babel/babel · error · Error

The `annexB` option can only be set to `false`.

Error message

The `annexB` option can only be set to `false`.

What it means

This Error is thrown by getOptions() in @babel/parser when normalizing options. The `annexB` option defaults to true (Annex B web-browser syntax is on); Babel only allows opting out, so any value that is not exactly `false` (true, truthy strings, numbers, objects) is rejected with 'The `annexB` option can only be set to `false`.' It prevents ambiguous enablement of an already-default feature.

Source

Thrown at packages/babel-parser/src/options.ts:259

    // When enabled, the parser will support Annex B syntax.
    // https://tc39.es/ecma262/#sec-additional-ecmascript-features-for-web-browsers
    annexB: true,
  };
}

// Interpret and default an options object

export function getOptions(opts?: Options | null): OptionsWithDefaults {
  // https://github.com/babel/babel/pull/16918
  // `options` is accessed frequently, please make sure it is a fast object.
  // `%ToFastProperties` can make it a fast object, but the performance is the same as the slow object.
  const options: any = createDefaultOptions();

  if (opts == null) {
    return options;
  }
  if (opts.annexB != null && opts.annexB !== false) {
    throw new Error("The `annexB` option can only be set to `false`.");
  }

  for (const key of Object.keys(options) as (keyof Options)[]) {
    if (opts[key] != null) options[key] = opts[key];
  }

  if (options.startLine === 1) {
    if (opts.startIndex == null && options.startColumn > 0) {
      options.startIndex = options.startColumn;
    } else if (opts.startColumn == null && options.startIndex > 0) {
      options.startColumn = options.startIndex;
    }
  } else if (opts.startColumn == null || opts.startIndex == null) {
    throw new Error(
      "With a `startLine > 1` you must also specify `startIndex` and `startColumn`.",
    );
  }

View on GitHub (pinned to 06b6eae39d)

Solutions

1. Omit annexB entirely (defaults to true), or set it to `false` to disable Annex B syntax.
2. Sanitize boolean options to undefined when they should use the default.
3. Fix the option producer so it only emits annexB when the user explicitly disabled it.

Example fix

// before
parse(code, { annexB: true }); // throws

// after
parse(code, {}); // annexB defaults to true
// or, to disable Annex B:
parse(code, { annexB: false });

const opts = { ...userOpts };
if ('annexB' in opts && opts.annexB !== false) {
  delete opts.annexB; // rely on default (true)
}
parse(code, opts);

const isAnnexBOption = (v: unknown): boolean => v === undefined || v === false;

When it happens

Trigger: Passing `{ annexB: true }`, `{ annexB: 'yes' }`, `{ annexB: 1 }`, or `{ annexB: {} }` to parse()/parseExpression() options. The check is `opts.annexB != null && opts.annexB !== false`.

Common situations: Generated config that sets every boolean option to true; migrating from a tool that accepted annexB as a string; copying options objects and flipping flags wholesale; a wrapper that defaults missing booleans to true.

Related errors

• With a `startLine > 1` you must also specify `startIndex` an
• The `allowAwaitOutsideFunction` option cannot be used with `
• `sourceType: 'commonjs'` implies `allowReturnOutsideFunction
• ${msg(access(loc, 1))} must be an object, false, or undefine
• ${descriptor}: '${option}' is not a valid top-level option.

AI-assisted analysis of babel/babel@06b6eae39d (2026-08-03). Data as JSON: /data/errors/5bbc8846ef75902c.json. Report an issue: GitHub.