How to Use a Ruby Loop Inside of Haml's :JavaScript Region

Is there a way to use a Ruby loop inside of HAML's :javascript region?

this one works

%script
  - 10.upto(20) do |i|
    document.getElementById('aDiv').innerHTML += '#{i}';

how to run ruby in haml in javascript definition?

First, let's review what you seem to know:

Ruby requires you to define local variables before you use them.
You can run Ruby code on lines outside of a filter using - ....
You use #{...} markup to interpolate Ruby code inside a filter.

You say you want to run each, but presumably you want output from this; since the result of #{...} is turned into a string and put in your code, what you really want (probably) is map:

%html
  %head
    :javascript
      var foo = [];
      #{
        limit = rand(4)+3
        array = (0..limit).to_a
        array.map{ |i| "foo[#{i}] = #{rand(12)};" }.join ' '
      }
      console.log(foo.length);
    %body

Running the above code gives this output:

<html>
  <head>
    <script type='text/javascript'>
      //<![CDATA[
        var foo = [];
        foo[0] = 2; foo[1] = 0; foo[2] = 11; foo[3] = 8; foo[4] = 0; foo[5] = 1;
      //]]>
    </script>
    <body></body>
  </head>
</html>

As you can see, the big #{...} block (which may span multiple lines) runs arbitrary Ruby code. The result of the last expression (in this case the map{...}.join) is converted to a string and placed in the output.

Inline ruby in :javascript haml tag?

You can use the string interpolation syntax (#{...}) :

:javascript
   var tab = #{@tab}

Take care of correctly escaping, however.

In HAML, how to write comments in :javascript region so that comments do not show to the public?

#{}- blocks are evaluated, so you can write

#{ # this is a ruby comment, but still a comment (newline is required)
}

How to iterate over an array with HAML?

You forgot the hash symbol.

- @fonts.each do |font|
  %link{:href=>"//fonts.googleapis.com/css?family=#{font}",:rel=>"stylesheet",:type=>"text/css"}
                                                  ^ here