V8engine_Example.Rmd

```{r}
library(knitr)
knit_engines$set(V8 = runr:::V8engine)
```

By default the Javascript context is a R variable named `.context`. You can set another name by doing `opts_chunk$set(V8.context=mycontext)`. For instance:
```{r}
library(V8)
ct <- new_context()
opts_chunk$set(V8.context=ct)
```


For example, we write the Javascript code below in a chunk with the option `engine='V8'`:

```
var x=[]
for(i = 0; i<3; i++){
  x[i]=i;
}
x.push([3,4])
JSON.stringify(x)
```

See it in action:

```{r, engine='V8'}
var x=[]
for(i = 0; i<3; i++){
  x[i]=i;
}
x.push([3,4])
JSON.stringify(x)
```

In the previous chunk, the `results` option is set to its default value. You can use `results='hide'` to hide the output, or `results='last'` to show only the output generated by the last line:

```{r, engine='V8', results='last'}
// this chunk has the options engine='V8' and results='last'
var x=[]
x.push([1,2])
x.push([3,4])
JSON.stringify(x)
```


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 :

```{r}
( x.in.R <- ct$get("x") )
```

- Double `x` in R and assign in Javascript context :

```{r}
ct$assign("double_x", 2*x.in.R)
```

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

```{r, engine='V8'}
// this chunk has the option engine='V8'
JSON.stringify(double_x);
```


You can load a Javascript library as follows:

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

As you can see, it works:

```{r, engine='V8'}
// this chunk has the option engine='V8'
JSON.stringify(_.flatten(x))
```

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

```{r, eval=FALSE}
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.