Docker containerization for Node.js in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when this Dockerfile runs?

Consider this Dockerfile for a Node.js app. What will be the output when the container starts?

Node.js

FROM node:20-alpine
WORKDIR /app
COPY package.json ./
RUN npm install
COPY . .
CMD ["node", "index.js"]

A The container runs the Node.js app defined in index.js after installing dependencies.

B The container fails because it does not copy package-lock.json before npm install.

C The container runs but skips npm install because package.json is missing.

D The container runs but uses the default Node.js REPL instead of the app.

💡 Hint

Think about the order of commands and what files are copied before npm install.

The Dockerfile copies package.json first, runs npm install to install dependencies, then copies the rest of the app files. Finally, it runs the app with node index.js.

📝 Syntax

intermediate

Identify the syntax error in this Dockerfile snippet

Which option correctly fixes the syntax error in this Dockerfile snippet?

Node.js

FROM node:20
WORKDIR /app
COPY package.json ./
RUN npm install
COPY . .
CMD node index.js

A Replace COPY . . with COPY ./ ./app

B Add a missing colon after RUN npm install:

C Add a semicolon after WORKDIR /app;

D Change CMD node index.js to CMD ["node", "index.js"]

💡 Hint

Look at how CMD expects its arguments in JSON array format.

CMD should be an array of strings to avoid shell parsing issues. Using CMD ["node", "index.js"] is the correct syntax.

🔧 Debug

advanced

Why does this container fail to start?

This Dockerfile builds a Node.js app but the container fails immediately after starting. What is the cause?

Node.js

FROM node:20
WORKDIR /app
COPY . .
RUN npm install
CMD ["node", "server.js"]

A COPY . . copies files after npm install, so node_modules is overwritten.

B npm install runs before copying package.json, so dependencies are missing.

C The CMD command is missing the shell form, causing a syntax error.

D The working directory /app does not exist, causing a runtime error.

💡 Hint

Consider the order of COPY and RUN commands and what files get overwritten.

Since COPY . . happens before npm install, node_modules is created. But if COPY . . happens after npm install, it overwrites node_modules, causing missing dependencies.

❓ state_output

advanced

What is the value of NODE_ENV inside the container?

Given this Dockerfile snippet, what will be the value of the environment variable NODE_ENV inside the running container?

Node.js

FROM node:20
ENV NODE_ENV=production
WORKDIR /app
COPY package.json ./
RUN npm install
COPY . .
CMD ["node", "app.js"]

A "NODE_ENV"

B undefined

C "production"

D "development"

💡 Hint

Look at the ENV instruction and how environment variables are set in Dockerfiles.

The ENV instruction sets NODE_ENV to "production" for all subsequent instructions and the running container.

🧠 Conceptual

expert

Which Dockerfile layering strategy optimizes build speed for Node.js apps?

To speed up Docker builds for a Node.js app, which layering order is best?

A Copy package.json and package-lock.json, run npm install, then copy app source files.

B Copy all source files first, then run npm install.

C Run npm install before copying any files.

D Copy only node_modules folder, then copy source files.

💡 Hint

Think about which files change frequently and which don't.

Copying package.json and package-lock.json first and running npm install caches dependencies. Then copying source files avoids reinstalling dependencies if only source changes.