Run JavaScript in a V8 context

Description

The v8 function (formerly called new_context) creates a new V8 context. A context provides an execution environment that allows separate, unrelated, JavaScript code to run in a single instance of V8, like a tab in a browser.

Usage

v8(global = "global", console = TRUE, typed_arrays = TRUE)

Arguments

Details

V8 contexts cannot be serialized but creating a new contexts and sourcing code is very cheap. You can run as many parallel v8 contexts as you want. R packages that use V8 can use a separate V8 context for each object or function call.

The ct$eval method evaluates a string of raw code in the same way as eval would do in JavaScript. It returns a string with console output. The ct$get, ct$assign and ct$call functions on the other hand automatically convert arguments and return value from/to JSON, unless an argument has been wrapped in JS(), see examples. The ct$validate function is used to test if a piece of code is valid JavaScript syntax within the context, and always returns TRUE or FALSE.

JSON is used for all data interchange between R and JavaScript. Therefore you can (and should) only exchange data types that have a sensible JSON representation. All arguments and objects are automatically converted according to the mapping described in Ooms (2014), and implemented by the jsonlite package in fromJSON and toJSON.

The name of the global object (i.e. global in node and window in browsers) can be set with the global argument. A context always have a global scope, even when no name is set. When a context is initiated with global = NULL, the global environment can be reached by evaluating this in the global scope, for example: ct$eval("Object.keys(this)").

Methods

console()

starts an interactive console

eval(src)

evaluates a string with JavaScript source code

validate(src)

test if a string of JavaScript code is syntactically valid

source(file)

evaluates a file with JavaScript code

get(name, ...)

convert a JavaScript to R via JSON. Optional arguments (...) are passed to fromJSON to set JSON coercion options.

assign(name, value)

copy an R object to JavaScript via JSON

call(fun, ...)

call a JavaScript function with arguments .... Arguments which are not wrapped in JS() automatically get converted to JSON

reset()

resets the context (removes all objects)

References

A Mapping Between JSON Data and R Objects (Ooms, 2014): http://arxiv.org/abs/1403.2805

Examples

# Create a new context
ctx <- v8();

# Evaluate some code
ctx$eval("var foo = 123")
ctx$eval("var bar = 456")
ctx$eval("foo+bar")

# Functions and closures
ctx$eval("JSON.stringify({x:Math.random()})")
ctx$eval("(function(x){return x+1;})(123)")

# Objects (via JSON only)
ctx$assign("mydata", mtcars)
ctx$get("mydata")
ctx$get("mydata", simplifyVector = FALSE)

# Assign JavaScript
ctx$assign("foo", JS("function(x){return x*x}"))
ctx$assign("bar", JS("foo(9)"))
ctx$get("bar")

# Validate script without evaluating
ctx$validate("function foo(x){2*x}") #TRUE
ctx$validate("foo = function(x){2*x}") #TRUE
ctx$validate("function(x){2*x}") #FALSE

# Use a JavaScript library
ctx$source(system.file("js/underscore.js", package="V8"))
ctx$call("_.filter", mtcars, JS("function(x){return x.mpg < 15}"))

# Example from underscore manual
ctx$eval("_.templateSettings = {interpolate: /\\{\\{(.+?)\\}\\}/g}")
ctx$eval("var template = _.template('Hello {{ name }}!')")
ctx$call("template", list(name = "Mustache"))

# Call anonymous function
ctx$call("function(x, y){return x * y}", 123, 3)

## Not run: CoffeeScript
ct2 <- v8()
ct2$source("http://coffeescript.org/extras/coffee-script.js")
jscode <- ct2$call("CoffeeScript.compile", "square = (x) -> x * x", list(bare = TRUE))
ct2$eval(jscode)
ct2$call("square", 9)

# Interactive console
ct3 <- v8()
ct3$console()
//this is JavaScript
var test = [1,2,3]
JSON.stringify(test)
exit
## End(Not run)