User Interfaces:Implemented

Accessing JavaScript Objects From Ruby

Oct 25, 2010

Awhile back, I wrote a post on how to access Ruby objects from inside your JavaScript enviroment when using The Ruby Racer. It showed just some of the many ways that you can access Ruby state and call Ruby code from within JavaScript. However, the story doesn’t end there. The Ruby Racer is, after all, a two way bridge, and I thought it would be useful to document some of the ways in which you can access your JavaScript environment from within Ruby.

Let’s start with the Context#eval method, which is used to execute JavaScript code from Ruby. Its the most intuitive way, and what gets used most often for examples.

Given a string, it compiles it as JavaScript source and then executes it inside the context. Nothing crazy there:

V8::Context.new do |cxt|
  cxt.eval('1 + 1') #=> 2
  cxt.eval('foo = {bar: "bar", baz: "baz"}')
  cxt.eval('foo.bar') #=> "bar"
  cxt.eval('foo.baz') #=> "baz"
  cxt.eval('new Object()') #=> [object Object]
end

Among other things, the context captures what functions and objects are defined in the global scope whenever anything gets eval()‘d. So, for example, the Object constructor used in the last line is stored in the context.

As a tool for embedding however, eval() is the most blunt and (often) inefficient method available. For this reason The Ruby Racer, sports an extensive Ruby API for interacting with JavaScript objects which includes accessing properties, calling functions and methods, as well as creating new instances. In fact, I rarely use eval() at all to manipulate a JavaScript context except for loading source files into the interpreter.

The Ruby API consists of V8::Object and all of its subclasses:

V8::Context.new do |cxt|
  cxt.eval('new Object()').class #=> V8::Object
  cxt.eval('(function() {})').class #=> V8::Function
  cxt.eval('new Array()).class #=> V8::Array
end

Objects

The most fundamental operations in dealing with JavaScript objects are the getting and setting of values. Coincidentally, this is also a fundamental concept in Ruby, so it comes as no great surprise that we can re-use those constructs in Ruby that deal with property access to mirror those same operations on their JavaScript counterparts: the []()/[]=() and foo()/foo=() methods:

given the following context:

cxt = V8::Context.new
order = cxt.eval('order = {eggs: "over-easy"}')

we can read the eggs property of our order in several differnt ways. Via hash access (note that both string and symbol are acceptable as key values):

order['eggs'] #=> "over-easy"
order[:eggs] #=> "over-easy"

Values can be set with the hash-style access as well, and changes made from Ruby will be reflected accordingly on the JavaScript side

order['eggs'] = "sunny side up"
cxt.eval('order.eggs') #=> "sunny side up"

For property names that are also valid Ruby method names, you can access them just like you would with Ruby properties declared with attr_reader or attr_accessor. Again, changes made to the object in this way will be reflected in JavaScript:

  order.eggs #=> "sunny side up"
  order.eggs = "scrambled"
  cxt.eval('order.eggs') #=> "scrambled"
end

In the cases where the property name is not a valid Ruby method name, hash-style access is mandatory:

order['Extra $%#@! Mayonaise!'] = true

V8::Object also includes Enumerable and allows you to access all properties of a given object.

order.each do |key, value|
  puts "#{key} -> #{value}"
end

#outputs:
eggs -> scrambled
Extra $%#@! Mayonaise! -> true

As a convenience, V8::Context delegates the hash access functions to the JavaScript object which serves as its global scope.

order == cxt['order'] #=> true
order == cxt.scope['order'] #=> true

Arrays

Not much to see here, but before you move along: A V8::Array is like every other V8::Object except it has a length property, and it enumerates over the items stored at indices instead of the key, value pairs of its properties.

array = cxt.eval('["green", "red", "golden"]')
array.length #=> 3
array.map {|color| "#{color} slumbers"} #=> ['green slumbers', 'red slumbers', 'golden slumbers']

Functions

Consider the classic “Circle” example from every object oriented playbook you’ll ever read. In JavaScript, the way to implement the circle “class” is with a constructor function.

circle = cxt.eval<<-JS
  function Circle(radius) {
    this.radius = radius
    this.area = function() {
      return this.radius * this.radius * Math.PI
    }
    this.circumference = function() {
      return 2 * Math.PI * this.radius
    }
  }
  new Circle(5)
JS

Now that we have the Circle constructor defined, and we have a reference to an instance of it in Ruby, we can call its methods just as though it were a normal Ruby object. Of course, only we know that under the covers the implementation is actually in JavaScript:

circle.class            #=> V8::Object
circle.radius           #=> 5
circle.area()           #=> 25Π
circle.circumference()  #=> 10Π

In JavaScript, methods are just object properties that happen to be functions. Therefore, you can get a reference to the actual function value just as you could from JavaScript simply by accessing the property by name. The resulting value is an instance of V8::Function.

area =                circle['area']
circumference =       circle['circumference']
area.class            #=> V8::Function
circumference.class   #=> V8::Function

Once you have a reference to a V8::Function, there are two ways to call the underlying JavaScript code (Actually there are three, but the third will be covered in the next section). These are the call() and methodcall() methods. To understand the difference between these two methods, it helps to understand how JavaScript functions themselves are invoked. In the event, there is the option to pass an object which will serve as the implicit invocant or this value. If no this is provided, the function will use the global scope in its place. Take for example the following JavaScript

var circle = new Circle(5)
var area = circle.area  //=> [object Function]
//no invocant, corresponds to ruby call()
area()  //=> NaN, there is no global 'radius'.
//call with invocant, corresponds ruby methodcall()
area.apply(circle)  //=> 25Π

The same code in Ruby:

area = circle['area']
area.class              #=> V8::Function
area.call()             #=> NaN
area.methodcall(circle) #=> 25Π

Because of this mechanism, JavaScript method invocation is much more flexible than Ruby in the sense that virtually any object can be used as the invocant of any function provided it has the requisite properties to satisfy the function’s requirements:

area.methodcall(:radius => 5) #=> 25Π
other_circle = OpenStruct.new
other_circle.radius = 10
area.methodcall(other_circle) #=> 100Π

A very powerful construct indeed.

Constructors

But wait, there’s more! Any JavaScript function can be either invoked normally, or, combined with the new keyword, as a constructor. It’s what we used in the original eval() block to create the instance of Circle whose methods we were messing about with.

That was not quite necessary since we can invoke functions as constructors from Ruby too. This is done with the new method of V8::Function.

Circle = cxt['Circle']
circle2 = Circle.new(3)
circle2.radius        #=> 3
circle2.area          #=> 9Π
circle2.circumference #=> 6Π

Interestingly, when used as a constructor, a JavaScript function is almost completely indistinguishable from a Ruby class. This apparent duality between classes and functions is behind the decision to represent Ruby classes reflected into JavaScript as functions

The driving goal in all of these cases is the ability to author intuitive, tightly bound JavaScript, which necessarily doesn’t involve evaluating strings to get what you want done. In closing, if there is something missing from this API you think would be useful, I would love to hear about it


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