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devDependencies

Learn development packages with devDependencies.

devDependencies

devDependencies is for packages needed only during development: test frameworks, linters, build tools. When running in production these packages are not needed.

For example, jest is a test framework; tests are not run in production. So it is added to devDependencies with npm install --save-dev jest. express runs in production, so it goes into dependencies. When you install with npm install --production, devDependencies are skipped, which keeps the production image small. This way packages are separated by purpose and the deployment is optimized.

The --save-dev flag

The way to add a package to devDependencies is the --save-dev flag. Its short form is -D. When you run npm install --save-dev jest, jest is both downloaded and added to the devDependencies section of package.json.

Without a flag, npm install jest adds the package to dependencies, meaning it counts as a package loaded in production. That is the wrong place for a test framework. It is important to think about which section fits: is it needed in production? If the answer is no, use --save-dev. Then npm install --production skips this package.

Practice Print whether jest is in devDependencies

Define a string named content with the value {"dependencies":{"express":"^4.18.0"},"devDependencies":{"jest":"^29.0.0"}}. Parse it into an object with JSON.parse. Print "jest" in pkg.devDependencies with console.log.

Starter code

index.js
// Define the package.json content (with dependencies and devDependencies)
// Parse it into an object with JSON.parse
// Print whether jest is in devDependencies
// write here

Sample solution

index.js
const content = '{"dependencies":{"express":"^4.18.0"},"devDependencies":{"jest":"^29.0.0"}}';
const pkg = JSON.parse(content);
console.log("jest" in pkg.devDependencies);

Expected output

true

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Match each package to its section

Term Meaning
jest Test framework (dev)
express Web framework (prod)
eslint Code linter (dev)
lodash Utility library (prod)

2 Which one belongs in devDependencies?

index.js
devDependencies
  • mongoose (database)
  • express (web framework)
  • lodash (utility)
  • jest (test framework) Correct answer

Explanation: Test frameworks are needed only during development.

3 Add to devDependencies

Question 
npm install ____ jest
Answer 
npm install --save-dev jest

Explanation: --save-dev adds jest to devDependencies.

All lessons in Intermediate

1 Events

  1. EventEmitter
  2. on and emit
  3. once
  4. removeListener
  5. error event
  6. Summary - Events

2 Streams

  1. What are streams?
  2. Readable stream
  3. Writable stream
  4. pipe
  5. Transform stream
  6. Summary - Streams

3 HTTP server

  1. http.createServer
  2. Request and response
  3. Routing
  4. JSON response
  5. Query string
  6. Summary - HTTP server

4 Project: Helper library

  1. Module design
  2. String helpers
  3. Number helpers
  4. Date helpers
  5. Putting it together

5 Express basics

  1. What is Express?
  2. express()
  3. app.get
  4. app.post
  5. app.listen
  6. Summary - Express basics

6 Middleware

  1. What is middleware?
  2. app.use
  3. Router middleware
  4. Error middleware
  5. Body parser
  6. Summary - Middleware

7 Routing

  1. Route parameters
  2. Query parameters
  3. express.Router
  4. Route groups
  5. Method chaining
  6. Summary - Routing

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 Async patterns

  1. Async iteration
  2. Async generators
  3. Concurrency limit
  4. Retry pattern
  5. Debounce and throttle
  6. Summary - Async patterns

10 Buffer and binary

  1. What is Buffer?
  2. Buffer.from
  3. Buffer.concat
  4. toString and encoding
  5. Buffer slice
  6. Summary - Buffer

11 Process and CLI

  1. process.argv
  2. process.env
  3. process.exit
  4. process.stdin/stdout
  5. Child process
  6. Summary - Process

12 Project: Library API

  1. Model design
  2. Express app
  3. CRUD routes
  4. Middleware
  5. Putting it together

13 npm and packages

  1. npm install
  2. package.json scripts
  3. devDependencies
  4. Semver
  5. npm publish
  6. Summary - npm

14 Error handling

  1. The Error class
  2. try/catch
  3. Custom errors
  4. Error middleware
  5. Unhandled rejection
  6. Summary - Error handling

15 Testing basics

  1. node:test
  2. assert
  3. describe and it
  4. Before and after
  5. Mock
  6. Summary - Testing basics

16 Debugging

  1. node inspect
  2. Chrome DevTools
  3. console methods
  4. util.inspect
  5. Source maps
  6. Summary - Debugging

17 Date and time

  1. The Date object
  2. Date methods
  3. Intl.DateTimeFormat
  4. performance.now
  5. setTimeout and setInterval
  6. Summary - Date and time

18 Crypto

  1. crypto module
  2. createHash
  3. randomBytes
  4. HMAC
  5. AES
  6. Summary - Crypto

19 URL and query string

  1. URL object
  2. URLSearchParams
  3. URL properties
  4. URL.resolve
  5. querystring
  6. Summary - URL and query string

20 Intermediate final

  1. Project design
  2. Express API
  3. fs persistence
  4. Middleware
  5. Putting it together