JavaScript Type Conversion

Data Types

5 Data Types

string, number, boolean, object, function

3 Object Types

object, array, date

2 Other Types

null, undefined

Type Conversion

Number/Boolean/Date -> String

String(x)　　// x can be any number, expression or variable

x.toString()

String/Boolean -> Number

Number(x)

Number(' ') => 0

Number('') => 0

Number('1  2') => NaN　　Number('1+2') => NaN

Number(false) => 0　　Number(true) => 1

Operator '+'

+ can convert a variable to a number

+ '5' => 5

+ 'a' => NaN

Implicit Type Conversion

+ can also be applied in the cancatening the strings.

Infinity+(-Infinity) => NaN

+0+(+0) => +0, (-0)+(-0) => -0, (+0)+(-0) => +0

var c = a+b

if a is string:

　　if b is string: return the concatenation of a+b 　　// 'x' + 'y' => 'xy'

　　if b is not string : return a+b.toString

if a is number:

　　if b is string: return a.toString+b 　　// 100+'23' => '10023' 　　'3' + 4 + 5  =>  '345'　　3 + 4 + '5' => '75'

== will perform implict conversion on the varibale before comparing

string == number => Number(string) == number

boolean == ?　=> Number(boolean) == ?

object == ?(not obj)　=> valueof(object) == ?

null == undefined => return true

NaN == ? => return false // NaN == NaN => return false *NaN is unequal to every value including itself

object1 == object2 => compare if they point to the same object

Similiar with  <,  >,  <=,  >=

&& and ||

null && ? => null　　NaN && ? => NaN // if theie is a null/NaN/undefined evaulted value, return null/NaN/undefined

null && NaN => null　　NaN && null => NaN　　undefined && null => undefined

same as the cases in '||'

a && b

if a evalutes to be true: return b (original value before evalution)

else: return a (original value before evalution)

a || b

if a evalutes to be false: return b (original value before evalution)

else: return a (original value before evalution)

name = other_name || 'default'