Working with package.json

The best way to manage locally installed npm packages is to create a package.json file.

A package.json file:

• lists the packages that your project depends on.
• allows you to specify the versions of a package that your project can use using semantic versioning rules.
• makes your build reproducible, and therefore much easier to share with other developers.

Requirements

A package.json must have:

• "name"
  • all lowercase
  • one word, no spaces
  • dashes and underscores allowed
• "version"
  • in the form of x.x.x
  • follows semver spec

For example:

{
  "name": "my-awesome-package",
  "version": "1.0.0"
}

Creating a package.json

There are two basic ways to create a package.json file.

1. Run a CLI questionnaire

To create a package.json with values that you supply, run:

> npm init

This will initiate a command line questionnaire that will conclude with the creation of a package.json in the directory in which you initiated the command.

2. Create a default package.json

To get a default package.json, run npm init with the --yes or -y flag:

> npm init --yes

This method will generate a default package.json using information extracted from the current directory.

> npm init --yes
Wrote to /home/ag_dubs/my_package/package.json:

{
  "name": "my_package",
  "description": "",
  "version": "1.0.0",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "repository": {
    "type": "git",
    "url": "https://github.com/ashleygwilliams/my_package.git"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "bugs": {
    "url": "https://github.com/ashleygwilliams/my_package/issues"
  },
  "homepage": "https://github.com/ashleygwilliams/my_package"
}

• name: the current directory name
• version: always 1.0.0
• description: info from the readme, or an empty string ""
• main: always index.js
• scripts: by default creates an empty test script
• keywords: empty
• author: empty
• license: ISC
• bugs: info from the current directory, if present
• homepage: info from the current directory, if present

You can also set several config options for the init command. Some useful ones:

> npm set init.author.email "wombat@npmjs.com"
> npm set init.author.name "ag_dubs"
> npm set init.license "MIT"

NOTE:

If there is no description field in the package.json, npm uses the first line of the README.md or README instead. The description helps people find your package when searching npm, so it's definitely useful to make a custom description in the package.json to make your package easier to find.

How to Customize the package.json questionnaire

If you expect to create many package.json files, you might wish to customize the questions asked during the init process, so that the files always contain key information that you expect. You can customize the fields as well as the questions that are asked.

To do this, you create a custom .npm-init.js in your home directory ~/.npm-init.js.

A simple .npm-init.js might look something like this:

module.exports = {
  customField: 'Custom Field',
  otherCustomField: 'This field is really cool'
}

Running npm init with this file in your home directory would output a package.json that included these lines:

{
  customField: 'Custom Field',
  otherCustomField: 'This field is really cool'
}

You can also customize the questions by using the prompt function.

  module.exports = prompt("what's your favorite flavor of ice cream, buddy?", "I LIKE THEM ALL");

To learn more about how to create advanced customizations, check out the docs for init-package-json

Specifying Dependencies

To specify the packages your project depends on, you need to list the packages you'd like to use in your package.json file. There are 2 types of packages you can list:

• "dependencies": These packages are required by your application in production.
• "devDependencies": These packages are only needed for development and testing.

Manually editing your package.json

You can manually edit your package.json. You'll need to create an attribute in the package object called dependencies that points to an object. This object will hold attributes that name the packages you'd like to use. It will point to a semver expression that specifies the versions of that project that are compatible with your project.

If you have dependencies you only need to use during local development, follow the same instructions as above but use the attribute called devDependencies.

For example, the project below uses any version of the package my_dep that matches major version 1 in production and requires any version of the package my_test_framework that matches major version 3, but only for development:

{
  "name": "my_package",
  "version": "1.0.0",
  "dependencies": {
    "my_dep": "^1.0.0"
  },
  "devDependencies" : {
    "my_test_framework": "^3.1.0"
  }
}

The --save and --save-dev install flags

The easier (and more awesome) way to add dependencies to your package.json is to do so from the command line, flagging the npm install command with either --save or --save-dev, depending on how you'd like to use that dependency.

To add an entry to your package.json's dependencies:

npm install <package_name> --save

To add an entry to your package.json's devDependencies:

npm install <package_name> --save-dev

Managing dependency versions

npm uses Semantic Versioning, or, as we often refer to it, SemVer, to manage versions and ranges of versions of packages.

If you have a package.json file in your directory and you run npm install, npm will look at the dependencies that are listed in that file and download the latest versions, using semantic versioning.

Learn More

To understand more about the power of package.json, see the video "Installing npm packages locally" which you can find in Chapter 8.

To learn more about semantic versioning, see Getting Started "Semver" page.

Last modified February 13, 2023           Found a typo? Send a pull request!