05 - Using a `package.json`

Using a package.json

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

A package.json file affords you a lot of great things:

1. It serves as documentation for what packages your project depends on.
2. It allows you to specify the versions of a package that your project can use using semantic versioning rules.
3. Makes your build reproducible which means that its way easier to share with other developers.

Requirements

As a bare minimum, 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

To create a package.json run:

> npm init

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

The --yes init flag

The extended CLI Q&A experience is not for everyone, and often if you are comfortable with using a package.json you'd like a more expedited experience.

You can get a default package.json by running npm init with the --yes or -y flag:

> npm init --yes

This will ask no questions, and instead 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",
  "description": "",
  "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, else an empty string ""
• main: always index.js
• scripts: by default creates a 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 on npm search, so it's definitely useful to make a custom description in the package.json to make your package more discoverable.

Customizing the init process

It is also possible to totally customize the information created and the questions asked during the init process. This is done by creating a custom .npm-init.js. By default, npm will look in your home directory. ~/.npm-init.js

A simple .npm-init.js could 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 similiar to this:

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

Customizing the questions is also possible, by using the prompt function.

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

To learn more on how to create more advanced customizations, checkout the docs for init-package-json

Specifying Packages

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 named after the packages you'd like to use, that point to a semver expression that specifies what versions of that project are compatible with your project.

If you have dependencies you only need to use during local development, you will follow the same instructions as above but in an 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, then npm will look at the dependencies that are listed in that file and download the latest versions satisfying semver rules for all of those.

To learn more about semantic versioning, check out our Getting Started "Semver" page.