library(knitr)
knit_engines$set(V8 = function(options) {
  require(V8)
  context <- ifelse(!is.null(options$V8.context), options$V8.context, "ct")  
  if(!exists(context)){
    assign(context, new_context(), envir = .GlobalEnv)
    if(!is.null(options$V8.libraries)){
      for(library in options$V8.libraries){
        eval(parse(text=sprintf('%s$source("%s")', context, library))) 
      }
    }
  }
  code <- as.character(c(options$code))
  if(!eval(parse(text=sprintf("%s$validate(code)", context)))) stop("unvalid javascript code")
  output <- eval(parse(text=sprintf("%s$eval(code)", context))) 
  code <-  knitr:::wrap.source(list(src= paste(code, collapse = "\n")), options)
  if(options$results=="hide") return(code)
  else return(c(code, knitr:::wrap.character(output, options)))
})

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]]

By default the Javascript context is a R variable named ct. You can set another name by doing opts_chunk$set(V8.context="mycontext"). Then using the context variable you can import a variable from the Javascript context to R or vice-versa (see ?V8::V8 for details) :

• 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)

The Javascript JSON.stringify function is nice for displaying an object in a V8 chunk:

JSON.stringify(double_x);

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

You can load a Javascript library like this :

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

As you can see, it works:

JSON.stringify(_.flatten(x))

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

Another way is to give the libraries as a character vector in the V8.libraries chunk option:

opts_chunk$set(V8.libraries=c(system.file("js/underscore.js", package="V8"), "http://coffeescript.org/extras/coffee-script.js"))

The libraries given by this way are loaded only once, at the moment the Javascript context is created.