codemod from ForInStatement to ForStatement

I have a codemod where I want to transform

for (var key in foo){}

into

for (var keys = 0; key < foo; key++){}

I managed this far:

return j(file.source)
    .find(j.ForInStatement)
    .replaceWith(p => {

        var prop = p.node.left.declarations[0].id;

        var v = [j.variableDeclarator(prop, null)];
        var varDec = j.variableDeclaration('var', v);
        var binary = j.binaryExpression('<', prop, j.identifier('foo'));
        var uE = j.updateExpression('++', prop, false);
        var block = j.blockStatement([]);

        var forIn = j.forStatement(varDec, binary, uE, block);
        return forIn;
    })
    .toSource();

and that brought me this far:

for (var key; key < foo; key++)
  {}

I'm still a bit lost creating things from scratch...

Question:

• Am I doing this correct? feels quite verbose, but maybe it is the way to go.
• How to finish? I am missing for example foo.length...

PS: Felix, if you are reading this, do point me where can I send pull requests to improve the docs! I am happy to help with that while I trying to learn this API. Oh, and Felix, what a nice tool you guys made!

Creating AST nodes directly is indeed verbose. However, jscodeshift exports some helper "methods" that allow you to generate AST nodes from a string and let you interpolate existing AST nodes (tagged templates ftw!).

Your above example could be simplified to:

  return j(file.source)
    .find(j.ForInStatement)
    .replaceWith(p => {
      var prop = p.node.left.declarations[0].id;
      return statement`for (var ${prop} = 0; ${prop} < foo.length; ${prop}++) ${p.node.body}`;
    })
    .toSource();

DEMO

Note though that those template methods (expression, statement, statements) have their limitations (you can only interpolate AST nodes where you can put an identifier).

---

If you wanted to create foo.length manually, you would have to create a MemberExpression:

j.memberExpression(
  prop,
  j.identifier('length')
);