Error @babel/plugin-proposal-private-property-in-object without declaring it in its
dependencies (or devDependencies) in the package.json file

Error @babel/plugin-proposal-private-property-in-object without declaring it in its dependencies (or devDependencies) in the package.json file


This @babel/plugin-proposal-private-property-in-object version is not meant to
be imported. Something is importing
@babel/plugin-proposal-private-property-in-object without declaring it in its
dependencies (or devDependencies) in the package.json file.
Add "@babel/plugin-proposal-private-property-in-object" to your devDependencies
to work around this error. This will make this message go away.



The error happens when it runs rails server in a Ruby on Rails 6 application.

Current configuration


In the terminal:

npm install --save-dev @babel/plugin-proposal-private-property-in-object

The command npm install --save-dev @babel/plugin-proposal-private-property-in-object is used to install the Babel plugin @babel/plugin-proposal-private-property-in-object as a development dependency in a Node.js project.

Babel is a popular JavaScript compiler used for transpiling modern JavaScript code into an older version of JavaScript that is compatible with a wider range of browsers.

Solution explanation

Here's what the @babel/plugin-proposal-private-property-in-object plugin does and why you might use it:

  • Private class properties

In JavaScript, class properties are typically public, meaning they can be accessed and modified from outside the class. However, there is a proposal in JavaScript to support private class properties, which are properties that can only be accessed and modified from within the class where they are defined. These private properties are denoted by a # symbol before the property name.

  • Babel Plugin:

The @babel/plugin-proposal-private-property-in-object plugin is a Babel plugin that allows you to use private class properties syntax in your JavaScript code, even if it's not natively supported by the JavaScript runtime in your target environment.

  • Transpilation

When you use this Babel plugin, it transpiles your JavaScript code that contains private class properties into equivalent code that can run in environments that don't yet support private class properties natively. It essentially converts your code into a form that is compatible with older browsers or environments.

  • Development Dependency

You install this plugin as a development dependency (--save-dev) because it's primarily used during the development and build process, not at runtime. It's part of your build toolchain, and it helps ensure your code can be executed in a wider range of environments when you bundle and deploy your application.

Stop the server

In order to stop the Rails Server, let's press the following command:


Restart the server

rails server


Access a specific page


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Final thoughts

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