Repository: eslint/eslint
File: lib/config/config.js (JavaScript)

Code before:
        schema.validate(config);

        // first, copy all the other keys over
        Object.assign(this, otherKeys);

        // ensure that a language is specified
        if (!language) {
            throw new TypeError("Key 'language' is required.");
        }

        // copy the rest over
        this.plugins = plugins;
        this.language = language;

        // Check language value
        const { pluginName: languagePluginName, objectName: localLanguageName } = splitPluginIdentifier(language);

        this.#languageName = language;

        if (!plugins || !plugins[languagePluginName] || !plugins[languagePluginName].languages || !plugins[languagePluginName].languages[localLanguageName]) {
            throw new TypeError(`Key "language": Could not find "${localLanguageName}" in plugin "${languagePluginName}".`);
        }

        this.language = plugins[languagePluginName].languages[localLanguageName];

        if (this.language.defaultLanguageOptions || languageOptions) {
            this.languageOptions = flatConfigSchema.languageOptions.merge(
                this.language.defaultLanguageOptions,
                languageOptions
            );
        }

        // Validate language options
        if (this.languageOptions) {
            try {
                this.language.validateLanguageOptions(this.languageOptions);
            } catch (error) {
                throw new TypeError(`Key "languageOptions": ${error.message}`, { cause: error });
            }
        }

        // Check processor value
        if (processor) {
            this.processor = processor;

            if (typeof processor === "string") {
                const { pluginName, objectName: localProcessorName } = splitPluginIdentifier(processor);

                this.#processorName = processor;

                if (!plugins || !plugins[pluginName] || !plugins[pluginName].processors || !plugins[pluginName].processors[localProcessorName]) {

Review comment (suggestion):
We only need to check if it's undefined so I think `??` is more appropriate.

```suggestion
        if (this.language.defaultLanguageOptions ?? languageOptions) {
```

Code after:
        schema.validate(config);

        // first, copy all the other keys over
        Object.assign(this, otherKeys);

        // ensure that a language is specified
        if (!language) {
            throw new TypeError("Key 'language' is required.");
        }

        // copy the rest over
        this.plugins = plugins;
        this.language = language;

        // Check language value
        const { pluginName: languagePluginName, objectName: localLanguageName } = splitPluginIdentifier(language);

        this.#languageName = language;

        if (!plugins || !plugins[languagePluginName] || !plugins[languagePluginName].languages || !plugins[languagePluginName].languages[localLanguageName]) {
            throw new TypeError(`Key "language": Could not find "${localLanguageName}" in plugin "${languagePluginName}".`);
        }

        this.language = plugins[languagePluginName].languages[localLanguageName];

        if (this.language.defaultLanguageOptions ?? languageOptions) {
            this.languageOptions = flatConfigSchema.languageOptions.merge(
                this.language.defaultLanguageOptions,
                languageOptions
            );
        } else {
            this.languageOptions = {};
        }

        // Validate language options
        try {
            this.language.validateLanguageOptions(this.languageOptions);
        } catch (error) {
            throw new TypeError(`Key "languageOptions": ${error.message}`, { cause: error });
        }

        // Check processor value
        if (processor) {
            this.processor = processor;

            if (typeof processor === "string") {
                const { pluginName, objectName: localProcessorName } = splitPluginIdentifier(processor);

                this.#processorName = processor;

                if (!plugins || !plugins[pluginName] || !plugins[pluginName].processors || !plugins[pluginName].processors[localProcessorName]) {