Types

Numbers

Strings

Booleans

null & undefined

Objects

Types

are

kinds of values

that can be

represented & manipulated

in a programming language

JavaScript types 

can be categorized as follows

primitives / objects

&

primitives / objects

primitives

objects

any value that does not belong to the primitive type

NumberS

JavaScript has a single number type

there is no distinction between integer values and floating-point values

  all numbers are represented in 64-bit floating-point format

Number literals

Integer literals

base 10

a sequence of digits

base 16

15                 //=> 15 in base 10
0xF                //=> 15 in base 16
017                //=> 15 in base 8

Floating-point literals

0.15    //=> 0.15
.15     //=> 0.15
0.15e+2 //=> 15 (0.15 * (10 * 10))
0.15e-2 //=> 0.0015 (0.15 / (10 * 10))

Arithmetic operations

work with numbers is done using basic arithmetic operators

+ addition

- subtraction

* multiplication

/ division

% modulo (remainder)

Math.pow(2, 53)          Math.round(.6)           Math.ceil(.6)          Math.floor(.6)
Math.abs(-5)             Math.max(x,y,z)          Math.min(x,y,z)        Math.random()
Math.PI                  Math.E                   Math.sqrt(3)           Math.pow(3, 1/3)
Math.sin(0)              Math.log(10)             Math.exp(3)           Math.log(100)/Math.LN10  Math.log(512)/Math.LN2

in JavaScript there are no errors

in cases of 

overflow

underflow

or

division by zero

overflow

when the result of a numeric operation is larger

than the largest representable number the result is

Infinity

when a negative value becomes larger than the largest
representable negative number the result is
-Infinity

adding / subtracting / multiplying / dividing

infinite values

by anything else

results in an infinite value

underflow

15 / 1.8e+308    //=> 0
-15 / 1.8e+308   //=> -0

15/-0 === 15/0   //=> false

division by zero

is not an error

it simply returns Infinity or -Infinity

BUT

there is one exception

zero divided by zero evaluates to NaN

Predefined global variables

isFinite(10);       //=> true
isFinite(0.1);      //=> true
isFinite(10.1e+7);  //=> true
isFinite(Infinity); //=> false
isFinite(10/0);     //=> false
isFinite(NaN);      //=> false

NaN

isNaN(NaN);   //=> true
isNaN('ten'); //=> true
isNaN({});    //=> true
isNaN(10);    //=> false
isNaN('10');  //=> false
isNaN(0xF);   //=> false

rounding errors

only a

finite number of real numbers can be represented

with the binary floating point format

which means that when working with them this will often be only

an approximation

the values often are very close to the correct value

adequate for almost any purpose

0.2 + 0.1                           //=> 0.30000000000000004
((0.2 * 100) + (0.1 * 100)) / 100   //=> 0.3

Strings

Strings

are JavaScript’s type for representing text

a string is an immutable ordered sequence of 16-bit values

which typically represents a Unicode character

  the length of a string is the number of 16-bit values it contains

empty string

is the string with the 

length equal to 0

  there is no special type that represents a single element of a string

String Literals

  to represent a string literally simply enclose the characters of the string

within a matched pair of single or double quotes (' or ")

Escape sequences

the backslash character (\) combined with the character that follows

represents a character that is otherwise not represented in the string

Working with strings

concatenation

using the + operator

joins two strings

appending the second to the first

length property

used to determine the length of a string

var str = 'JavaScript';
str.length //=> 10

invoking methods methods on strings

var str = 'JavaScript';
str.charAt(0);          //=> "J"
str.substring(5,10);    //=> "cript"
str.split('');          //=> ["J", "a", "v", "a", "S", "c", "r", "i", "p", "t"]
str.toLowerCase();      //=> "javascript"
str.replace('J', 'j');  //=> "javaScript"

Booleans

Booleans

  there are only two values possible for this type

true / false

they are usually the result of comparisons

'JavaScript' === 15 //=> false
15 === 15           //=> true
'15' != 15          //=> false

any value can be converted to a boolean value

Boolean operators

&& (AND)

evaluates to a truthy value if and only if both of its operands are truthy

otherwise it evaluates to a falsy value

10 < 15 && 15 > 10 //=> true

| | (OR)

evaluates to a truthy value if either one (or both) of its operands is truthy

and evaluates to a falsy value if both operands are falsy

15 < 10 || 15 > 10 //=> true 

! (NOT)

evaluates to true if its operand is falsy and evaluates to false if its operand is truthy

15 < 10 === !true //=> true

Null

&

undefined

Null

is a keyword that evaluates to a special value

used to indicate the absence of a value

it is typically regarded as the 

sole member of its own type

&

  it can be used to indicate no value for

numbers, strings or objects

UNDEFINED

undefined is system-level unexpected 

error-like absence of value

null is program-level expected

normal absence of value

despite the differences null & undefined both

var person = { 
  name: 'John Doe',
  children: null
};

person.children.length;  //=> TypeError: Cannot read property 'length' of null
obj.children[0]          //=> TypeError: Cannot read property '0' of null

Objects

Object types

are

any values that does not belong to the primitive types

an ordinary object

is a collection of properties

where each property has a name or key 

&

a value

var obj = {
  keyOne: 'valueOne',
  keyTwo: 'valuerTwo',
  keyN: 'valueN'
};

Object Literals

the most common things to do with objects are

create 

&

 set / query / delete / test / enumerate
their properties

special kind of objects

arrays

functions

global object

wrapper objects 

ARRAYS

var arr = [1000, 'string', function () {}, { name: 'JavaScript'}, [1, 2]];

FUNCTIONs

a function is

an object that has executable code associated with it

it may be invoked to run that executable code and return a computed value

JavaScript defines a special language syntax for working with them

The Global Object

is a regular object that serves a very important purpose

its properties are the globally defined symbols 

that are available to a JavaScript program

when the JavaScript interpreter starts it creates a new 

global object and gives it an initial set of properties that define

global properties like undefined, Infinity, NaN

global functions like isNaN(), parseInt(), eval()

constructor functions like Date(), RegExp(), String(), Object(),  Array()

global objects like Math and JSON

besides all predefined global values

global object also holds program-defined globals

WRAPPER objects

strings / numbers / booleans

are not objects

so why do they have properties?

because whenever you try to refer to a property

JavaScript

converts the string value to an object 

as if by calling 

new String() / new Number() / new Boolean()

in conclusion

wrapper objects

are

temporary objects 

created when you access 

a property of a 

string / number / boolean

it is possible to explicitly create wrapper objects

by invoking the String(), Number(), or Boolean() constructors

new String('JavaScript');       //=> String {0: "J", 1: "a", 2: "v", 3: "a", 4: "S", 5: "c", 6: "r", 
     //           7: "i", 8: "p", 9: "t", length: 10}

JavaScript converts wrapper objects into the wrapped

primitive value as necessary

var s = "test", n = 1, b = true; //=> a string, number, and boolean value
var S = new String(s);           //=> a string object
var N = new Number(n);           //=> a number object
var B = new Boolean(b);          //=> a boolean object 

the objects usually

but not always, behave just like the values

== equality operator treats a value and its wrapper object as equal

=== strict equality operator distinguishes them

the typeof operator will also show the difference

between a primitive value and its wrapper objects