[Bug]: generating source maps fails due to sourcesContent being undefined

[christian-bromann]

💻

• Would you like to work on a fix?

How are you using Babel?

Programmatic API (babel.transform, babel.parse)

Input code

I am trying to transform code via:

const { transformSync } = require('@babel/core');
const codeMap = transformSync(code, babelOpts);

where code is:

 var __defProp = Object.defineProperty;
var __getOwnPropDesc = Object.getOwnPropertyDescriptor;
var __decorateClass = (decorators, target, key, kind) => {
  var result = kind > 1 ? void 0 : kind ? __getOwnPropDesc(target, key) : target;
  for (var i = decorators.length - 1, decorator; i >= 0; i--)
    if (decorator = decorators[i])
      result = (kind ? decorator(target, key, result) : decorator(result)) || result;
  if (kind && result)
    __defProp(target, key, result);
  return result;
};
import { LitElement, css, html } from "lit";
import { customElement, property } from "lit/decorators.js";
export let SimpleGreeting = class extends LitElement {
  constructor() {
    super(...arguments);
    // eslint-disable-next-line indent
    this.name = "World";
  }
  // Render the UI as a function of component state
  render() {
    return html`<p>Hello, ${this.name}! ${this.getQuestion()}</p>`;
  }
  getQuestion() {
    return "How are you today?";
  }
};
// Define scoped styles right with your component, in plain CSS
SimpleGreeting.styles = css`
    :host {
        color: blue;
    }`;
__decorateClass([
  property()
], SimpleGreeting.prototype, "name", 2);
SimpleGreeting = __decorateClass([
  customElement("simple-greeting")
], SimpleGreeting);

and babelOpts is:

{
  configFile: false,
  babelrc: false,
  ast: true,
  filename: '/webdriverio/examples/wdio/browser-runner/components/LitComponent.ts',
  inputSourceMap: {
    version: 3,
    sources: [
      '/webdriverio/examples/wdio/browser-runner/components/LitComponent.ts'
    ],
    mappings: ';;;;;;;;;;;AAAA,SAAS,YAAY,KAAK,YAAY;AACtC,SAAS,eAAe,gBAAgB;AAGjC,WAAM,iBAAN,cAA6B,WAAW;AAAA,EAAxC;AAAA;AAUH;AAAA,gBAAgB;AAAA;AAAA;AAAA,EAGhB,SAAS;AACL,WAAO,iBAAiB,KAAK,SAAS,KAAK,YAAY;AAAA,EAC3D;AAAA,EAEA,cAAe;AACX,WAAO;AAAA,EACX;AACJ;AAAA;AApBa,eAEF,SAAS;AAAA;AAAA;AAAA;AAQhB;AAAA,EAFC,SAAS;AAAA,GARD,eAUT;AAVS,iBAAN;AAAA,EADN,cAAc,iBAAiB;AAAA,GACnB;',
    names: []
  },
  sourceMaps: true,
  compact: false,
  comments: true,
  parserOpts: {
    allowReturnOutsideFunction: true,
    sourceType: 'module',
    plugins: [
      'asyncGenerators',
      'bigInt',
      'classProperties',
      'classPrivateProperties',
      'classPrivateMethods',
      'dynamicImport',
      'importMeta',
      'numericSeparator',
      'objectRestSpread',
      'optionalCatchBinding',
      'topLevelAwait'
    ]
  },
  plugins: [ [ [Function (anonymous)] ] ]
}

Configuration file name

No response

Configuration

n/a

Current and expected behavior

It currently fails with latest release on:

 Cannot read properties of undefined (reading '0')
/examples/wdio/browser-runner/components/LitComponent.ts

    at new SourceMap (/packages/wdio-browser-runner/node_modules/@babel/generator/lib/source-map.js:31:99)
    at new Generator (/packages/wdio-browser-runner/node_modules/@babel/generator/lib/index.js:13:35)
    at generate (/packages/wdio-browser-runner/node_modules/@babel/generator/lib/index.js:88:15)
    at generateCode (/packages/wdio-browser-runner/node_modules/@babel/core/lib/transformation/file/generate.js:48:39)
    at run (/packages/wdio-browser-runner/node_modules/@babel/core/lib/transformation/index.js:39:33)
    at run.next (<anonymous>)
    at transform (/packages/wdio-browser-runner/node_modules/@babel/core/lib/transform.js:22:41)
    at transform.next (<anonymous>)
    at evaluateSync (/packages/wdio-browser-runner/node_modules/gensync/index.js:251:28)
    at sync (/packages/wdio-browser-runner/node_modules/gensync/index.js:89:14)
    at stopHiding - secret - don't use this - v1 (/packages/wdio-browser-runner/node_modules/@babel/core/lib/errors/rewrite-stack-trace.js:46:12)
    at transformSync (/packages/wdio-browser-runner/node_modules/@babel/core/lib/transform.js:43:76)
    at Instrumenter.instrumentSync (/packages/wdio-browser-runner/node_modules/istanbul-lib-instrument/src/instrumenter.js:102:25)
    at TransformContext.transform (file:///packages/wdio-browser-runner/node_modules/vite-plugin-istanbul/dist/index.mjs:109:35)
    at Object.transform (file:///packages/wdio-browser-runner/node_modules/vite/dist/node/chunks/dep-ae3fae1b.js:41674:44)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async loadAndTransform (file:///packages/wdio-browser-runner/node_modules/vite/dist/node/chunks/dep-ae3fae1b.js:39479:29

because when creating the trace map at

this._inputMap = new _traceMapping.TraceMap(opts.inputSourceMap);

the returned object has sourcesContent: undefined,.

Environment

@babel/generator: 7.21.0
Node.js v18.7.0
NPM v8.15.0
macOS

Possible solution

No response

Additional context

This happened during recent Babel updates. It is hard for me to say what is the root cause of this. I am working on WebdriverIO and we use Babel as part of vite-plugin-istanbul to generate code coverage reports.

You can reproduce this by checking out https://github.com/webdriverio/component-testing-examples/tree/main/create-react-app and run any of the example directories.

I am aware that this is not necessary an issue of Babel but would love to understand what the root cause of this is and am happy to work on a fix if someone can give me any pointers.

[babel-bot]

Hey @christian-bromann! 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.

[liuxingbaoyu]

Thanks for the detailed report, this should be fixed by #15445.
cc @stefcameron

The reason for the problem is that we didn't handle the absence of sourcesContent in the input source map in the latest version.

[stefcameron]

Thank you @liuxingbaoyu

[christian-bromann]

Awesome, good to know it is being addressed, good job team.

[JLHwung]

Fixed in @babel/generator 7.21.1

[christian-bromann]

Thanks for the quick turnaround folks 👏