lazy-require.snippet.js

/*
Below is an incomplete snippet of code which is used to implement the Lazy Require Calls
method described here http://spadgos.github.io/rupy13/#/29

Basically, it takes an AST like this:

  var Foo = require('foo');
  function bar() {
    Foo.quux();
  }

...into this:

  var Foo;
  function bar() {
    (Foo || (Foo = require('foo'))).quux();
  }

The `ASTHelpers.transform` method is simply a wrapper for the standard Uglify transform,
which executes the callback only on nodes which match the given `type`. Hopefully it
should be understandable enough.
*/


// Get the AST subtree to perform a lazy require call. Basically converts a reference to a variable which has been
// required into `variable || (variable = require("path/to/require"))`
function getReplacementRequireVar(name, req) {
  var miniAST = uglify.parse(uglify.string_template('{name} || ({name} = require("{req}"))', {
    name: name,
    req: req
  }));
  // miniAST.body == [ SimpleStatement ]
  return miniAST.body[0].body;
}

// see if this symbol is the same as the one defined at the top of the file
function matches (symbolDec) {
  return symbolDec.start === this.start;
}

/**
 * Finds all variables assigned to a require call and rewrites them to only make the call to require when necessary.
 */
function rewriteRequires(ast) {
  var map = {};
  ASTHelpers.transform(ast, [{
    type: 'VarDef',
    handler: function (node) {
      if (node.value
          && node.value instanceof uglify.AST_Call
          && node.value.expression.name === 'require'
          && node.value.expression.global()) {

        map[node.name.name] = {
          module: node.value.args[0].value,
          node: node
        };
        node.value = null; // delete the require call
      }
    }
  }]);

  ASTHelpers.transform(ast, [{
    type: 'SymbolRef',
    handler: function (node) {
      var reqInfo = map[node.name];

      if (reqInfo && node.thedef.orig.some(matches, reqInfo.node)) {
        return getReplacementRequireVar(node.name, reqInfo.module);
      }
    }
  }]);
}