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Which versions does this cheat sheet target?
The examples use Express 5, Node.js 18 or later, TypeScript, and Mongoose v8-style model typing. Express 5’s documented minimum runtime is Node.js 18. Express 4 and Express 5 are not interchangeable: moving an existing Express 4 app to 5 can require changes for breaking compatibility differences.
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| Choice | Runtime and behavior | Practical implication |
|---|---|---|
| Express 4 | The Express 5 migration guidance does not establish a Node.js 18 minimum for Express 4. The migration guide warns that apps may need changes when moving to Express 5. | Do not assume a 4.x app can be upgraded by changing only its package version; check the migration guidance for affected behavior. |
| Express 5 | Node.js 18 or later is required. Returned route-handler and middleware promise rejections are forwarded to error handling. | Use a compatible Node runtime and make sure asynchronous work is returned from the handler; see the error-handling section below. |
How do you set up Express with TypeScript?
Express is JavaScript software; TypeScript definitions are installed separately. Add the Express and Node type packages, then compile with TypeScript so type checking is an explicit step.
npm install express mongoose
npm install --save-dev typescript @types/express @types/node
npx tsc --initFor an ESM-style Node project, a minimal configuration can use Node-oriented module resolution, strict checking, and a separate output directory. Pair this with "type": "module" in package.json so Node treats the emitted JavaScript as ESM.
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{
"compilerOptions": {
"target": "ES2022",
"module": "NodeNext",
"moduleResolution": "NodeNext",
"rootDir": "src",
"outDir": "dist",
"strict": true,
"esModuleInterop": true,
"skipLibCheck": true
},
"include": ["src/**/*.ts"]
}Compile and run the emitted JavaScript:
npx tsc
node dist/index.jsExpress’s installation guide also documents running TypeScript files directly with Node, but only for Node.js 22.18.0 or later (or 23.6.0 or later on the v23 line) and TypeScript 5.8 or later. Node’s native type stripping does not type-check the program; use npx tsc for that check. Direct execution is therefore not a substitute for type checking.
Packages without bundled TypeScript definitions may need their own community-maintained @types package. Check middleware dependencies individually rather than assuming Express’s types cover them.
How should Express middleware and routes pass control?
Express processes middleware in order during a request-response cycle. A middleware function can act on the request or response, finish the response, or hand control to the next middleware. If it does neither of the last two, the request hangs.
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const app = express();
app.use(express.json());
app.get('/health', (_req, res) => {
res.json({ ok: true });
});express.json() parses JSON request bodies before later handlers use them. A route handler passed directly to an Express route method can often receive inferred request and response types; for example, a route parameter such as req.params.id is inferred as a string from the path. Separately declared handlers and error middleware may need explicit annotations.
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Middleware checklist
- Register middleware before the routes that rely on it.
- Send a response when the current handler has completed the request.
- Call
next()when handing control onward without ending the response. - Pass an error to
next(err)when callback-based or detached asynchronous work fails.
How do you connect Mongoose to MongoDB?
For a local MongoDB server, use 127.0.0.1 in the connection URI:
import mongoose from 'mongoose';
await mongoose.connect('mongodb://127.0.0.1:27017/myapp');Node.js 18 and later prefer IPv6 addresses in some localhost resolution cases. If localhost resolves to ::1 but MongoDB is not listening on IPv6, the connection can fail; 127.0.0.1 avoids that particular mismatch. The database server must still be running and reachable.
For an application entry point, await the connection before starting to accept requests, so the server does not report itself ready while MongoDB is unavailable.
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import express from 'express';
import mongoose from 'mongoose';
const app = express();
async function main() {
await mongoose.connect('mongodb://127.0.0.1:27017/myapp');
app.listen(3000);
}
main().catch((err: unknown) => {
console.error('Application startup failed:', err);
process.exitCode = 1;
});Do not put real database credentials directly in source code. MongoDB connection URIs support credentials and options such as authSource; select values that match the database’s configuration.
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How do you keep Mongoose TypeScript types aligned with the schema?
Mongoose can infer types from schemas in many situations, but when you define a document interface, you are responsible for keeping it consistent with the schema. TypeScript interfaces do not validate incoming data at runtime; the Mongoose schema supplies the runtime schema behavior.
import mongoose, { Schema, Types } from 'mongoose';
interface User {
name: string;
email?: string;
}
const userSchema = new Schema<User>({
name: { type: String, required: true },
email: { type: String }
});
const UserModel = mongoose.model<User>('User', userSchema);Here, name is required in both the interface and schema, while email is optional. If a schema requires a field that the interface marks optional, Mongoose may not report that mismatch for you. Review the two declarations together whenever a field’s required status changes.
When should you use a query versus an aggregation?
Use an ordinary Mongoose query when it expresses the operation you need. Use an aggregation pipeline when the operation depends on pipeline stages or produces a result shape assembled across stages. The distinction matters because Mongoose treats the two paths differently.
| Behavior | Mongoose query | Aggregation |
|---|---|---|
| Best fit | Ordinary document filters and query operations. | Multi-stage transformations or pipeline-shaped results. |
| Type casting | Mongoose may cast query filter values to the schema type. | Mongoose does not cast pipeline stages; values must already have the database type needed by the stage. |
| Returned value | Typically hydrated Mongoose documents, with document behavior. | Plain JavaScript objects, not hydrated Mongoose documents. |
For example, a query can cast a string filter to an ObjectId when the schema expects one. An aggregation $match does not do that conversion for you:
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const id = new Types.ObjectId('507f1f77bcf86cd799439011');
const rows = await UserModel.aggregate([
{ $match: { _id: id } },
{ $project: { name: 1, email: 1 } }
]);Make the pipeline’s value type explicit. If the value starts as a string, convert it to mongoose.Types.ObjectId when the stored field is an ObjectId. Because aggregate results are plain objects, do not call Mongoose document methods on them or assume they have been hydrated.
How should an Express route handle errors?
Express catches synchronous throws in routes and middleware. In Express 5, a rejection from a promise returned by a handler is forwarded to error handling. That forwarding depends on returning the promise: detached work and callback APIs need an explicit error path.
app.get('/users/:id', async (req, res) => {
const user = await UserModel.findById(req.params.id);
if (!user) {
res.status(404).json({ error: 'User not found' });
return;
}
res.json(user);
});For a callback-based operation, pass the error to next from the callback. For a promise chain, return the chain and route its rejection with .catch(next) when explicit forwarding is needed.
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app.get('/legacy', (req, res, next) => {
legacyOperation((err, result) => {
if (err) return next(err);
res.json(result);
});
});Put error middleware after routes. It has four arguments in this order: (err, req, res, next). If headers have already been sent, delegate with next(err) so Express’s default error handler can finish handling the failure.
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app.use((err: unknown, req: express.Request, res: express.Response, next: express.NextFunction) => {
if (res.headersSent) {
next(err);
return;
}
res.status(500).json({ error: 'Internal server error' });
});Do not expose internal exception details in a production response; log or handle diagnostic information separately from the message sent to the client.
What does “Auth” mean for this cheat sheet?
Authentication is not one interchangeable Express snippet. The title does not specify whether the application should use server-side sessions, signed tokens, OAuth, or another design, and the Express session-middleware documentation establishes package availability—not a complete authentication system. It does not, by itself, determine cookie security settings, a production session store, password handling, or the application’s login flow.
Before implementing authentication, decide which architecture and security requirements the app actually needs. If choosing session middleware, account for its separately installed package and community-maintained TypeScript declarations; do not treat installation alone as authentication.
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