Installing packages (dependencies vs devDependencies) in Node.js - Visual Walkthrough

Concept Flow - Installing packages (dependencies vs devDependencies)

Start: Need a package

↓

Choose package type

↓

dependencies

↓

Install package

↓

Save in package.json

↓

Use package in app or dev

↓

Run npm install to get packages

This flow shows choosing between dependencies and devDependencies, installing the package, saving it in package.json, and using it in the project.

Execution Sample

Node.js

npm install express
npm install --save-dev jest

Installs 'express' as a runtime dependency and 'jest' as a development-only dependency.

Execution Table

Step	Command	Package Type	package.json Entry	Effect
1	npm install express	dependencies	"dependencies": { "express": "^version" }	express added for runtime use
2	npm install --save-dev jest	devDependencies	"devDependencies": { "jest": "^version" }	jest added for development only
3	npm install	all	Reads package.json	Installs all dependencies and devDependencies locally
4	Run app	runtime	Uses dependencies only	Only dependencies are needed to run app
5	Run tests	development	Uses devDependencies	devDependencies used during testing
6	Exit	-	-	Installation complete, packages ready

💡 All packages installed and saved correctly in package.json under proper sections

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
package.json dependencies	{}	{"express": "^version"}	{"express": "^version"}	{"express": "^version"}	{"express": "^version"}
package.json devDependencies	{}	{}	{"jest": "^version"}	{"jest": "^version"}	{"jest": "^version"}
node_modules folder	empty	express installed	express + jest installed	express + jest installed	express + jest installed

Key Moments - 3 Insights

Why do we use --save-dev for some packages?

Packages installed with --save-dev go into devDependencies (see execution_table step 2). They are only needed during development, like testing tools, not when running the app.

What happens if I run npm install without arguments?

npm install reads package.json and installs all dependencies and devDependencies locally (see execution_table step 3). This prepares the project for both running and development.

Are devDependencies installed in production?

By default, devDependencies are not installed in production environments. Only dependencies are installed to keep production lightweight (see execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution table, after which step is 'jest' added to devDependencies?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Check the 'package.json Entry' column for devDependencies in the execution_table rows.

At which step does npm install add all packages to node_modules?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Look for the step where 'node_modules folder' shows both express and jest installed in variable_tracker.

If you only want packages needed to run the app, which package.json section do you check?

A dependencies

B devDependencies

C scripts

D peerDependencies

💡 Hint

Refer to execution_table step 4 where runtime uses only dependencies.

Concept Snapshot

npm install package: adds package to dependencies for runtime use
npm install --save-dev package: adds package to devDependencies for development only
package.json tracks these sections separately
npm install installs all packages listed
Use dependencies to run app, devDependencies for development tasks

Full Transcript

This lesson shows how to install packages in Node.js projects using npm. When you run 'npm install express', express is added to dependencies in package.json and is needed to run the app. When you run 'npm install --save-dev jest', jest is added to devDependencies and is only needed during development, like testing. Running 'npm install' without arguments installs all packages listed in both dependencies and devDependencies. In production, only dependencies are installed to keep the app lightweight. This helps keep development tools separate from runtime code.

Practice

1. Which command installs a package as a regular dependency needed to run your Node.js app?

easy

A. npm uninstall express

B. npm install --save-dev express

C. npm install express

D. npm update express

Solution

1. Step 1: Understand the difference between dependencies and devDependencies

   Dependencies are packages needed when your app runs, while devDependencies are only needed during development.

2. Step 2: Identify the correct npm command for dependencies

   The command npm install express installs the package as a regular dependency by default.

3. Final Answer:

   npm install express -> Option C

4. Quick Check:

   Regular dependencies use npm install without flags [OK]

Hint: Use plain npm install for runtime dependencies [OK]

Common Mistakes:

• Using --save-dev for packages needed at runtime
• Confusing uninstall with install commands
• Assuming npm update installs packages

2. Which of the following commands correctly installs a package as a development dependency?

easy

A. npm install lodash

B. npm install --save-dev jest

C. npm install --prod jest

D. npm install --global jest

Solution

1. Step 1: Recognize the flag for devDependencies

   The flag --save-dev tells npm to install the package as a development dependency.

2. Step 2: Match the command with the correct flag

   npm install --save-dev jest installs Jest as a devDependency, which is used for testing during development.

3. Final Answer:

   npm install --save-dev jest -> Option B

4. Quick Check:

   Dev dependencies use --save-dev flag [OK]

Hint: Use --save-dev flag for development-only packages [OK]

Common Mistakes:

• Omitting --save-dev for dev dependencies
• Using --prod which is not a valid flag for devDependencies
• Installing globally instead of locally

3. Given the following package.json snippet, which package will NOT be installed when running npm install --production?

{
  "dependencies": {
    "express": "^4.18.2"
  },
  "devDependencies": {
    "mocha": "^10.2.0"
  }
}

medium

A. mocha

B. express

C. both express and mocha

D. neither express nor mocha

Solution

1. Step 1: Understand the effect of --production flag

   The --production flag tells npm to install only the packages listed under dependencies, skipping devDependencies.

2. Step 2: Identify which packages are devDependencies

   Here, mocha is under devDependencies, so it will NOT be installed with npm install --production.

3. Final Answer:

   mocha -> Option A

4. Quick Check:

   DevDependencies skipped with --production flag [OK]

Hint: npm install --production skips devDependencies [OK]

Common Mistakes:

• Assuming all packages install regardless of flags
• Confusing dependencies with devDependencies
• Thinking --production installs devDependencies

4. You run npm install --save-dev eslint but later realize you need eslint as a regular dependency. What is the best way to fix this?

medium

A. Run npm install eslint to add it as a dependency

B. Run npm install --save-prod eslint

C. Manually edit package.json to move eslint from devDependencies to dependencies

D. Run npm uninstall eslint then npm install eslint

Solution

1. Step 1: Understand npm behavior with existing packages

   Running npm install eslint alone will not move it from devDependencies to dependencies if already installed as devDependency.

2. Step 2: Properly remove and reinstall as dependency

   Uninstalling first removes it from devDependencies, then reinstalling adds it to dependencies.

3. Final Answer:

   Run npm uninstall eslint then npm install eslint -> Option D

4. Quick Check:

   Uninstall then install moves package between dependency types [OK]

Hint: Uninstall then reinstall to change dependency type [OK]

Common Mistakes:

• Trying to install without uninstalling first
• Editing package.json manually without reinstalling
• Using non-existent --save-prod flag

5. You want to install webpack and babel packages. webpack is needed to run your app, but babel is only needed during development. Which commands correctly install them with proper dependency types?

hard

A. npm install webpack && npm install --save-dev babel

B. npm install webpack && npm install babel

C. npm install --save-dev webpack && npm install babel

D. npm install --save-dev webpack babel

Solution

1. Step 1: Identify runtime vs development packages

   Webpack is needed at runtime, so it should be a regular dependency. Babel is only for development, so it should be a devDependency.

2. Step 2: Use correct npm commands for each

   Use npm install webpack for runtime dependency and npm install --save-dev babel for development dependency.

3. Final Answer:

   npm install webpack && npm install --save-dev babel -> Option A

4. Quick Check:

   Runtime = npm install, Dev = npm install --save-dev [OK]

Hint: Install runtime normally, dev tools with --save-dev [OK]

Common Mistakes:

• Installing all packages as devDependencies
• Installing dev tools as regular dependencies
• Using --save-dev for runtime packages