R: Run JavaScript in a V8 context

V8 {V8} | R Documentation

## 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)

    engine_info()

### Arguments

`global` | character vector indicating name(s) of the global environment. Use NULL for no name.
`console` | expose `console` API (`console.log`, `console.warn`, `console.error`).
`typed_arrays` | (deprecated) enable typed arrays in legacy libv8. Deprecated because typed arrays are natively supported in recent versions of libv8.

### Details

A V8 context cannot be saved or duplicated, but creating a new context 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 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)")`.

### V8 Context Methods

    ## ctx <- v8()
    <V8 engine 9.7.106.18>
     $assign(name, value, auto_unbox = TRUE, ...)
     $call(fun, ..., auto_unbox = TRUE, await = FALSE)
     $console()
     $eval(src, serialize = FALSE, await = FALSE)
     $get(name, ..., await = FALSE)
     $reset()
     $source(file)
     $validate(src)

The `ct$eval` method evaluates a string of JavaScript code in the same way as `eval` in JavaScript. By default `eval()` returns a string with console output; but when the `serialize` parameter is set to `TRUE` it serializes the JavaScript return object to a JSON string or a raw buffer.

The `ct$get`, `ct$assign` and `ct$call` functions automatically convert arguments and return value between R and JavaScript (using JSON). To pass literal JavaScript arguments that should not be converted to JSON, wrap them in `JS()`, see examples.

If a call to `ct$eval()`,`ct$get()`, or `ct$call()` returns a JavaScript promise, you can set `await = TRUE` to wait for the promise to be resolved. It will then return the result of the promise, or an error in case the promise is rejected.

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.

In an interactive R session you can use `ct$console()` to switch to an interactive JavaScript console. Here you can use `console.log` to print objects, and there is some support for JS tab-completion. This is mostly for testing and debugging, it may not work perfectly in every IDE or R-frontend.

### Data Interchange

JSON is used for data interchange between R and JavaScript. Therefore you can (and should) only exchange data types that have a sensible JSON representation. One exception is raw vectors which are converted to/from Uint8Array buffers, see below. All other arguments and objects are automatically converted according to the mapping described in Ooms (2014), and implemented by the jsonlite package in `fromJSON()` and `toJSON()`.

As for version 3.0 of this R package, Raw vectors are converted to `Uint8Array` typed arrays, and vice versa. This makes it possible to efficiently copy large chunks binary data between R and JavaScript, which is useful for running wasm or emscripten.

### Note about Linux and Legacy V8 engines

This R package can be compiled against modern (V8 version 6+) libv8 API, or the legacy libv8 API (V8 version 3.15 and below). You can check `V8::engine_info()` to see the version that is running. The legacy version does not support modern JS (ES6) or WASM, but it is still the default on older versions of Ubuntu and CentOS. The latest versions of all major Linux distributions now provide a modern version of V8. For Ubuntu 16.04 and 18.04 we provide backports of libv8 (via libnode-dev), see the readme for details.

### References

A Mapping Between JSON Data and R Objects (Ooms, 2014): https://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")
    outlist <- ctx$get("mydata", simplifyVector = FALSE)
    outlist[1]

    # 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$source("http://coffeescript.org/v1/browser-compiler/coffee-script.js")
    jscode <- ct2$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)

[Package V8 version 4.0.0 Index]