title: Unfamiliar use of square brackets in calling a function
body: <p>In the middle of this <a href="http://hangar.runway7.net/javascript/guide">page</a>, I find the code below.</p>
  
  <pre><code>var plus = function(x,y){ return x + y };
  var minus = function(x,y){ return x - y };
  
  var operations = {
    '+': plus,
    '-': minus
  };
  
  var calculate = function(x, y, operation){
      return operations[operation](x, y);
  }
  
  calculate(38, 4, '+');
  calculate(47, 3, '-');
  </code></pre>
  
  <p>Now while I can trace how it works, I've never seen this use of square brackets before. It certainly doesn't look like it's creating an array or referencing a member of an array. Is this common? If so, where are some other examples?</p>