title: How to know if a variable has been declared in Babel 6
body: <p>For example</p>
  
  <pre><code>var a = function() { };
  var b = function() { 
    a(); // Identifier.name a - declared already
  };
  </code></pre>
  
  <p>and </p>
  
  <pre><code>var b = function() { 
    a(); // Identifier.name a - NOT declared already
  };
  </code></pre>
  
  <p>Here's the visitor</p>
  
  <pre><code>export default function ({types: t }) {
    return {
      visitor: {
        CallExpression: function (p) {
          var nameToCheckIfDeclared = p.node.callee.name;
          // ???
        }
      }
    };
  }
  </code></pre>