coffeescript - Append Array to Array of Arrays -

i'm trying write append function, such not mutate input list:

append([ [1], [2] ], [3]) should equal [ [1], [2], [3] ].

i tried:

append = (arrayofarrays, newelem) ->   [first, remainder...] = arrayofarrays   [first, remainder.concat(newelem)] 

but not work intended:

append [[1],[2]], [3] == [[1],[[2],3]]

the reason why it's not working is:

1. you appending remainder, instead of new array. , concat expects array parameter, combine other array
2. as in case item appended using concat array, need nest inside array.

i advise using slice(0) method duplicate initial array, , concat new array it, contains new array:

# duplicate array of arrays, , append new array it, returning new array. # @params list [array<array>] array of arrays # @params newitem [array] new array # @return [array<array>] duplicate of array new array appended append = (list, newitem) ->   list.slice(0).concat [newitem]  = [[], [1,1], [2]] append a, [3,3] # => [[], [1,1], [2], [3,3]] # => [[], [1,1], [2]] 

slice make copy of part of array. slice(0) start @ 0th index, , go end, making copy of array.

concat joins 2 arrays. want keep new array intact. therefore have wrap in array gets merged main array.