Intermediate everything in Backend Dev, page 4
Propagating async errors to Express error handlers
Express does not auto-catch rejected promises, so catch and call next(err), or wrap handlers in an asyncHandler that forwards rejections; Express 5 awaits handlers automatically.
Securing Express with Passport local strategy
Configure LocalStrategy with a verify callback, call passport.authenticate as route middleware, and set up serializeUser/deserializeUser for sessions.
Session-based versus token-based authentication
Sessions store server-side state with a cookie id, tokens carry self-contained claims with no server store, weigh revocation versus scalability, especially across services.
Mongoose pre('save') hooks for password hashing
Pre('save') runs before persistence; use it to hash the password, guarding with isModified, calling next() or returning.
Database migrations with the Sequelize CLI
Migrations are version-controlled scripts with up/down so teams apply identical schema changes; use sequelize-cli to generate, edit with addColumn, then db:migrate.
Mongoose populate() for referenced documents
Populate() replaces stored ObjectIds with the referenced documents, needs a ref in the schema, called via .populate('author').
404 vs 500: missing resource vs server failure
A missing resource returns 404 Not Found (client asked for something absent); a database failure returns 500 Internal Server Error (server-side fault).
Idempotency: PUT vs POST in REST
Idempotent means repeated identical requests leave the same server state; PUT is idempotent, POST is not. Use PUT to overwrite a resource at a known URL.
Modularize routes with express.Router
Create a Router per resource in its own file, define routes on it, export it, and mount with app.use('/products', router).
Middleware execution order and sharing data via req
Middleware runs top-down in registration order; each calls next(); attach data like req.user that later handlers read.
Express error-handling middleware signature
(err, req, res, next) with err first, identified by arity, placed last after all routes.
Application-level vs router-level middleware
App.use() binds to the app and runs everywhere; router.use() binds to a Router and runs only for that router's routes.
Express route order and matching
Express checks routes in definition order, /items/new matches the literal route first, if /:id came first new would be captured as an id.
res.send vs res.json vs res.end
Send is flexible and sets content type by type, json serializes and sets JSON content type, end is the raw http terminator with no body helpers.
Parsing JSON bodies with express.json
Register express.json via app.use so it reads the request stream and populates req.body before handlers run, place it before routes.
fs.watch vs fs.watchFile
Watch uses OS event notifications, efficient but inconsistent across platforms, watchFile polls stat at an interval, reliable but slower.
path.resolve vs path.join
Join concatenates and normalizes relative segments, resolve builds an absolute path right to left from cwd and resets on any absolute segment.
Reading a POST body from the request stream
Body arrives in chunks via data events, accumulate them, on the end event concatenate and JSON.parse inside try/catch.
Counting lines in a 5GB log file efficiently
ReadFile loads all 5GB into RAM and may exceed buffer limits, instead stream with createReadStream plus readline and count line by line.
Comparing the three async error-handling styles
Callbacks pass err as first arg; Promises route errors to catch; async/await uses try/catch; an unhandled rejection can crash the Node process.
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