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What is inside
20 chapters, in the order the interviews tend to ask them:
The Node.js runtime and the event loop
Modules, packages and project tooling
TypeScript across the MERN stack
Document modelling and schema design
CRUD operations and the query language
Aggregation pipelines and reporting
Indexes, plans and collection performance
Replication, transactions and horizontal scale
Mongoose schemas and the object document mapper
Mongoose queries, relations and projections
MongoDB security and access control
Express routing and the middleware chain
REST API design for a MERN service
Request validation and centralised error handling
Authentication and authorisation with Express
The React client shell and routing
Server state, caching and mutation in the client
Forms, validation and accessible client contracts
Testing a MERN application end to end
Deploying and operating a MERN application
How the answers are built
Each chapter opens with the checklist the question is really testing. A few from this pack:
Each phase drains its own queue, and after every callback Node empties the nextTick queue and then the microtask queue, so a promise continuation beats both a timer and an I/O callback registered in the same turn.
Node decides a file's module system from the nearest package.json type field, with .mjs and .cjs as per file overrides, so one missing field can change how every file in a directory loads.
A type is erased at compile time, so it describes intent for your own code and says nothing about the bytes on the wire, which is why validation still has to happen where data enters.
Embedding against referencing is a decision about access pattern and growth bound, not about which arrangement looks tidier on paper.
A query narrows and is safe to repeat, while an update mutates and may not be idempotent, so an increment applied twice counts twice, and that difference decides whether a retry is safe.
A pipeline is a sequence of stages, each consuming the output of the last, so a $match on a field that an earlier $project already removed matches nothing and returns an empty result rather than an error.
Questions this pack covers
How does phase ordering decide the order between setImmediate and setTimeout?
What actually runs on the libuv thread pool rather than on the main thread?
What actually breaks when you mix require and import in the same project?
How does Node decide whether a given file is an ES module?
How do you share a type between the API and the client in a MERN app?
What does a discriminated union buy you over a boolean flag?
When should a related document be embedded rather than referenced?
How does the document size limit constrain an embedded array?
Each one is answered in full in the pack, with the reasoning written out rather than a bullet list.
