gen-codemod

Generate codemod by comparing two JavaScript files.

Usage

gen-codemod INITIAL.js DESIRED.js > my-transform.js

INITIAL.js is the initial, pre-codemod JavaScript you'd like to change.

DESIRED.js is the desired, post-codemod JavaScript you'd like to have.

gen-codemod compares these two files and creates a codemod to get from INITIAL.js to DESIRED.js.

Example

When upgrading from sinon.js v1 -> v2, every instance of sinon.stub(obj, method, fn) needs to be changed to sinon.stub(obj, method).andCallsFake(fn). We can generate this codemod as follows:

echo "sinon.stub(A, B, C)" > sinon-v1.js
echo "sinon.stub(A, B).andCallFake(C)" > sinon-v2.js
gen-codemod sinon-v1.js sinon-v2.js

Note: Single uppercase letters, such as A, B, C, etc, are used as variables and match any AST node.

This outputs:

module.exports = function(file, api) {
  const j = api.jscodeshift;

  const rootProperties = {
    type: 'CallExpression',
    callee: {
      type: 'MemberExpression',
      object: { type: 'Identifier', name: 'sinon' },
      property: { type: 'Identifier', name: 'stub' },
      computed: false
    },
    arguments: [{}, {}, {}]
  };

  function getTransform(path) {
    return j.callExpression(
      j.memberExpression(
        j.callExpression(
          j.memberExpression(
            j.identifier('sinon'),
            j.identifier('stub'),
            false
          ),
          [getANode(path), getBNode(path)]
        ),
        j.identifier('andCallsFake'),
        false
      ),
      [getCNode(path)]
    );
  }

  function getANode(path) {
    return path.node.arguments[0];
  }
  function getBNode(path) {
    return path.node.arguments[1];
  }
  function getCNode(path) {
    return path.node.arguments[2];
  }

  return j(file.source)
    .find(j.CallExpression, rootProperties)
    .replaceWith(getTransform)
    .toSource();
};

To run your codemod, use jscodeshift:

gen-codemod sinon-v1.js sinon-v2.js > my-transform.js
npx jscodeshift -t my-transform.js PROJECT_PATH

Install

npm i -g gen-codemod

Limitations

Can't change multiple AST expressions at once (only reads the first AST node from the initial / desired JavaScript files).