Babel AST transform drops Recast's `original` property

[Nick Heiner]

Bug Report

Maybe you don't consider this a bug, but I thought I'd flag it anyway.

Current behavior
Recast (0.20.4) works by attaching an original property to AST nodes. babel.transformFromAst drops that property, preventing Recast from working as expected.

This is demonstrated by the following script:

const babel = require('@babel/core');
const recast = require('recast');
const _ = require('lodash');

const filename = 'file.js';
const source = `
function f(
  a,
  b,
  c
) {
  return d;
}
`;

const babelOpts = {
  filename,
  ast: true
};

const parser = {
  parse(source, opts) {
    return babel.parse(source, {
      ...babelOpts,
      ..._.omit(
        opts, 
        'jsx', 'loc', 'locations', 'range', 'comment', 'onComment', 'tolerant', 'ecmaVersion'
      )
    });
  }
};

const ast = recast.parse(source, {parser});
const result = babel.transformFromAstSync(ast, source, babelOpts);

const printedFromRecastParse = recast.print(ast).code;
const printedFromBabelTransform = recast.print(result.ast).code;

console.log('printedFromRecastParse', printedFromRecastParse);
console.log();
console.log('printedFromBabelTransform', printedFromBabelTransform);

This produces:

printedFromRecastParse
function f(
  a,
  b,
  c
) {
  return d;
}


printedFromBabelTransform "use strict";
function f(a, b, c) {
  return d;
}

Expected behavior
Program output:

printedFromRecastParse
"use strict";
function f(
  a,
  b,
  c
) {
  return d;
}


printedFromBabelTransform "use strict";
function f(
  a,
  b,
  c
) {
  return d;
}

Environment

  System:
    OS: macOS 11.2.1
  Binaries:
    Node: 12.14.0 - ~/.nvm/versions/node/v12.14.0/bin/node
    Yarn: 1.22.5 - /usr/local/bin/yarn
    npm: 6.13.4 - ~/.nvm/versions/node/v12.14.0/bin/npm
  npmPackages:
    @babel/core: 7.13.1 => 7.13.1
    @babel/plugin-syntax-pipeline-operator: 7.12.13 => 7.12.13
    @babel/plugin-transform-typescript: 7.13.0 => 7.13.0
    @babel/preset-env: 7.11.5 => 7.11.5
    @babel/preset-typescript: 7.10.4 => 7.10.4
    babel-jest: 26.3.0 => 26.3.0
    eslint: 7.9.0 => 7.9.0
    jest: 26.4.2 => 26.4.2

• Monorepo: no
• How you are using Babel: Standalone

Possible Solution
This workaround addresses the issue, but it's a bit cumbersome. Here's the snippet above with the workaround applied:

const babel = require('@babel/core');
const recast = require('recast');
const _ = require('lodash');

const filename = 'file.js';
const source = `
function f(
  a,
  b,
  c
) {
  return d;
}
`;

const babelOpts = {
  filename,
  ast: true,
  code: false
};

const parser = {
  parse(source, opts) {
    return babel.parse(source, {
      ...babelOpts,
      ..._.omit(
        opts, 
        'jsx', 'loc', 'locations', 'range', 'comment', 'onComment', 'tolerant', 'ecmaVersion'
      )
    });
  }
};


const ast = recast.parse(source, {parser});

const setAst = babel => ({
  visitor: {
    Program(path) {
      path.replaceWith(ast.program);
    }
  }
});
const result = babel.transformSync('', {
  ...babelOpts,
  plugins: [setAst]
});

const printedFromRecastParse = recast.print(ast).code;
const printedFromBabelTransform = recast.print(result.ast).code;

console.log('printedFromRecastParse', printedFromRecastParse);
console.log();
console.log('printedFromBabelTransform', printedFromBabelTransform);

Additional context

[Babel Bot]

Hey @NickHeiner! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack community that typically always has someone willing to help. You can sign-up here for an invite."

[Nicolò Ribaudo]

You can try passing cloneInputAst: false to transformFromAst's options.

[Nick Heiner]

That fixes it! Thanks.