JavaScript GlossaryS2C Home   « JavaScript Glossary

Glossary Term Description
Data TypesRecognized JavaScript values and their ranges.
ExpressionsAny valid unit of code that resolves to a value.
LiteralsLiteral notation for various entities.
Operator PrecedenceOperator symbols order of preference.

Data Typesgo to top of page Top

The following and suprisingly small list of data types and their ranges are recognized in JavaScript.

Data Type Range or Example Notes
Booleantrue or falseBoolean constructor which is a wrapper for a boolean value or
boolean primitive.
nullnullSpecial keyword also a primitive value denoting a null value.
JavaScript is case-sensitive, so other variants such as Null and NULL are not recognized.
NumberMAX_VALUE 1.7976931348623157e+308
-MAX_VALUE -1.7976931348623157e+308

MIN_VALUE 5e-324
-MIN_VALUE -5e-324
Maximum positive number value that can be represented in JavaScript.
Maximum negative number value that can be represented in JavaScript.

Minimum positive number value that can be represented in JavaScript.
Minimum negative number value that can be represented in JavaScript.
String'string text', "string text"String constructor for strings and character sequences.
undefinedundefinedGlobal variable and primitive value.

Data Type Conversion

JavaScript is not a strongly typed language such as Java, but is dynamically typed so after variable creation you can store data of any type within the variable by using the equals symbol (=) via assignment. You can also set a variables initial value at creation as well. An example of dynamic typing is shown below:

// Firstly we assign a numeric to aVar
var aVar = 1;

// Here we move a string to the variable which is fine as javaScript is dynamically typed
aVar = 'move in a string';

Where expressions involve numeric and string values and use the + operator, JavaScript will convert numeric values to strings and treat the expression as a string concatenation. The JavaScript interpreter uses automatic type conversion when combining string and numbers, which means all numbers are converted to strings before concatenation takes place.

// Create a string and numeric variables.
var aString = '5';
var aNumber = 5;

// Concatenation ends up with a string holding the value  '55' not 10.
alert(aString + aNumber);

Expressionsgo to top of page Top

An expression is any valid unit of code that resolves to a value whether the result is assigned to a variable or not as in the following examples:

// Expression assignment of 3
var aVar = 3;

// Expression value of 3
1 + 2;

We can split expressions into four categories which are listed below with typical usage:

Category Description Typical Usage
ArithmeticEvaluates to a number.Arithmetic Operators
(+, -, *, /, %, ++, --, unary +, unary -)
LogicalEvaluates to true or false.Logical Operators
(&&, ||, !)
ObjectEvaluates to an object.Special Operators
StringEvaluates to a character string.String Operators
(+ and +=)

Literalsgo to top of page Top

Literals are strings of characters that we use to represent actual value in JavaScript. For certain objects we can use literal notation instead of the new special operator to create instances of an object. The following subsections give examples of literal usage and where appropriate normal and literal instantiation for the objects in question.

Literal Type Description
Array LiteralArray literal syntax can be used to create and initialize arrays.
Boolean LiteralUsed to set true and false values in booleans.
Floating-point LiteralNon-integer numbers that have a fractional component.
IntegerOctal (base 8), decimal (base 10) and hexadecimal (base 16).
Object LiteralObject literal syntax can be used to create and initialize objects.
String LiteralString literal syntax can be used to create and initialize strings.

Array Literal

Array literal syntax can be used to create arrays by using square brackets [ ], which enclose a list of zero or more expressions where each expression represents an element within the array. The array is created from the enclosed expressions and initialized with the specified values as its elements. The length is derived from the number of expressions specified along with any delimiting commas.

// Create an array with three elements.
var anArray = new Array('one', 'two', 'three');
alert(anArray + ' - ' + anArray.length);

// Create an empty array using literal syntax.
var anEmptyArray = []

// Create an array with three elements using literal syntax.
var weekendDays = ['Fri', 'Sat', 'Sun'];

// Create an array with three elements using literal syntax and delimiter.
// The second element will have a value of undefined.
var weekendDays = ['Fri', , 'Sun'];

Boolean Literal

Boolean literal values are true and false and can be used to set values within a Boolean object or boolean primitive.

The code below illustrates the use of boolean literal values as well as highlighting the difference between the Boolean constructor and the boolean primitives true and false.

// Create a Boolean object.
aBoolean = new Boolean(false);
if (aBoolean) {
  alert('set false, but evaluates true'); // This code is executed
} else {
  alert('set false'); // This code is NOT executed

// Set a boolean primitive.
bBoolean = false;
if (bBoolean) {
  alert('set false, but evaluates true'); // This code is NOT executed
} else {
  alert('set false'); // This code is executed

Floating-point Literal

Floating-point literals represent non-integer numbers that have a fractional component. In JavaScript A floating-point literal can consist of up to four parts comprised from the following syntax:


Component Description Examples
Decimal integerCan be prefixed signed with a + or -.1, -99, +123
Decimal pointUsed to separate the integer and fractional decimal..
Fractional decimal integerUsed to set fractional decimal.1, 10, 100
ExponentE or e followed by an integer which can be prefixed signed with a + or -.e1, -E4, +e12

The above components can be mixed and matched to form a floating-point literal, but the floating-point literal must have at least one digit and either a decimal point or e or E.

Following are a few examples of well formed floating-point literals:

-1.23, .986, -5e-324, 1.7976931348623157e+308, .2E34 and 1E-234.


We can express Integers in octal (base 8), decimal (base 10) and hexadecimal (base 16).

Base Description Examples
OctalSequence of digits with a leading 0 indicates it is in octal.
Octal integers can only include the digits 0-7.
Octal integer literals have been deprecated and removed although JavaScript still supports them for backward compatibility.
-01, 012, +0123
DecimalSequence of digits without a leading 0.-1, 12, +123
HexadecimalSequence of digits with a leading 0x or 0X indicates it is in hexadecimal.
Hexadecimal integers can include digits 0-9 and the letters a-f and A-F.
-0xC1D3, 0X12, 0xfff

Object Literal

Object literal syntax can be used to create and initialize objects. This style of coding using matching braces { } to mark object boundaries, with property names followed by a colon and values delimited by commas, is known as JavaScript Object Notation. This is more commonly known as JSON which is popular with page authors because of its advantages over object assignment. Following are examples of object instantiation using assignment and object literal syntax:

 * Create an Address object via assignment.
var anAddress = new Object();
anAddress.number = 12;
anAddress.street = 'Picnic Street'; = 'Toytown';
anAddress.owner = homeOwner;
homeOwner.firstName = 'Teddy';
homeOwner.lastName = 'Bear';

 * Create an Address object using object literal notation.
 * Visually more informative of object structure and less code than the 
 * code from above. For more complex objects the visual and compactual 
 * differences are accentuated :). 
var anAddress = {
  number: 12,
  street: 'Picnic Street',
  town: 'Toytown',
  homeOwner: {
    firstName: 'Teddy',
    lastName: 'Bear'

String Literal

String literals are string primitives which can be used to create and initialize strings and are composed of zero or more characters enclosed in single or double quotation marks. The enclosing quotation marks must be of the same type and cannot be mixed. So strings must be delimited by single quotation marks '' or double quotation marks "".

Any string primitives have full access to String methods as JavaScript automaticially converts any string primitives to temporary String objects when a String method is called on the string primitive.

// Create a String object
aString = new String('happychappy'); 

// Create a string literal
bString = 'happychappy'; 

Operator Precedencego to top of page Top

The table below shows the order of precedence that is invoked when JavaScript interprets operator symbols. The list includes some Special Operators which are discussed in greater detail in the relevant parts of the reference section.

When using multiple operators in an expression it is always best practice to use parentheses to explicitly state the order of precedence for clarity and readability. The table lists order of precedence from highest to lowest.

Precedence Operators Operation Associativity
[ ]
newobject creationright
2()function callleft
3++prefix/postfix incrementn/a
--prefix/postfix decrement
4+unary plusright
-unary minus
!boolean logical NOT
~bitwise NOT
6+addition or string concatenationleft
7<<bitwise left shiftleft
>>bitwise right shift
>>>bitwise signed right shift
8/less thanleft
<=less than or equal to
>greater than
>=greater than or equal to
inproperty test
instanceofprototype test
9==equal toleft
!=not equal to
===strict equal to
!==strict not equal to
10&boolean logical AND or bitwise ANDleft
11^boolean logical XOR or bitwise XORleft
12|boolean logical OR or bitwise ORleft
13&&short-circuit boolean logical ANDleft
14||short-circuit boolean logical ORleft
15? :conditionalright
+=shorthand addition
-=shorthand subtraction
/=shorthand division
*=shorthand mutiplication
%=shorthand modulus
&=shorthand boolean logical AND or bitwise AND
|=shorthand boolean logical OR or bitwise OR
^=shorthand boolean logical XOR or bitwise XOR
<<=shorthand bitwise left shift
>>=shorthand bitwise right shift
>>>=shorthand bitwise signed right shift

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