If you work with JavaScript, or you’ve ever interacted with a JavaScript project, Node.js or a frontend project, you surely met the package.json file.

What’s that for? What should you know about it, and what are some of the cool things you can do with it?

The package.json file is kind of a manifest for your project. It can do a lot of things, completely unrelated. It’s a central repository of configuration for tools, for example. It’s also where npm and yarn store the names and versions for all the installed packages.

The file structure

Here’s an example package.json file:

It’s empty! There are no fixed requirements of what should be in a package.json file, for an application. The only requirement is that it respects the JSON format, otherwise it cannot be read by programs that try to access its properties programmatically.

If you’re building a Node.js package that you want to distribute over npm things change radically, and you must have a set of properties that will help other people use it. We’ll see more about this later on.

This is another package.json:

It defines a name property, which tells the name of the app, or package, that’s contained in the same folder where this file lives.

Here’s a much more complex example, which was extracted from a sample Vue.js application:

there are lots of things going on here:

• version indicates the current version
• name sets the application/package name
• description is a brief description of the app/package
• main set the entry point for the application
• private if set to true prevents the app/package to be accidentally published on npm
• scripts defines a set of node scripts you can run
• dependencies sets a list of npm packages installed as dependencies
• devDependencies sets a list of npm packages installed as development dependencies
• engines sets which versions of Node.js this package/app works on