jamiebuilds_the-super-tiny-compiler/3-traverser.js

/**
 * ============================================================================
 *                                 ⌒(❀>◞౪◟<❀)⌒
 *                               THE TRAVERSER!!!
 * ============================================================================
 */

/**
 * So now we have our AST, and we want to be able to visit different nodes with
 * a visitor. We need to be able to call the methods on the visitor whenever we
 * encounter a node with a matching type.
 *
 *   traverse(ast, {
 *     Program(node, parent) {
 *       // ...
 *     },
 *
 *     CallExpression(node, parent) {
 *       // ...
 *     },
 *
 *     NumberLiteral(node, parent) {
 *       // ...
 *     },
 *   });
 */

// So we define a traverser function which accepts an AST and a
// visitor. Inside we're going to define two functions...
function traverser(ast, visitor) {

  // A `traverseArray` function that will allow us to iterate over an array and
  // call the next function that we will define: `traverseNode`.
  function traverseArray(array, parent) {
    array.forEach(child => {
      traverseNode(child, parent);
    });
  }

  // `traverseNode` will accept a `node` and its `parent` node. So that it can
  // pass both to our visitor methods.
  function traverseNode(node, parent) {

    // We start by testing for the existence of a method on the visitor with a
    // matching `type`.
    let methods = visitor[node.type];

    // If there is an `enter` method for this node type we'll call it with the
    // `node` and its `parent`.
    if (methods && methods.enter) {
      methods.enter(node, parent);
    }

    // Next we are going to split things up by the current node type.
    switch (node.type) {

      // We'll start with our top level `Program`. Since Program nodes have a
      // property named body that has an array of nodes, we will call
      // `traverseArray` to traverse down into them.
      //
      // (Remember that `traverseArray` will in turn call `traverseNode` so  we
      // are causing the tree to be traversed recursively)
      case 'Program':
        traverseArray(node.body, node);
        break;

      // Next we do the same with `CallExpression` and traverse their `params`.
      case 'CallExpression':
        traverseArray(node.params, node);
        break;

      // In the cases of `NumberLiteral` and `StringLiteral` we don't have any
      // child nodes to visit, so we'll just break.
      case 'NumberLiteral':
      case 'StringLiteral':
        break;

      // And again, if we haven't recognized the node type then we'll throw an
      // error.
      default:
        throw new SyntaxError(`Unrecognized node ${node.type}: ${JSON.stringify(node)}}`);
    }

    // If there is an `exit` method for this node type we'll call it with the
    // `node` and its `parent`.
    if (methods && methods.exit) {
      methods.exit(node, parent);
    }
  }

  // Finally we kickstart the traverser by calling `traverseNode` with our ast
  // with no `parent` because the top level of the AST doesn't have a parent.
  traverseNode(ast, null);
}

// Just exporting our traverser to be used in the final compiler...
module.exports = traverser;