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 = f()
is an assignment expression that assigns the value of f()
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 = f() 
x = f() 
Addition assignment  x += f() 
x = x + f() 
Subtraction assignment  x = f() 
x = x  f() 
Multiplication assignment  x *= f() 
x = x * f() 
Division assignment  x /= f() 
x = x / f() 
Remainder assignment  x %= f() 
x = x % f() 
Exponentiation assignment  x **= f() 
x = x ** f() 
Left shift assignment  x <<= f() 
x = x << f() 
Right shift assignment  x >>= f() 
x = x >> f() 
Unsigned right shift assignment  x >>>= f() 
x = x >>> f() 
Bitwise AND assignment  x &= f() 
x = x & f() 
Bitwise XOR assignment  x ^= f() 
x = x ^ f() 
Bitwise OR assignment  x = f() 
x = x  f() 
Logical AND assignment  x &&= f() 
x && (x = f()) 
Logical OR assignment  x = f() 
x  (x = f()) 
Logical nullish assignment  x ??= f() 
x ?? (x = f()) 
Assigning to properties
If a variable refers to an object, then the lefthand side of an assignment expression may make assignments to properties of that variable. For example:
let obj = {};
obj.x = 3;
console.log(obj.x); // Prints 3.
console.log(obj); // Prints { x: 3 }.
const key = "y";
obj[key] = 5;
console.log(obj[key]); // Prints 5.
console.log(obj); // Prints { x: 3, y: 5 }.
For more information about objects, read Working with Objects.
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;
Evaluation and nesting
In general, assignments are used within a variable declaration (i.e., with const
, let
, or var
) or as standalone statements).
// Declares a variable x and initializes it to the result of f().
// The result of the x = f() assignment expression is discarded.
let x = f();
// Declares a variable x and initializes it to the result of g().
// The result of the x = g() assignment expression is discarded.
x = g(); // Reassigns the variable x to the result of g().
However, like other expressions, assignment expressions like x = f()
evaluate into a result value.
Although this result value is usually not used, it can then be used by another expression.
Chaining assignments or nesting assignments in other expressions can result in surprising behavior. For this reason, some JavaScript style guides discourage chaining or nesting assignments). Nevertheless, assignment chaining and nesting may occur sometimes, so it is important to be able to understand how they work.
By chaining or nesting an assignment expression, its result can itself be assigned to another variable. It can be logged, it can be put inside an array literal or function call, and so on.
let x;
const y = (x = f()); // Or equivalently: const y = x = f();
console.log(y); // Logs the return value of the assignment x = f().
console.log(x = f()); // Logs the return value directly.
// An assignment expression can be nested in any place
// where expressions are generally allowed,
// such as as array literals’ elements or as function calls’ arguments.
console.log([ 0, x = f(), 0 ]);
console.log(f(0, x = f(), 0));
The evaluation result matches the expression to the right of the =
sign in the
“Meaning” column of the table above. That means that x = f()
evaluates into
whatever f()
’s result is, x += f()
evaluates into the resulting sum x + f()
,
x **= f()
evaluates into the resulting power x ** y
, and so on.
In the case of logical assignments, x &&= f()
,
x = f()
, and x ??= f()
, the return value is that of the
logical operation without the assignment, so x && f()
,
x  f()
, and x ?? f()
, respectively.
When chaining these expressions without parentheses or other grouping operators like array literals, the assignment expressions are grouped right to left (they are rightassociative), but they are evaluated left to right.
Note that, for all assignment operators other than =
itself,
the resulting values are always based on the operands’ values before
the operation.
For example, assume that the following functions f
and g
and the variables x
and y
have been declared:
function f () {
console.log('F!');
return 2;
}
function g () {
console.log('G!');
return 3;
}
let x, y;
Consider these three examples:
y = x = f()
y = [ f(), x = g() ]
x[f()] = g()
Evaluation example 1
y = x = f()
is equivalent to y = (x = f())
,
because the assignment operator =
is rightassociative.
However, it evaluates from left to right:
 The assignment expression
y = x = f()
starts to evaluate.
The
y
on this assignment’s lefthand side evaluates into a reference to the variable namedy
.  The assignment expression
x = f()
starts to evaluate.
The
x
on this assignment’s lefthand side evaluates into a reference to the variable namedx
. 
The function call
f()
prints "F!" to the console and then evaluates to the number2
.  That
2
result fromf()
is assigned tox
.

The

The assignment expression
x = f()
has now finished evaluating; its result is the new value ofx
, which is2
.  That
2
result in turn is also assigned toy
.

The

The assignment expression
y = x = f()
has now finished evaluating; its result is the new value ofy
– which happens to be2
.x
andy
are assigned to2
, and the console has printed "F!".
Evaluation example 2
y = [ f(), x = g() ]
also evaluates from left to right:
 The assignment expression
y = [ f(), x = g() ]
starts to evaluate.
The
y
on this assignment’s lefthand evaluates into a reference to the variable namedy
. 
The inner array literal
[ f(), x = g() ]
starts to evaluate. The function call
f()
prints "F!" to the console and
2
. 2. The assignment expressionx = g()
starts to evaluate.
The
x
on this assignment’s lefthand side evaluates into a reference to the variable namedx
. 
The function call
g()
prints "G!" to the console and then evaluates to the number3
.  That
3
result fromf()
is assigned tox
.  The assignment expression
x = g()
has now finished evaluating;
x
, which is3
. That3
result becomes the next element in the inner array literal (after the2
from thef()
).  The function call

The inner array literal
[ f(), x = g() ]
has now finished evaluating; its result is an array with two values:[ 2, 3 ]
.  That
[ 2, 3 ]
array is now assigned toy
.

The

The assignment expression
y = [ f(), x = g() ]
has now finished evaluating; its result is the new value ofy
– which happens to be[ 2, 3 ]
.x
is now assigned to3
,y
is now assigned to[ 2, 3 ]
, and the console has printed "F!" then "G!".
Evaluation example 3
x[f()] = g()
also evaluates from left to right.
(This example assumes that x
is already assigned to some object.
For more information about objects, read Working with Objects.)
 The assignment expression
x[f()] = g()
starts to evaluate.
The
x[f()]
property access on this assignment’s lefthand starts to evaluate.
The
x
in this property access evaluates into a reference to the variable namedx
. 
Then the function call
f()
prints "F!" to the console and then evaluates to the number2
.

The

The
x[f()]
property access on this assignment has now finished evaluating; its result is a variable property reference:x[2]
. 
Then the function call
g()
prints "G!" to the console and then evaluates to the number3
. 
That
3
is now assigned tox[2]
. (This step will succeed only ifx
is assigned to an object.)

The

The assignment expression
x[f()] = g()
has now finished evaluating; its result is the new value ofx[2]
– which happens to be3
.x[2]
is now assigned to3
, and the console has printed "F!" then "G!".
Avoid assignment chains
Chaining assignments or nesting assignments in other expressions can result in surprising behavior. For this reason, chaining assignments in the same statement is discouraged).
In particular, putting a variable chain in a const
, let
, or var
statement often does not work.
Only the outermost/leftmost variable would get declared;
any other variables within the assignment chain are not declared by the const
/let
/var
statement.
For example:
let z = y = x = f();
This statement seemingly declares the variables x
, y
, and z
.
However, it only actually declares the variable z
.
y
and x
are either invalid references to nonexistent variables (in strict mode) or, worse, would implicitly create global variables for x
and y
in sloppy mode.
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 a comparison operator but rather is 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];
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.
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.
delete Math.PI; // returns false (cannot delete nonconfigurable properties)
const myObj = {h: 4};
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]);