Rest Spread

Rest Spread




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Rest Spread
console . log ( multiply ( 1 , 2 , 3 , 4 , 5 )) ;
ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”
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function multiplyAll ( ... args ) {
ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”
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function welcomeSite ( siteName , bookName , ... bookTitles ) {
 console . log ( ' Welcome to ' + siteName + 'โ€™s ' + bookName + ' book' ) ; // Welcome to W3Docsโ€™s JS book
 // the rest go into bookTitles array
 console . log ( bookTitles [ 0 ]) ; // Arrays
 console . log ( bookTitles [ 1 ]) ; // Functions
 console . log ( bookTitles . length ) ; // 2
ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”
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function f(arg1, ...rest, arg2) { // arg2 after ...rest!!!!
 // error
}
 console . log ( arguments . length ) ;
showSiteBooks ( "Javascript" , "Git" ) ;
ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”
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showSiteBooks ( "Javascript" , "Git" ) ;
ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”
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 let showArgs = ( ) => console . log ( arguments [ 0 ]) ;
ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”
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console . log ( Math . max ( ... arr )) ; // 8 (spread makes array into a list of arguments)
ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”
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let arr1 = [ 2 , - 3 , 5 , 4 ] ;
let arr2 = [ 9 , 2 , - 7 , 2 ] ; 
console . log ( Math . max ( ... arr1 , ... arr2 )) ; // 9
ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”
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let arr1 = [ 2 , - 3 , 5 , 4 ] ;
let arr2 = [ 9 , 2 , - 7 , 2 ] ; 
console . log ( Math . max ( 3 , ... arr1 , 6 , ... arr2 , 18 )) ; // 18
ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”
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let merged = [ 1 , ... arr , 3 , ... arr2 ] ;
console . log ( merged ) ; // 1,5,4,9,3,6,8,12 (1, then arr, then 3, then arr2)
ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”
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console . log ([ ... str ]) ; // W,3,D,o,c,s
ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”
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// Array.from converts an iterable into an array
console . log ( Array . from ( str )) ; // W,3,D,o,c,s
ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”ื”
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JavaScript has a range of helpful features allowing developers to work with arrays and function parameters much effortlessly. In this chapter, the rest parameters and spread syntax are covered.
The rest parameter gives you a robust way to work with an indefinite quantity of parameters.
For better understanding what rest parameter is, letโ€™s start from the beginning.
You can call a function with an unspecified number of arguments, no matter how it can be defined.
No error can occur here, because of โ€œenormousโ€ arguments. But, only the first two are counted in the output.
You can include the rest of the parameters in the function definition using three dots, like this: โ€ฆ . It must be followed by the array name containing them. The literal meaning of the dots is โ€œgather the remaining parameters into an array.โ€
Now, letโ€™s cover this example:
You have the option of getting the first parameters as variables gathering only the remaining part.
In the example below, the first two arguments get into variables, and the rest - into bookTitles array:
Please, take into account that the rest parameters should be at the end. As they gather all the remaining arguments, there is no use in them, and they can cause an error:
We can distinguish a unique array-like object, called arguments . It includes all arguments by their index.
The curious thing about arguments is that although it is both array-like and iterable, it is not an array. The array methods can not work for it. So, arguments.map(...) canโ€™t be called. Moreover, it always comprises the overall arguments. They canโ€™t be captured partially.
It would be best if you remembered that arrow functions donโ€™t have "arguments" . If you decide to access the object of arguments from an arrow function, you can get this:
The spread syntax allows an array or a string to expand in the places where zero or more arguments or elements are expected. In other words, if you have a string, an array, or an object, and you wish to use all the values, you can spread them into function calls, new objects, or new arrays with a short syntax. 
Although the spread syntax looks like rest parameters, also using three dots โ€ฆ , it does the opposite.
Whenever you use ...arr in the function call, it expands an iterable object arr to the list of arguments.
The case of Math.max looks as follows:
You can pass different iterables as well, this way:
You can integrate the spread syntax with normal values, as follows:
It is also desirable to use the spread syntax for merging arrays:
You can also use the spread syntax for turning the string into an array of characters.
The spread syntax uses iterators for gathering elements the same way as does for..of .
The Array.from can also be used for that task, as it converts an iterable into an array, like this:
It gives the same result as [...str] .
But the subtle difference between the Array.from(obj) and [...obj] is that the first one operates on both iterables and array-likes, but the spread syntax can work only with iterables.
As you can notice, the three dots โ€ฆ in the code is either rest parameters or the spread syntax.
In this chapter, we represented an easy way of distinguishing between them:
Together they allow navigating between an array and a list of parameters smoothly.

ุนุฑุจูŠ English Espaรฑol Franรงais Indonesia Italiano ๆ—ฅๆœฌ่ชž ํ•œ๊ตญ์–ด ะ ัƒััะบะธะน Tรผrkรงe ะฃะบั€ะฐั—ะฝััŒะบะฐ ็ฎ€ไฝ“ไธญๆ–‡

 function sum ( a , b ) { 
 return a + b ; 
 } 

 alert ( sum ( 1 , 2 , 3 , 4 , 5 ) ) ; 
 

 function sumAll ( ... args ) { // args is the name for the array 
 let sum = 0 ; 

 for ( let arg of args ) sum += arg ; 

 return sum ; 
 } 

 alert ( sumAll ( 1 ) ) ; // 1 
 alert ( sumAll ( 1 , 2 ) ) ; // 3 
 alert ( sumAll ( 1 , 2 , 3 ) ) ; // 6 
 

 function showName ( firstName , lastName , ... titles ) { 
 alert ( firstName + ' ' + lastName ) ; // Julius Caesar 

 // the rest go into titles array 
 // i.e. titles = ["Consul", "Imperator"] 
 alert ( titles [ 0 ] ) ; // Consul 
 alert ( titles [ 1 ] ) ; // Imperator 
 alert ( titles . length ) ; // 2 
 } 

 showName ( "Julius" , "Caesar" , "Consul" , "Imperator" ) ; 
 
The rest parameters must be at the end

 function f ( arg1 , ... rest , arg2 ) { // arg2 after ...rest ?! 
 // error 
 } 
 

 function showName ( ) { 
 alert ( arguments . length ) ; 
 alert ( arguments [ 0 ] ) ; 
 alert ( arguments [ 1 ] ) ; 

 // it's iterable 
 // for(let arg of arguments) alert(arg); 
 } 

 // shows: 2, Julius, Caesar 
 showName ( "Julius" , "Caesar" ) ; 

 // shows: 1, Ilya, undefined (no second argument) 
 showName ( "Ilya" ) ; 
 
Arrow functions do not have "arguments"

 function f ( ) { 
 let showArg = ( ) => alert ( arguments [ 0 ] ) ; 
 showArg ( ) ; 
 } 

 f ( 1 ) ; // 1 
 

 alert ( Math . max ( 3 , 5 , 1 ) ) ; // 5 
 

 let arr = [ 3 , 5 , 1 ] ; 

 alert ( Math . max ( arr ) ) ; // NaN 
 

 let arr = [ 3 , 5 , 1 ] ; 

 alert ( Math . max ( ... arr ) ) ; // 5 (spread turns array into a list of arguments) 
 

 let arr1 = [ 1 , - 2 , 3 , 4 ] ; 
 let arr2 = [ 8 , 3 , - 8 , 1 ] ; 

 alert ( Math . max ( ... arr1 , ... arr2 ) ) ; // 8 
 

 let arr1 = [ 1 , - 2 , 3 , 4 ] ; 
 let arr2 = [ 8 , 3 , - 8 , 1 ] ; 

 alert ( Math . max ( 1 , ... arr1 , 2 , ... arr2 , 25 ) ) ; // 25 
 

 let arr = [ 3 , 5 , 1 ] ; 
 let arr2 = [ 8 , 9 , 15 ] ; 

 let merged = [ 0 , ... arr , 2 , ... arr2 ] ; 

 alert ( merged ) ; // 0,3,5,1,2,8,9,15 (0, then arr, then 2, then arr2) 
 

 let str = "Hello" ; 

 alert ( [ ... str ] ) ; // H,e,l,l,o 
 

 let str = "Hello" ; 

 // Array.from converts an iterable into an array 
 alert ( Array . from ( str ) ) ; // H,e,l,l,o 
 

 let arr = [ 1 , 2 , 3 ] ; 

 let arrCopy = [ ... arr ] ; // spread the array into a list of parameters 
 // then put the result into a new array 

 // do the arrays have the same contents? 
 alert ( JSON . stringify ( arr ) === JSON . stringify ( arrCopy ) ) ; // true 

 // are the arrays equal? 
 alert ( arr === arrCopy ) ; // false (not same reference) 

 // modifying our initial array does not modify the copy: 
arr . push ( 4 ) ; 
 alert ( arr ) ; // 1, 2, 3, 4 
 alert ( arrCopy ) ; // 1, 2, 3 
 

 let obj = { a : 1 , b : 2 , c : 3 } ; 

 let objCopy = { ... obj } ; // spread the object into a list of parameters 
 // then return the result in a new object 

 // do the objects have the same contents? 
 alert ( JSON . stringify ( obj ) === JSON . stringify ( objCopy ) ) ; // true 

 // are the objects equal? 
 alert ( obj === objCopy ) ; // false (not same reference) 

 // modifying our initial object does not modify the copy: 
obj . d = 4 ; 
 alert ( JSON . stringify ( obj ) ) ; // {"a":1,"b":2,"c":3,"d":4} 
 alert ( JSON . stringify ( objCopy ) ) ; // {"a":1,"b":2,"c":3} 
 
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