library(knitr)
split_results <- function (x, sep) # function borrowed from runr package
{
    x = strsplit(paste(x, collapse = "\n"), sep)[[1]]
    res = vector("list", length(x))
    for (i in seq_along(x)) {
        el = gsub("^\n+|\n+$", "", x[i])
        res[[i]] = if (i%%2 == 1) 
            structure(list(src = el), class = "source")
        else paste(el, collapse = "\n")
    }
    res
}
knit_engines$set(V8 = function(options) {
  code <- as.character(c(options$code))
  output <- ct$eval(code) 
  code <- lapply(split_results(code, sep="this_is_a_stupid_string"), function(x) knitr:::wrap.source(x, options))
  if(options$results=="hide") return(code)
  else return(c(code, knitr:::wrap.character(output, options)))
})

Load V8 and define a new context named ct:

library(V8)
ct <- new_context()

Define x in Javascript context using the chunk option engine='V8':

var x=[];
x.push([1,2]);
x.push([3,4]);
x.push([5,6]);
JSON.stringify(x)

## [[1,2],[3,4],[5,6]]

Get x in R :

( x.in.R <- ct$get("x") )

##      [,1] [,2]
## [1,]    1    2
## [2,]    3    4
## [3,]    5    6

Double x in R and assign in Javascript context :

ct$assign("double_x", 2*x.in.R)

JSON.stringify(double_x);

## [[2,4],[6,8],[10,12]]

Load a Javascript library :

ct$source(system.file("js/underscore.js", package="V8"))

JSON.stringify(_.flatten(x))

## [1,2,3,4,5,6]