integer:
Before: 1110 0110 1111 1010 0000 0000 0000 0110 0000 0000 0001 After: 1010 0000 0000 0000 0110 0000 0000 0001
 Each bit in the first operand is paired with the corresponding bit in the second operand: first bit to first bit, second bit to second bit, and so on.
 The operator is applied to each pair of bits, and the result is constructed bitwise.
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  Result  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 (leftmost) bit set to 1 represent negative numbers
(two'scomplement representation). ~x
evaluates to the same value that
x  1
evaluates to.
Bitwise shift operators
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 thirtytwobit integers and return a result
of either type Number
or BigInt
: specifically, if the type
of the left operand is BigInt
, they return BigInt
;
otherwise, they return Number
.
The shift operators are listed in the following table.
Operator  Description  Example 

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. 
Signpropagating
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. 
Zerofill
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 nonnegative numbers, zerofill right shift and
signpropagating right shift yield the same result. 
Logical operators
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 nonBoolean values, they may return a nonBoolean value. The
logical operators are described in the following table.
Operator  Usage  Description 

Logical
AND (&& ) 
expr1 && expr2 
Returns expr1 if it can be converted to false ;
otherwise, returns expr2 . Thus, when used with Boolean values,
&& returns true if both operands are true;
otherwise, returns false . 
Logical OR
( ) 
expr1  expr2 
Returns expr1 if it can be converted to true ;
otherwise, returns expr2 . Thus, when used with Boolean values,
 returns true if either operand is true; if both are
false, returns false . 
Logical NOT
(! ) 
!expr 
Returns 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
Shortcircuit evaluation
As logical expressions are evaluated left to right, they are tested for possible "shortcircuit" evaluation using the following rules:
false
&& anything is shortcircuit evaluated to false.true
 anything is shortcircuit 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.
String operators
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.
For example,
console.log('my ' + 'string'); // console logs the string "my string".
The shorthand assignment operator += can also be used to concatenate strings.
For example,
var mystring = 'alpha';
mystring += 'bet'; // evaluates to "alphabet" and assigns this value to mystring.
Conditional (ternary) operator
The 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
If 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.
For example,
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
The comma
operator (,
) 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 2dimensional 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]);
Unary operators
A unary operation is an operation with only one operand.
delete
The
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.
If the delete
operator succeeds, it removes the property from the object.
Trying to access it afterwards will yield undefined
. The
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 false (cannot delete if created implicitly)
delete y; // returns false (cannot delete if declared with var)
delete Math.PI; // returns false (cannot delete nonconfigurable properties)
delete myobj.h; // returns true (can delete userdefined properties)
Deleting array elements
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
not reindexed. 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
The typeof
operator is used in either of the following ways:
typeof operand
typeof (operand)
The typeof
operator returns a string indicating the type of the
unevaluated operand. 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();
The 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
, the 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
The void
operator is used in either of the following ways:
void (expression)
void expression
The 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.
Relational operators
A relational operator compares its operands and returns a Boolean value based on whether the comparison is true.
in
The in
operator returns 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)
// builtin 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
The instanceof
operator returns 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
, and objectType
is an object type, such as
Date
or Array
.
Use instanceof
when you need to confirm the type of an object at runtime.
For example, when catching exceptions, you can branch to different exceptionhandling
code depending on the type of exception thrown.
For example, the following code uses instanceof
to determine whether
theDay
is a Date
object. Because theDay
is a
Date
object, the statements in the if
statement execute.
var theDay = new Date(1995, 12, 17);
if (theDay instanceof Date) {
// statements to execute
}
Operator precedence
The 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.
Operator type  Individual operators 

member  . [] 
call / create instance  () new 
negation/increment  ! ~  + ++  typeof void delete 
multiply/divide  * / % 
addition/subtraction  +  
bitwise shift  << >> >>> 
relational  < <= > >= in instanceof 
equality  == != === !== 
bitwiseand  & 
bitwisexor  ^ 
bitwiseor   
logicaland  && 
logicalor   
conditional  ?: 
assignment 
= += = *= /= %= <<= >>= >>>= &= ^= = &&= = ??=

comma  , 
A more detailed version of this table, complete with links to additional details about each operator, may be found in JavaScript Reference.
Expressions
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.
The code 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:
 Arithmetic: evaluates to a number, for example 3.14159. (Generally uses arithmetic operators.)
 String: evaluates to a character string, for example, "Fred" or "234". (Generally uses string operators.)
 Logical: evaluates to true or false. (Often involves logical operators.)
 Primary expressions: Basic keywords and general expressions in JavaScript.
 Lefthandside expressions: Left values are the destination of an assignment.
Primary expressions
Basic keywords and general expressions in JavaScript.
this
Use the 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);">
Grouping operator
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
Lefthandside expressions
Left values are the destination of an assignment.
new
You can use the new
operator to create an instance of a userdefined object type or of one of the
builtin object types. Use new
as follows:
var objectName = new objectType([param1, param2, ..., paramN]);
super
The super keyword is used to call functions on an object's parent. It is useful with classes to call the parent constructor, for example.
super([arguments]); // calls the parent constructor.
super.functionOnParent([arguments]);