Opal is a source-to-source compiler, so there is no VM as such and the compiled code aims to be as fast and efficient as possible, mapping directly to underlying javascript features and objects where possible.
self is mostly compiled to this. Methods and blocks are implemented as javascript functions, so their this value will be the right self value. Class bodies and the top level scope use a self variable to improve readability.
true and false are compiled directly into their native boolean equivalents. This makes interaction a lot easier as there is no need to convert values to opal specific values. It does mean that there is only a Boolean ruby class available, not seperate TrueClass and FalseClass classes.
nil is compiled to a nil javascript variable. nil is a real object which allows methods to be called on it. Opal cannot send methods to null or undefined, and they are considered bad values to be inside ruby code.
nil # => nil true # => true false # => false self # => self
Ruby strings are compiled directly into javascript strings for performance as well as readability. This has the side effect that Opal does not support mutable strings - i.e. all strings are immutable.
For performance reasons, symbols compile directly into strings. Opal supports all the symbol syntaxes, but does not have a real Symbol class. Symbols and Strings can therefore be used interchangeably.
"hello world!" # => "hello world!" :foo # => "foo" <<-EOS # => "\nHello there.\n" Hello there. EOS
In Opal there is a single class for numbers; Numeric. To keep opal as performant as possible, ruby numbers are mapped to native numbers. This has the side effect that all numbers must be of the same class. Most relevant methods from Integer, Float and Numeric are implemented on this class.
42 # => 42 3.142 # => 3.142
Ruby arrays are compiled directly into javascript arrays. Special ruby syntaxes for word arrays etc are also supported.
[1, 2, 3, 4] # => [1, 2, 3, 4] %w[foo bar baz] # => ["foo", "bar", "baz"]
Inside a generated ruby script, a function __hash is available which creates a new hash. This is also available in javascript as Opal.hash and simply returns a new instance of the Hash class.
{ :foo => 100, :baz => 700 } # => __hash("foo", 100, "baz", 700) { foo: 42, bar: [1, 2, 3] } # => __hash("foo", 42, "bar", [1, 2, 3])
Similar to hash, there is a function __range available to create range instances.
1..4 # => __range(1, 4, true) 3...7 # => __range(3, 7, false)
As per ruby, Opal treats only false and nil as falsy, everything else is a truthy value including "", 0 and []. This differs from javascript as these values are also treated as false.
For this reason, most truthy tests must check if values are false or nil.
Taking the following test:
val = 42 if val return 3.142; end
This would be compiled into:
var val = 42; if (val !== false && val !== nil) { return 3.142; }
This makes the generated truthy tests (if statements, and checks and or statements) a litle more verbose in the generated code.
Instance variables in Opal work just as expected. When ivars are set or retrieved on an object, they are set natively without the @ prefix. This allows real javascript identifiers to be used which is more efficient then accessing variables by string name.
@foo = 200 @foo # => 200 @bar # => nil
This gets compiled into:
this.foo = 200; this.foo; // => 200 this.bar; // => nil
As described above, a compiled ruby source gets generated into a string of javascript code that is wrapped inside an anonymous function. This looks similar to the following:
(function($opal) { var $klass = $opal.klass, self = $opal.top; // generated code })(Opal);
As a complete example, assuming the following code:
puts "foo"
This would compile directly into:
(function($opal) { var $klass = $opal.klass, self = $opal.top; self.$puts("foo"); })(Opal);
Most of the helpers are no longer present as they are not used in this example.
If you write the generated code as above into a file app.js and add that to your HTML page, then it is obvious that "foo" would be written to the browser's console.
Because Opal does not aim to be fully compatible with ruby, there are some instances where things can break and it may not be entirely obvious what went wrong.
As opal just generates javascript, it is useful to use a native debugger to work through javascript code. To use a debugger, simply add an x-string similar to the following at the place you wish to debug:
# .. code `debugger` # .. more code
The x-strings just pass the debugger statement straight through to the javascript output.
All local variables and method/block arguments also keep their ruby names except in the rare cases when the name is reserved in javascript. In these cases, a $ suffix is added to the name (e.g. try => try$).