v a r l e t i = ' a l p h a ' ; . . . f o r ( l e t i = 0 ; i < 1 0 ; i + + ) { . . . } / / i = = = ' a l p h a ' . . . i f ( i = = = ' a l p h a ' ) { l e t i = 1 0 ; . . . } / / i = = ' a l p h a ' caveat : no hoisting characteristics i f ( c o n d i t i o n ) { c o n s o l e . l o g ( v a l u e ) ; / / R e f e r e n c e E r r o r ! l e t v a l u e = " b l u e " ; }
be initialized at declaration c o n s t M A X _ I T E M S = 3 0 ; c o n s t N A M E ; / / S y n t a x e r r o r : m i s s i n g i n i t i a l i z a t i o n constants are block scoped i f ( c o n d i t i o n ) { c o n s t M A X _ I T E M S = 5 ; . . . }
o n r e n d e r ( o b j , o p t i o n s = g e t D e f a u l t s ( ) ) { / / e s 5 : o p t i o n s = o p t i o n s | | g e t D e f a u l t s ( ) ; . . . } . . . r e n d e r ( m y O b j ) ; / / - > r e n d e r ( m y O b j , g e t D e f a u l t R e n d e r O p t i o n s ( ) ) caveat : u n d e f i n e d will be replaced by default parameter f u n c t i o n r e n d e r ( o b j , o p t i o n s = g e t D e f a u l t s ( ) , c a l l b a c k ) { . . . } r e n d e r ( m y O b j , u n d e f i n e d , m y C a l l b a c k ) ; / / - > r e n d e r ( m y O b j , g e t D e f a u l t s ( ) , m y C a l l b a c k )
r r a y instead of a r g u m e n t s f u n c t i o n f o o ( f i r s t , . . . r e s t ) { r e t u r n r e s t . c o n c a t ( ' l a s t ' ) ; } c o n s o l e . l o g ( f o o ( 1 , 2 , 3 ) ) ; / / - > [ 2 , 3 , ' l a s t ' ] caveat : no other named arguments can follow in function declaration f u n c t i o n f o o ( f i r s t , . . . n u m b e r s , l a s t ) { / / - > S y n t a x e r r o r : C a n ' t h a v e a n a m e d p a r a m e t e r a f t e r r e s t p a r a m e t e r s
v = [ 2 5 , 5 0 ] , m a x ; m a x = M a t h . m a x ( . . . v ) ; / / e s 5 : M a t h . m a x ( v [ 0 ] , v [ 1 ] ) ; m a x = M a t h . m a x ( 1 7 , . . . v , 2 7 ) ; / / e s 5 : M a t h . m a x ( 1 7 , v [ 0 ] , v [ 1 ] , 2 7 ) ; a r r a y 2 . p u s h ( . . . v ) ; / / e s 5 : a r r a y 2 . p u s h ( v [ 0 ] , v [ 1 ] ) ; Tip : convert array like structures into a true A r r a y on the fly [ . . . d o c u m e n t . q u e r y S e l e c t o r A l l ( " d i v . b i g " ) ] . f o r E a c h ( / / j Q u e r y ( ' d i v . b i g ' ) e l m = > e l m . c l a s s L i s t . r e m o v e ( ' b i g ' ) / / . r e m o v e C l a s s ( ' b i g ' ) ) ;
n c t i o n c r e a t e P e r s o n ( n a m e , a g e ) { r e t u r n { n a m e , / / e s 5 : n a m e : n a m e , f a m i l y N a m e , / / e s 5 : f a m i l y N a m e : f a m i l y N a m e , f u l l N a m e ( ) { / / e s 5 : f u l l N a m e : f u n c t i o n ( ) { r e t u r n t h i s . n a m e + ' ' + t h i s . f a m i l y N a m e ; } } ; Compound property names v a r p r o p e r t y = " f o o " , v a l u e = " b a r " ; v a r o b j = { / / e s 5 : v a r o b j = { } ; [ p r o p e r t y + ' _ b a r ' ] : v a l u e / / o b j [ p r o p e r t y + ' _ b a r ' ] = v a l u e ; }
a y 's [ a , b ] = [ b , a ] ; / / e s 5 : l e t t m p = a ; a = b ; b = t m p ; l e t [ x , y ] = f ( ) ; / / - > f ( ) r e t u r n s a n a r r a y [ 2 ] c o n s o l e . l o g ( ' x = ' , x , ' , y = ' + y ) ; / / e s 5 : v a r m a t c h = / ( \ d + ) - ( d + ) - ( \ d + ) / . m a t c h ( ' 2 0 1 4 - 6 - 3 0 ' ) ; / / v a r y e a r = m a t c h [ 1 ] , m o n t h = m a t c h [ 2 ] , d a y = m a t c h [ 3 ] ; l e t [ , y e a r , m o n t h , d a y ] = / ( \ d + ) - ( \ d + ) - ( \ d + ) / . m a t c h ( ' 2 0 1 4 - 6 - 3 0 ' ) ; syntactic sugar for O b j e c t 's v a r { p r o t o c o l , h o s t n a m e , p o r t } = d o c u m e n t . l o c a t i o n ; / / d i r e c t m a p p i n g v a r { h o s t n a m e : s e r v e r , p o r t } = d o c u m e n t . l o c a t i o n ; / / m a p p e d t o { s e r v e r , p o r t } Wait - there's more fun ! l e t s o u r c e = { a : 1 , b : { c : 2 } } ; . . . l e t { a : x , b : { c : y } } = s o u r c e ; / / x = 1 , y = 2 . . . [ s o u r c e ] . f o r E a c h ( f u n c t i o n ( { a : x , b : { c : y } } ) { c o n s o l e . l o g ( x , y ) ; / / - > " 1 , 2 " } ) ;
1 0 1 Binary literals making use of octal/binary values less error prone / / E C M A S c r i p t 3 v a r n u m b e r = 0 7 1 ; / / 5 7 i n d e c i m a l v a r v a l u e 1 = p a r s e I n t ( " 7 1 " ) ; / / 7 1 v a r v a l u e 2 = p a r s e I n t ( " 0 7 1 " ) ; / / 5 7 / / E C M A S c r i p t 5 v a r n u m b e r = 0 7 1 ; / / 5 7 i n d e c i m a l v a r v a l u e 1 = p a r s e I n t ( " 7 1 " ) ; / / 7 1 v a r v a l u e 2 = p a r s e I n t ( " 0 7 1 " ) ; / / 7 1 v a r v a l u e 3 = p a r s e I n t ( " 0 7 1 " , 8 ) ; / / 5 7 / / E C M A S c r i p t 6 v a r v a l u e 1 = 0 o 7 1 ; / / 5 7 i n d e c i m a l v a r v a l u e 2 = 0 b 1 0 1 ; / / 5 i n d e c i m a l f u n c t i o n g e t V a l u e ( ) { " u s e s t r i c t " ; r e t u r n 0 7 1 ; / / s y n t a x e r r o r } caveat : parsing octal/binary values / / p a r s e I n t ( ) d o e s n ' t h a n d l e s t r i n g s t h a t l o o k l i k e o c t a l o r b i n a r y l i t e r a l s c o n s o l e . l o g ( p a r s e I n t ( " 0 o 7 1 " ) ) ; / / 0 c o n s o l e . l o g ( p a r s e I n t ( " 0 b 1 0 1 " ) ) ; / / 0 / / u s e t h e N u m b e r c o n s t r u c t o r i n s t e a d
g ( N u m b e r ( " 0 o 7 1 " ) ) ; / / 5 7 c o n s o l e . l o g ( N u m b e r ( " 0 b 1 0 1 " ) ) ; / / 5 Language enhancements ` h e l l o $ { n a m e } ` Template strings featuring multiline strings and expression substitution l e t u r l = " h t t p : / / e x a m p l e . c o m " ; l e t d e c o r a t i o n = { / / t a r g e t : " _ b l a n k " , l a b e l : " e x a m p l e " } ; l e t a n c h o r = ` < a u r l = " $ { u r l } " t a r g e t = " $ { d e c o r a t i o n . t a r g e t | | ' b l a n k ' } " > $ { d e c o r a t i o n . l a b e l } < / a > ` ; tags can be used for custom DSL building f u n c t i o n s a f e h t m l ( l i t e r a l s , . . . v a l u e s ) { / / p r o c e s s e v a l u a t e d l i t e r a l s a n d v a l u e s a n r e t u r n a s a f e h t m l e n c o d e d s t r i n g . . . } l e t a n c h o r = s a f e h t m l ` < a u r l = " $ { u r l } " t a r g e t = " $ { d e c o r a t i o n . t a r g e t | | ' b l a n k ' } " > $ { d e c o r a t i o n . l a b e l } < / a > ` ;
r y ( " b u t t o n " ) . o n ( " c l i c k " , e v e n t = > c o n s o l e . l o g ( e v e n t . t a r g e t ) ) ; l e t b a s e = ( ) = > d o c u m e n t . l o c a t i o n . h r e f ; / / n o a r g u m e n t s r e q u i r e p a r e n t h e s i s c o n s o l e . l o g ( b a s e ( ) ) ; l e t t o U p p e r = s = > s . t o U p p e r C a s e ( ) ; / / s i n g l e a r g u m e n t r e q u i r e d o e s n t r e q u i r e p a r e n t h e s i s c o n s o l e . l o g ( t o U p p e r ( " h u h u " ) ) ; / / " H U H U " v a r v a l u e s = [ 9 , 7 , 1 , 5 ] ; / / m u l t i p l e a r g u m e n t s r e q u i r e p a r e n t h e s i s v a l u e s . s o r t ( ( a , b ) = > a - b ) ; / / = > [ 1 , 5 , 7 , 9 ] ; v a l u e s . s o r t ( ( a , b ) = > { / / t r a d i t i o n a l s t y l e d a r r o w f u n c t i o n w i t h r e t u r n s t a t e m e n t r e t u r n a - b ; } ) ; *Implicit t h i s binding to definition scope. c a l l ( ) , a p p l y ( ) , b i n d ( ) will have no effect. Arrow functions cannot be used with n e w .
f . . . ) statement f o r ( l e t v o f [ 1 , 3 , 5 , 7 ] ) { c o n s o l e . l o g ( v ) ; / / - > p r i n t s o u t a r r a y v a l u e s } I t e r a t o r /I t e r a b l e support f o r ( l e t [ i n d e x , v a l u e ] o f [ " a " , " b " , " c " ] . e n t r i e s ( ) ) { . . . } A r r a y comprehension l e t s q u a r e d = [ f o r ( x o f [ 1 , 2 , 3 ) x * x ] ; / / - > [ 1 , 4 , 9 ] l e t s q u a r e d O d d = [ f o r ( x o f a r r ) i f ( x % 2 ) x * x ] ; / / - > [ 1 , 9 ] l e t v = [ f o r ( [ i n d e x , v a l u e ] o f [ " a " , " b " , " c " ] . e n t r i e s ( ) ) [ v a l u e . t o U p p e r C a s e ( ) , i n d e x ] ] ; / / v = [ [ " A " , 0 ] , [ " B " , 1 ] , [ " C " , 2 ] ]
* Generator A Generator is a function y i e l d ing an unlimited amount of values wrapped in a automagic I t e r a t o r instance. f u n c t i o n * g e n e r a t e ( ) { l e t i = 0 ; w h i l e ( i < 5 ) { y i e l d i + + ; } } / / u s i n g f o r . . . o f s t a t e m e n t f o r ( l e t v o f g e n e r a t e ( ) ) { c o n s o l e . l o g ( v ) ; / / - > 0 , 1 , 2 , 3 , 4 } / / u s i n g w h i l e s t a t e m e n t l e t i t e r = g e n e r a t e ( ) , n e x t ; w h i l e ( ! ( n e x t = i t e r . n e x t ( ) ) . d o n e ) { c o n s o l e . l o g ( n e x t . v a l u e ) ; / / - > 0 , 1 , 2 , 3 , 4 } Caveat r e t u r n "closes" the iterator returned by a generator function. The return expression value will be ignored.
* Generator y i e l d * delegates control to another G e n e r a t o r f u n c t i o n * w a l k ( n o d e ) { f o r ( l e t c h i l d o f n o d e . c h i l d r e n ( ) ) { y i e l d * w a l k ( c h i l d ) ; } y i e l d n o d e ; } f o r ( l e t n o f w a l k ( r o o t ) ) { c o n s o l e . l o g ( n ) ; }
* Generator Passing arguments between consumer and I t e r a t o r around f u n c t i o n * s u m m a r i z e ( s u m = 0 ) { w h i l e ( t r u e ) { s u m + = y i e l d s u m ; c o n s o l e . l o g ( s u m ) ; } } l e t t o t a l = s u m m a r i z e ( 1 0 ) ; t o t a l . n e x t ( ) ; f o r ( l e t v o f [ 1 , 3 , 6 ] ) { t o t a l . n e x t ( v ) ; } / / " 1 1 " / / " 1 4 " / / " 2 0 "
* Generator Bonus : use y i e l d * to execute async functions sequential ! f u n c t i o n r u n ( g e n ) { v a r i t e r = g e n ( m s g = > i t e r . n e x t ( m s g . t o U p p e r C a s e ( ) ) ) ; i t e r . n e x t ( ) ; / / r e s u m e ( a k a s t a r t ) } r u n ( f u n c t i o n * ( r e s u m e ) { c o n s o l e . l o g ( " b e f o r e " ) ; c o n s o l e . l o g ( y i e l d s e t T i m e o u t ( ( ) = > r e s u m e ( " h e l l o " ) , 1 0 0 0 ) ) ; / / s u s p e n d c o n s o l e . l o g ( y i e l d s e t T i m e o u t ( ( ) = > r e s u m e ( " w o r l d " ) , 1 0 0 0 ) ) ; / / s u s p e n d c o n s o l e . l o g ( " a f t e r " ) ; } ) ; / / " b e f o r e " / / " H E L L O " / / " W O R L D " / / " a f t e r "
a s s P o s i t i o n { c o n s t r u c t o r ( x , y ) { t h i s . d a t a = { x , y } ; } s e t x ( x ) { t h i s . d a t a . x = x ; } g e t x ( ) { r e t u r n t h i s . d a t a . x ; } t o S t r i n g ( ) { r e t u r n ` P o s i t i o n ( = $ { t h i s . x } , $ { t h i s . d a t a . y } ) ` ; } . . . } l e t p = n e w P o s i t i o n ( 0 , 2 0 ) ; p . x = 1 5 ; c o n s o l e . l o g ( ` p i s $ { p } ` ) ; / / " p i s P o s i t i o n ( = 1 5 , 2 0 ) " . . .
s S h a p e e x t e n d s P o s i t i o n { c o n s t r u c t o r ( x , y , w i d t h , h e i g h t ) { s u p e r ( x , y ) ; O b j e c t . a s s i g n ( t h i s . d a t a , { w i d t h , h e i g h t } ) ; } g e t d i m e n s i o n ( ) { r e t u r n { w i d t h : t h i s . d a t a . w i d t h , h e i g h t : t h i s . d a t a . h e i g h t } ; } t o S t r i n g ( ) { r e t u r n ` S h a p e ( = $ { s u p e r . t o S t r i n g ( ) } , $ { t h i s . d i m e n s i o n . w i d t h } , $ { t h i s . d i m e n s i o n . h e i g h t } ) ` ; } } l e t s = n e w S h a p e ( 0 , 2 0 , 1 0 0 , 7 0 } ) ; s . x = 1 5 ; s . w i d t h = 5 0 ; c o n s o l e . l o g ( ` s i s $ { s } ` ) ; / / " s i s S h a p e ( = P o s i t i o n ( = 1 5 , 2 0 ) , 5 0 , 7 0 ) " c o n s o l e . l o g ( s i n s t a n c e o f P o s i t i o n , s i n s t a n c e o f S h a p e ) ; / / " t r u e t r u e " Inheritance, s u p e r access
[ ] ; a r r [ 3 ] = " f o o " ; c o n s o l e . l o g ( a r r . l e n g t h ) ; / / " 4 " c l a s s M y A r r a y e x t e n d s A r r a y { c o n s t r u c t o r ( . . . a r g s ) { s u p e r ( . . . a r g s ) ; } } a r r = n e w M y A r r a y ( ) ; a r r [ 3 ] = " f o o " ; c o n s o l e . l o g ( a r r . l e n g t h ) ; / / E S 5 : " 0 " / / E S 6 : " 4 " ... true native JS Object inheritance
m a n { c o n s t r u c t o r ( n a m e ) { t h i s . n a m e = n a m e ; } h i ( ) { r e t u r n ` H e l l o , I a m $ { t h i s . n a m e } . ` ; } } c l a s s E m p l o y e e e x t e n d s H u m a n { t o S t r i n g ( ) { r e t u r n ` e m p l o y e e ( = $ { t h i s . n a m e } ) ` ; } } c l a s s M a n a g e r e x t e n d s E m p l o y e e { t o S t r i n g ( ) { r e t u r n ` m a n a g e r ( = $ { t h i s . n a m e } ) ` ; } } l e t m i k e = n e w E m p l o y e e ( " m i k e " ) ; c o n s o l e . l o g ( ` $ { m i k e } : " $ { m i k e . h i ( ) } " ` , m i k e i n s t a n c e o f M a n a g e r , m i k e i n s t a n c e o f E m p l o y e e ) ; / / ' e m p l o y e e ( = m i k e ) : " H e l l o , I a m m i k e . " f a l s e t r u e ' O b j e c t . s e t P r o t o t y p e O f ( m i k e , n e w M a n a g e r ( ) ) ; c o n s o l e . l o g ( ` $ { m i k e } : " $ { m i k e . h i ( ) } " ` , m i k e i n s t a n c e o f M a n a g e r , m i k e i n s t a n c e o f E m p l o y e e ) ; / / ' m a n a g e r ( = m i k e ) : " H e l l o , I a m m i k e . " t r u e t r u e ' ... O b j e c t . s e t P r o t o t y p e O f relink objects after creation via prototype chain
b j e c t . i s ( l , r ) fixes some inconsistences of JS equality. Values are considered as "equivalent". O b j e c t . i s ( + 0 , - 0 ) ; / / f a l s e O b j e c t . i s ( N a N , N a N ) ; / / t r u e O b j e c t . i s ( 5 , 5 ) ; / / t r u e O b j e c t . i s ( 5 , " 5 " ) ; / / f a l s e O b j e c t . a s s i g n ( t a r g e t , . . . s o u r c e s ) copies all direct properties (i.e. not inherited properties)into the target object. l e t b a s e = { a : 1 } ; l e t s o u r c e 1 = { b : 2 , c : 3 } ; O b j e c t . s e t P r o t o t y p e O f ( s o u r c e 1 , b a s e ) ; l e t s o u r c e 2 = { c : 4 , d : 5 } ; l e t t a r g e t = { a : - 1 , b : - 2 } ; O b j e c t . a s s i g n ( t a r g e t , s o u r c e 1 , s o u r c e 2 ) ; c o n s o l e . l o g ( t a r g e t ) ; / / " O b j e c t { a : - 1 , b : 2 , c : 4 , d : 5 } "
u m b e r . i s F i n i t e ( a r g ) always return false when passed a non-number value i s F i n i t e ( 2 5 ) ; / / t r u e i s F i n i t e ( " 2 5 " ) ; / / t r u e / / n e w i n E S 6 ! N u m b e r . i s F i n i t e ( 2 5 ) ; / / t r u e N u m b e r . i s F i n i t e ( " 2 5 " ) ; / / f a l s e N u m b e r . i s N a N ( a r g ) always return false when passed a non-number value i s N a N ( N a N ) ; / / t r u e i s N a N ( " N a N " ) ; / / t r u e N u m b e r . i s N a N ( N a N ) ; / / t r u e N u m b e r . i s N a N ( " N a N " ) ; / / f a l s e N u m b e r . i s I n t e g e r ( a r g ) N u m b e r . i s I n t e g e r ( 2 5 ) ; / / t r u e N u m b e r . i s I n t e g e r ( 2 5 . 0 ) ; / / t r u e N u m b e r . i s I n t e g e r ( 2 5 . 1 ) ; / / f a l s e Safe integers are numbers between -253 and 253. Outside of this "safe" range, binary representations end up reused for multiple numeric values. l e t s a f e = N u m b e r . M A X _ S A F E _ I N T E G E R , u n s a f e = s a f e + 1 ;
g ( N u m b e r . i s I n t e g e r ( s a f e ) , N u m b e r . i s S a f e I n t e g e r ( s a f e ) ) ; / / " t r u e , t r u e " c o n s o l e . l o g ( N u m b e r . i s I n t e g e r ( u n s a f e ) , N u m b e r . i s S a f e I n t e g e r ( u n s a f e ) ) ; / / " t r u e , f a l s e " N u m b e r . p a r s e I n t ( a r g ) and N u m b e r . p a r s e I n t ( a r g ) behave exactly the same as the global functions of the same name. Class enhancements M a t h .* M a t h . a c o s h ( a r g ) Returns the inverse hyperbolic cosine of x. M a t h . a s i n h ( a r g ) Returns the inverse hyperbolic sine of x. M a t h . a t a n h ( a r g ) Returns the inverse hyperbolic tangent of x M a t h . c b r t ( a r g ) Returns the cubed root of x. M a t h . c l z 3 2 ( a r g ) Returns the number of leading zero bits in the 32-bit integer representation of x. M a t h . c o s h ( a r g ) Returns the hyperbolic cosine of x. M a t h . e x p m 1 ( a r g ) Returns the result of subtracting 1 from the exponential function of x M a t h . f r o u n d ( a r g ) Returns the nearest single-precision float of x. M a t h . h y p o t ( . . . a r g s ) Returns the square root of the sum of the squares of each argument. M a t h . i m u l ( x , y ) Returns the result of performing true 32-bit multiplication of the two arguments. M a t h . l o g 1 p ( a r g ) Returns the natural logarithm of 1 + x. M a t h . l o g 1 0 ( a r g ) Returns the base 10 logarithm of x. M a t h . l o g 2 ( a r g ) Returns the base 2 logarithm of x. M a t h . s i g n ( a r g ) Returns -1 if the x is negative 0 if x is +0 or -0, or 1 if x is positive. M a t h . s i n h ( a r g ) Returns the hyperbolic sine of x. M a t h . t a n h ( a r g ) Returns the hyperbolic tangent of x. M a t h . t r u n c ( a r g ) Removes fraction digits from a float and returns an integer.
t r i n g . s t a r t s W i t h ( s t r , o f s ? ) v a r m s g = " H e l l o w o r l d ! " ; m s g . s t a r t s W i t h ( " H e l l o " ) ; / / t r u e m s g . s t a r t s W i t h ( " o " ) ; / / f a l s e m s g . s t a r t s W i t h ( " o " , 4 ) ; / / t r u e S t r i n g . c o n t a i n s ( s t r , o f s ? ) v a r m s g = " H e l l o w o r l d ! " ; m s g . c o n t a i n s ( " o " ) ; / / t r u e m s g . c o n t a i n s ( " x " ) ; / / f a l s e m s g . c o n t a i n s ( " o " , 8 ) ; / / f a l s e S t r i n g . e n d s W i t h ( n t i m e s ) v a r m s g = " H e l l o w o r l d ! " ; m s g . e n d s W i t h ( " ! " ) ; / / t r u e m s g . e n d s W i t h ( " w o r l d ! " ) / / t r u e m s g . e n d s W i t h ( " o " , 8 ) ; / / f a l s e S t r i n g . r e p e a t ( n t i m e s ) " # " . r e p e a t ( 5 ) ; / / " # # # # # "
r a y . f i n d ( c a l l b a c k , t h i s A r g ? ) , A r r a y . f i n d I n d e x ( c a l l b a c k , t h i s A r g ? ) [ ' h e l l o ' , ' w o r l d ' , ' ! ' ] . f i n d ( s = > s . s t a r t s W i t h ( ' w ' ) ) ; / / " w o r l d " [ ' h e l l o ' , ' w o r l d ' , ' ! ' ] . f i n d ( s = > s . s t a r t s W i t h ( ' k ' ) ) ; / / u n d e f i n e d [ ' h e l l o ' , ' w o r l d ' , ' ! ' ] . f i n d I n d e x ( s = > s . s t a r t s W i t h ( ' w ' ) ) ; / / 1 [ ' h e l l o ' , ' w o r l d ' , ' ! ' ] . f i n d I n d e x ( s = > s . s t a r t s W i t h ( ' k ' ) ) ; / / - 1 A r r a y . f r o m ( a r r a y L i k e , m a p F u n c ? , t h i s A r g ? ) converts array like or I t e r a t o r objects into an A r r a y l e t a r r = A r r a y . f r o m ( d o c u m e n t . q u e r y S e l e c t o r A l l ( ' p ' ) ) ; / / l e t a r r = [ . . . d o c u m e n t . q u e r y S e l e c t o r A l l ( ' p ' ) ] ; a s s h o r t a l t e r n a t i v e A r r a y . o f ( . . . a r g s ) is an less error prone alternative to the A r r a y constructor / / b e f o r e E S 6 n e w A r r a y ( 3 , 2 , 1 ) ; / / - > [ 3 , 2 , 1 ] n e w A r r a y ( 3 ) ; / / - > [ , , ] / / E S 6 a r r = A r r a y . o f ( 1 , 2 , 3 ) ; / / - > [ 1 , 2 , 3 ] a r r = A r r a y . o f ( 3 ) ; / / - > [ 3 ] A r r a y . e n t r i e s ( ) , A r r a y . k e y s ( ) , A r r a y . v a l u e s ( ) return I t e r a t o r 's A r r a y . f i l l ( v a l u e , s t a r t = 0 , e n d = t h i s . l e n g t h ) fills all the elements of an array from a start index to an end index with
t o r interface I t e r a t o r exposes a n e x t ( ) function . . . n e x t ( ) { . . . r e t u r n { v a l u e : { . . . } , d o n e : t r u e ; / / o r f a l s e f o r f i n i s h e d } ; } I t e r a t o r is a generic interface for object iteration using f o r ( . . o f . . ) loop l e t s e t = n e w S e t ( [ " f o o " , " b a r " ] ) ; f o r ( l e t i t e m o f s e t . e n t r i e s ( ) ) { . . . } . . . l e t m a p = n e w M a p ( [ [ " f o o " , " 1 " ] , [ " b a r " , " 2 " ] ] ) ; f o r ( l e t i t e m o f m a p . e n t r i e s ( ) ) { . . . }
interface Any class/object having a [ S y m b o l . i t e r a t o r ] property can be iterated using f o r ( . . o f . . ) loop l e t o b j = { * [ S y m b o l . i t e r a t o r ] ( ) { y i e l d 1 ; y i e l d 2 ; y i e l d 3 ; } } ; f o r ( l e t i o f o b j ) { c o n s o l e . l o g ( i ) ; } 1 2 3
e w M a p ( [ [ 1 , ' h u h u ' ] , [ ' 1 ' : ' e i n s ' ] , [ ' 1 . 0 ' : ' e i n s k o m m a n u l l ' ] ] ) ; c o n s o l e . l o g ( m . s i z e ) ; / / " 3 " m . s e t ( 1 . 0 , ' e i n s - k o m m a - n u l l ' ) ; / / - > m . s i z e = = = 4 M a p The M a p object is a true key/value map. Any value (both objects and primitive values) may be used as key. S e t The S e t object lets you store unique values of any type, whether primitive values or object references. v a r s = n e w S e t ( [ 1 , ' 1 ' ] ) ; c o n s o l e . l o g ( m . s i z e ) ; / / " 2 " s . d e l e t e ( 1 ) ; / / m . s i z e = = = 1 W e a k M a p , W e a k S e t W e a k M a p keys and W e a k S e t values are collections of objects only and not of arbitrary values of any type. If there is no other reference to an object stored in the W e a k S e t (i.e the key in a W e a k M a p ), they can be garbage collected. W e a k M a p and W e a k S e t are not iterable for that reason.
m b o l is a primitive type originally invented to create private object members. l e t f o o = S y m b o l ( ) , f o o 2 = S y m b o l ( " f o o " ) ; c o n s o l e . l o g ( f o o , f o o 2 ) ; / / S y m b o l ( ) S y m b o l ( f o o ) l e t o b j = { [ f o o ] : ' b a r ' , [ f o o 2 ] : ' b a r 2 ' , ' f o o ' : ' _ b a r ' } ; c o n s o l e . l o g ( O b j e c t . k e y s ( o b j ) ) ; / / [ " f o o " ] c o n s o l e . l o g ( O b j e c t . g e t O w n P r o p e r t y N a m e s ( o b j ) ) ; / / [ " f o o " ] c o n s o l e . l o g ( O b j e c t . g e t O w n P r o p e r t y S y m b o l s ( o b j ) ) ; / / [ S y m b o l V a l u e , S y m b o l V a l u e ] c o n s o l e . l o g ( o b j [ f o o ] ) ; / / " b a r " Caveats : Symbol object members will not be enumerated as object keys. Symbols are primitive values, new Symbol() throws an error when called. Shared symbols S y m b o l 's can be shared in a global registry by get/create them using S y m b o l . f o r ( n a m e ) and looked up using S y m b o l . k e y F o r ( n a m e ) l e t f o o = S y m b o l . f o r ( " f o o " ) ; / / s y m b o l ( f o o ) c o n s o l e . l o g ( S y m b o l . k e y F o r ( f o o ) ) ; / / " f o o " l e t f o o 2 = S y m b o l . f o r ( " f o o " ) ; / / s y m b o l ( f o o ) c o n s o l e . l o g ( f o o = = = f o o 2 ) ; / / t r u e
m b o l ( " f o o " ) ; / / u n s h a r e d s y m b o l c o n s o l e . l o g ( S y m b o l . k e y F o r ( f o o 3 ) ) ; / / u n d e f i n e d Additional classes P r o m i s e A promise represents the eventual result of an asynchronous operation. P r o m i s e is a builtin class implementing the Promises/A+ specification. v a r p r o m i s e = n e w P r o m i s e ( ( r e s o l v e , r e j e c t ) = > { / / d o s o m e t h i n g , p o s s i b l y a s y n c . . . i f ( / * e v e r y t h i n g t u r n e d o u t f i n e * / ) r e s o l v e ( " S t u f f w o r k e d ! " ) ; e l s e r e j e c t ( n e w E r r o r ( " I t b r o k e " ) ) ; } ) ; . . . p r o m i s e . t h e n ( r e s u l t = > c o n s o l e . l o g ( r e s u l t ) , e r r = > c o n s o l e . l o g ( e r r ) ) ; P r o m i s e s can be chained together. f u n c t i o n l o a d C h a p t e r ( c h a p t e r ) r e t u r n P r o m i s e . r e s o l v e ( $ . g e t ( ' m e t a d a t a ' ) ) . t h e n ( j q X H R = > P r o m i s e . r e s o l v e ( $ . g e t ( ' c h a p t e r s / ' + J S O N . p a r s e ( j q X H R . r e s p o n s e T e x t ) . u r l s [ c h a p t e r - 1 ] ) ) } ) . t h e n ( j q X H R = > J S O N . p a r s e ( j q X H R . r e s p o n s e T e x t ) ) . c o n t e n t s ) ) ; . . . l o a d C h a p t e r ( 4 ) . t h e n ( c o n t e n t s = > d o c u m e n t . b o d y . i n n e r H T M L = c o n t e n t s , e r r = > a l e r t ( ` F a i l e d t o r e t r i e v e c h a p t e r $ { c h a p t e r } : $ { e r r . m e s s a g e } ` ) ) ;
r o m i s e . r e s o l v e ( t h e n a b l e ) can cast anything promise-like (or thenable in promise-speak) into a native Promise object ( j Q u e r y . D e f e r r e d ( ) , Angular Deferred $ q . d e f e r ( ) etc.) v a r j s P r o m i s e = P r o m i s e . r e s o l v e ( j Q u e r y . a j a x ( ' / w h a t e v e r . j s o n ' ) ) ; Caveat : Only the first resolve/reject argument of casted promise-like objects will be available in resolve/reject callbacks. P r o m i s e . r e s o l v e ( o b j ) ; creates a promise that fulfills to obj. P r o m i s e . r e s o l v e ( { m s g : " I s u c c e e d e d ! " } ) . t h e n ( o b j = > c o n s o l e . l o g ( o b j . m s g ) ) ; / / " i s u c c e e d e d ! " P r o m i s e . r e j e c t ( o b j ) ; returns a promise that rejects to obj (obj should be an instanceof Error). P r o m i s e . r e j e c t ( n e w E r r o r ( " I f a i l e d : - ( " ) . t h e n ( m s g = > c o n s o l e . l o g ( " s u c c e s s ! " ) , e r r o r = > c o n s o l e . l o g ( ` S o r r y - $ { e r r o r . m e s s a g e } ` ) ) ; / / " S o r r y - I f a i l e d : - ( P r o m i s e . a l l ( a r r a y ) ; creates a promise that fulfills when every item in the array fulfills, and rejects if (and when) any item rejects. P r o m i s e . a l l ( [ $ . g e t ( " f o o . j s o n " ) , $ . g e t ( " b a r . j s o n " ) , $ . g e t ( " s u m m a r y . j s o n " ) ] ) . t h e n ( ( [ f o o X H R , b a r X H R , s u m m a r y X H R ] ) = > c o n s o l e . l o g ( f o o . r e s p o n s e T e x t , b a r . r e s p o n s e T e x t , s u m m a r y . r e s p o n s e T e x t ) , e r r = > c o n s o l e . l o g ( ` F a i l e d r e c e i v i n g $ { e r r . m e s s a g e } ` ) ) ; / / o u t p u t o f d a t a f r o m a l l a j a x r e q u e s t s
c e ( a r r a y ) ; return a Promise that fulfills as soon as any item fulfills, or rejects as soon as any item rejects, whichever happens f Additional classes P r o x y P r o x y 's let you track/modify/intercept almost any operation on a JS object. f u n c t i o n c r e a t e T r a c k e r ( o b j ) { v a r t o u c h e d = n e w S e t ( ) ; v a r p r o x y = P r o x y ( t a r g e t , { s e t ( t a r g e t , p r o p e r t y , v a l u e ) { t o u c h e d . a d d ( p r o p e r t y ) ; t a r g e t [ p r o p e r t y ] = v a l u e ; } } ) ; r e t u r n [ p r o x y , t o u c h e d ] ; } . . . v a r m o d e l { i d : 0 8 1 5 , n a m e : " J o h n " , ' g e n d e r : m a l e ' } ; v a r [ f a c a d e , t o u c h e d ] = c r e a t e T r a c k e r ( m o d e l ) ; f a c a d e . n a m e = " J o h n D o e " ; / / t o u c h e d = [ " n a m e " ] / / m o d e l . n a m e = " J o h n D o e " ; . . . i f ( t o u c h e d . s i z e ) { b a c k e n d . s a v e ( m o d e l , t o u c h e d ) ; / / s a v e b a c k a l l c h a n g e d p r o p e r t i e s } P r o x y supports many more handler like s e t , d e l e t e etc. for intercepting any possible modification of a JS object. See here for more.
with explicit e x p o r t s, i m p o r t specific exported names from those modules, and keep these names sepa / / l i b / f o o . j s e x p o r t d e f a u l t c l a s s F o o { . . . } ; e x p o r t c o n s t V E R S I O N = ' 1 . 0 ' ; e x p o r t v a r u t i l = { e s c a p e ( s ) { . . . } } ; / / l i b / b a r . j s e x p o r t f u n c t i o n b a r ( ) { . . . } ; e x p o r t c o n s t V E R S I O N = ' 1 . 2 ' ; i m p o r t F o o f r o m ' . / l i b / f o o ' ; / / i m p o r t d e f a u l t e x p o r t i m p o r t { u t i l , V E R S I O N a s U T I L _ V E R S I O N } f r o m ' . / l i b / f o o ' ; / / i m p o r t u t i l a s u t i l , V E R S I O N a s U T I L _ V E R S I i m p o r t { b a r , V E R S I O N a s B A R _ V E R S I O N } f r o m ' . / l i b / b a r ' ; c o n s o l e . l o g ( U T I L _ V E R S I O N , B A R _ V E R S I O N ) ; / / " 1 . 0 1 . 2 " u t i l . e s c a p e ( " h e l l o w o r l d " ) ;
* ) ES6 defines a module loader interface for loading scripts dynamically (in the browser) < s c r i p t > P r o m i s e . a l l ( [ S y s t e m . i m p o r t ( ' . / l i b / f o o ' ) , S y s t e m . i m p o r t ( ' . / l i b / b a r ' ) ] ) . t h e n ( f u n c t i o n ( [ f o o , b a r ] ) { v a r o b j = n e w f o o . F o o ( ) ; c o n s o l e . l o g ( f o o . V E R S I O N , b a r . V E R S I O N ) ; / / " 1 . 0 1 . 2 " f o o . u t i l . e s c a p e ( " h e l l o w o r l d " ) ; } ) . c a t c h ( e r r ) { t h r o w e ; } ) ; < / s c r i p t >
. o b s e r v e ( c h a n g e s ) lets you listen to changes on a object. Will speed up libraries like AngularJS, EmberJS or Backbone massively. v a r o b j = { } ; O b j e c t . o b s e r v e ( o b j , c h a n g e s = > { c h a n g e s . f o r E a c h ( c h a n g e = > { / / d o s t u f f w i t h c h a n g e . c o n s o l e . l o g ( c h a n g e . t y p e , c h a n g e . n a m e , c h a n g e . o l d V a l u e , c h a n g e . o b j e c t [ c h a n g e . n a m e ] ) ; } ) ; } ) ; . . . o b j . f o o = 1 ; / / " a d d f o o u n d e f i n e d 1 " o b j . f o o = 5 ; / / " u p d a t e f o o 1 5 " d e l e t e o b j . f o o ; / / " d e l e t e f o o 5 " a s y n c / a w a i t is syntactic sugar for working with P r o m i s e objects a s y n c f u n c t i o n l o a d C h a p t e r ( c h a p t e r ) { v a r m e t a d a t a = a w a i t $ . g e t ( ' m e t a d a t a ' ) ; v a r c h a p t e r = a w a i t $ . g e t ( ' c h a p t e r s / ' + J S O N . p a r s e ( m e t a d a t a . r e s p o n s e T e x t ) . u r l s [ c h a p t e r - 1 ] ) ; r e t u r n J S O N . p a r s e ( c h a p t e r . r e s p o n s e T e x t ) ) . c o n t e n t s ; } . . . ( a s y n c f u n c t i o n ( ) { t r y { c o n s o l e . l o g ( a w a i t l o a d C h a p t e r ( 4 ) ) ; } c a t c h ( e r r ) { c o n s o l e . l o g ( e r r . m e s s a g e ) ; } } ) ( ) ; / / s a m e e f f e c t a s I I F E a b o v e
r ( 4 ) . t h e n ( c h a p t e r = > c o n s o l e . l o g ( c h a p t e r ) , e r r = > c o n s o l e . l o g ( e r r . m e s s a g e ) ) ; What's next ? ES7 Type support f u n c t i o n i n d e n t ( t e x t : s t r i n g , c o u n t : n u m b e r = 4 , i n d e n t : s t r i n g = ' ' ) : s t r i n g { r e t u r n i n d e n t . r e p e a t ( c o u n t ) . c o n c a t ( t e x t ) ; } c o n s o l e . l o g ( " h u h u " ) ; / / " h u h u " c o n s o l e . l o g ( i n d e n t ( { } ) ) ; / / " E r r o r : I n v a l i d a r g u m e n t s g i v e n ! / / - 1 s t a r g u m e n t h a s t o b e a n i n s t a n c e o f s t r i n g , g o t { } " c o n s o l e . l o g ( i n d e n t ( " h u h u " , " 2 " ) ) ; / / " E r r o r : I n v a l i d a r g u m e n t s g i v e n ! / / - 2 n d a r g u m e n t h a s t o b e a n i n s t a n c e o f n u m b e r , g o t " 2 " Annotations will bring transparent Dependency Injection / / c o f f e e _ m a k e r . j s i m p o r t { I n j e c t } f r o m ' d i / a n n o t a t i o n s ' ; i m p o r t { H e a t e r } f r o m ' . / h e a t e r ' ; @ I n j e c t ( H e a t e r ) e x p o r t c l a s s C o f f e e M a k e r { c o n s t r u c t o r ( h e a t e r ) { h e a t e r . s t a r t ( ) ; } } / / - > [ n e w I n j e c t ( h e a d e r ) ] = = C o f f e e M a k e r . a n n o t a t i o n s ; / / m a i n . j s i m p o r t { I n j e c t o r } f r o m ' d i / i n j e c t o r ' ; i m p o r t { C o f f e e M a k e r } f r o m ' . / c o f f e e _ m a k e r ' ; f u n c t i o n m a i n ( ) { v a r c o f f e e M a k e r = ( n e w I n j e c t o r ( ) ) . g e t ( C o f f e e M a k e r ) ;
supports ES6 partially (see ECMAScript 6 compatibility table) NodeJS supports many ES6 features (- - h a r m o n y flag) es6-shim provides ES6 API for legacy JavaScript engines Native supported ES6 features is growing monthly
Traceur is a JavaScript.next-to-JavaScript-of-today compiler that allows you to use features from the future today. Traceur transpiles most* ES6 + some of ES7 features into today's Javascript (ES5). * except of non shimmable/transpilable features like P r o x y , O b j e c t . o b s e r v e ( ) , new R e g E x p flags ...
be used as commandline tool, Grunt task or GulpJS task : v a r g u l p = r e q u i r e ( ' g u l p ' ) , t r a c e u r = r e q u i r e ( ' g u l p - t r a c e u r ' ) ; g u l p . t a s k ( ' d e f a u l t ' , f u n c t i o n ( ) { r e t u r n g u l p . s r c ( ' s r c / a p p . j s ' ) . p i p e ( t r a c e u r ( { s o u r c e M a p : t r u e } ) ) . p i p e ( g u l p . d e s t ( ' d i s t ' ) ) ; } ) ; supports AMD, CommonJS, SystemJS or all-in-one ES5 code as output Sourcemap support for easy debugging provides new ES6 API's (S e t , M a p , P r o m i s e etc.) can be used to compile ES6 on the fly Testing ES6 today is easy using Mocha/Jasmine and Karma/Protractor : i m p o r t N e s t e d M a p f r o m " . . / . . / s r c / c o r e / n e s t e d m a p " ; d e s c r i b e ( " N e s t e d M a p " , ( ) = > { i t ( " c o n s t r u c t o r " , ( ) = > { l e t n m = n e w N e s t e d M a p ( ) ; e x p e c t ( n m . p a r e n t ) . t o B e N u l l ( ) ; l e t _ n m = n e w N e s t e d M a p ( n m ) ; e x p e c t ( _ n m . p a r e n t ) . t o B e ( n m ) ; } ) ; } ) ;
s t a l l t r a c e u r make Traceur the default JS loader for NodeJS / / t e s t . e s 6 i m p o r t { b } f r o m ' . / r e s o u r c e s / b ' ; c o n s o l e . l o g ( b ) ; / / r e s o u r c e s / b . j s e x p o r t v a r b = ' B B B ' ; / / a p p . j s v a r t r a c e u r = r e q u i r e ( ' t r a c e u r ' ) ; t r a c e u r . r e q u i r e . m a k e D e f a u l t ( f u n c t i o n ( f i l e n a m e ) { r e t u r n f i l e n a m e . e n d s W i t h ( ' . e s 6 ' ) ; } ) ; r e q u i r e ( ' . / t e s t . e s 6 ' ) ;
i p t s r c = " h t t p s : / / t r a c e u r - c o m p i l e r . g o o g l e c o d e . c o m / g i t / b i n / t r a c e u r . j s " t y p e = " t e x t / j a v a s c r i p t " > < / s c r i p t > < s c r i p t s r c = " h t t p s : / / t r a c e u r - c o m p i l e r . g o o g l e c o d e . c o m / g i t / s r c / b o o t s t r a p . j s " t y p e = " t e x t / j a v a s c r i p t " > < / s c r i p t < s c r i p t > t r a c e u r . o p t i o n s . e x p e r i m e n t a l = t r u e ; < / s c r i p t > < s c r i p t t y p e = " m o d u l e " > c l a s s G r e e t e r { c o n s t r u c t o r ( m e s s a g e ) { t h i s . m e s s a g e = m e s s a g e ; } g r e e t ( ) { l e t e l e m e n t = d o c u m e n t . q u e r y S e l e c t o r ( ' # m e s s a g e ' ) ; e l e m e n t . i n n e r H T M L = t h i s . m e s s a g e ; } } ; l e t g r e e t e r = n e w G r e e t e r ( ' H e l l o , w o r l d ! ' ) ; g r e e t e r . g r e e t ( ) ; < / s c r i p t > or using already transpiled files : < s c r i p t s r c = " b i n / t r a c e u r - r u n t i m e . j s " > < / s c r i p t > < s c r i p t s r c = " o u t / g r e e t e r . j s " > < / s c r i p t >
vanilla code much improved readability will result in better productivity less legacy dependencies (like jQuery, Underscore etc.) making your code more future proof less code rewrites over the years huge performance speedup when ES6 will be arrived natively at browsers ES6 modules simplify larger applications sourece code management ES6 adaption in browsers and NodeJS is increasing monthly CONTRA using ES6 today in legacy JS engines makes your toolchain more complicated Traceur is just a workaround : transpiled code is larger and slower than it's ES6 source. Native ES6 language features will not be used in generated code ES6 is a huge step requiring learning to use new language features the right way
My advise If your toolchain is well done (GulpJS, gulp-watch, Traceur, Jasmine/Protractor for testing) ES6 is a pleasure to use. Learning curve is steep. Traceur is getting very stable, output will be much faster. ES6 will be adapted by most browsers until 2016 - including Internet Explorer. NodeJS will support ES6 mostly in 2014. ES6 features will be adapted very fast by popular NodeJS modules (see Koa as an good ES6 example) because of its massive simplifying async features. Modern browser libraries like AngularJS 2.0 will be based on ES6 (not a guess - reality !) Start learning ! Use it in non critical/inhouse applications/modules to get experienced. Be prepared when everybody supports ES6.
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