R version 4.5.0 alpha (2025-03-23 r88038 ucrt)
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Platform: x86_64-w64-mingw32/x64

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> library(RJSONIO)
> 
> toJSON(list(1, 2, list(1, NA)))
[1] "[\n        1,\n       2,\n[\n        1,\nnull \n] \n]"
> 
> toJSON(list(1, 2, list(NA)))  # collapses the sub-list into the main vector.
[1] "[\n        1,\n       2,\n[\n null \n] \n]"
> 
> 
> e = new.env(); e$a = 1:10; e$bc = letters[1:3]
> cat(toJSON(e, pretty = TRUE))
{
	"a" : [
		1,
		2,
		3,
		4,
		5,
		6,
		7,
		8,
		9,
		10
	],
	"bc" : [
		"a",
		"b",
		"c"
	]
}> 
> 
> a = list(x=1, y=character(0))
> fromJSON( toJSON( a ) )
$x
[1] 1

$y
list()

> a = list(x=1, y=character(0), b = 1)
> fromJSON( toJSON( a ) )
$x
[1] 1

$y
list()

$b
[1] 1

> 
> 
> a = list(x = vector(), y = 123, z = "allo")
> fromJSON(toJSON(a))
$x
list()

$y
[1] 123

$z
[1] "allo"

> 
> 
> 
> proc.time()
   user  system elapsed 
   0.23    0.09    0.54