[Bug] TypeError: "e is not iterable" in JSON worker initialization with monaco-editor@0.55.x and webpack-plugin@7.1.0

Reproducible in monaco-editor-playground?

No - this is specific to the webpack plugin bundling setup.

Version

• monaco-editor: 0.55.1
• monaco-editor-webpack-plugin: 7.1.0
• @monaco-editor/react: 4.7.0
• Next.js: 15.2.4

Browser and OS info

Chrome (latest), macOS

Description

When using monaco-editor 0.55.x with the webpack plugin to load the JSON language, the JSON worker fails to initialize with a "TypeError: e is not iterable" error.

Steps to reproduce

1. Create a Next.js project with monaco-editor and the webpack plugin
2. Configure next.config.mjs:

import MonacoWebpackPlugin from 'monaco-editor-webpack-plugin';

const nextConfig = {
    webpack: (config, { isServer }) => {
        if (!isServer) {
            config.plugins.push(
                new MonacoWebpackPlugin({
                    languages: ['json', 'plaintext'],
                    filename: 'static/[name].worker.js',
                })
            );
        }
        return config;
    },
};

export default nextConfig;

3. Configure monaco loader to use the installed monaco-editor:

import { loader } from '@monaco-editor/react';

const monaco = await import('monaco-editor');
loader.config({ monaco });

4. Render a Monaco Editor with language="json"
5. Navigate to the page in Chrome

Actual behavior

The JSON worker fails to initialize with the following error:

json.worker.js:1 Uncaught TypeError: e is not iterable
    at json.worker.js:1:77543
    at rJ.initialize (json.worker.js:1:77630)
    at rJ._handleMessage (json.worker.js:1:76640)
    at Object.handleMessage (json.worker.js:1:76478)
    at rz._handleRequestMessage (json.worker.js:1:75261)
    at rz._handleMessage (json.worker.js:1:74713)
    at rz.handleMessage (json.worker.js:1:74601)
    at rJ.onmessage (json.worker.js:1:76570)
    at globalThis.onmessage (json.worker.js:1:253773)

Expected behavior

The JSON worker should initialize successfully and provide JSON language features.

Root Cause Analysis

The error occurs in the vscode-json-languageservice when the configure() method tries to iterate over settings. Looking at the source:

In jsonLanguageService.ts:

configure: (settings) => {
    jsonSchemaService.clearExternalSchemas();
    settings.schemas?.forEach(jsonSchemaService.registerExternalSchema.bind(jsonSchemaService));
    // ...
}

In jsonSchemaService.ts setSchemaContributions():

for (let schemaAssociation of schemaAssociations) { ... }

The worker receives initialization data via workerManager.js:

createData: {
    languageSettings: this._defaults.diagnosticsOptions,
    languageId: this._defaults.languageId,
    enableSchemaRequest: this._defaults.diagnosticsOptions.enableSchemaRequest
}

Suspected cause: Monaco 0.55.0 introduced breaking namespace changes (moving languages.json to json). The webpack plugin may be passing initialization data in an incompatible format, or there's a mismatch in how the bundled vscode-json-languageservice expects to receive configuration.

Workaround

Downgrading to monaco-editor@0.52.0 resolves the issue:

npm install monaco-editor@0.52.0

Additional context

• The error does not occur with monaco-editor 0.52.0
• The webpack plugin repository was archived in Nov 2023 and moved into the main monaco-editor repo
• Related changelog entry in 0.55.0: "Moves nested namespaces (languages.css, languages.html, languages.json, languages.typescript) to top level namespaces"