MEAN Cheat Sheet 2026

The 30 highest-yield MEAN facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. Which RxJS creation function converts a JavaScript Promise into an Observable? → from()
2. Which of the following best describes Node.js's single-threaded event loop model? → Node.js uses one thread but offloads I/O operations asynchronously via the event loop
3. What is the purpose of environment variables in a Node.js MEAN application? → Store configuration and secrets outside the codebase, accessed via process.env
4. In MongoDB, what is a capped collection? → A fixed-size collection that overwrites oldest documents when full
5. What is the default behavior of Node.js's `EventEmitter` when an 'error' event is emitted with no listener? → An exception is thrown and the process may crash
6. When two sibling Angular components (with no direct parent-child relationship) need to share state, what is the recommended approach? → Use a shared Angular service with a BehaviorSubject to hold and broadcast state
7. What is the purpose of MongoDB transactions introduced in version 4.0? → Provide ACID guarantees across multiple documents and collections
8. When passing an error to `next(err)` in Express, which middleware type intercepts it? → The first error-handling middleware (four parameters) defined after the error source
9. What is the purpose of Angular's `@ViewChild` decorator? → Gets a reference to a child component, directive, or DOM element in the template
10. How do you define a URL parameter in an Express route? → app.get('/users/:id', handler)
11. In MongoDB, what does the $group aggregation stage do? → Groups documents by a specified expression and accumulates values
12. Which year was MongoDB first released publicly? → 2009
13. What does the MongoDB `upsert` option do in an update operation? → Inserts a new document if no matching document exists
14. In order to convert Angular Components into native Custom Elements, which of the following APIs is used? → createCustomElement()
15. Which MongoDB write concern value ensures acknowledgment from a majority of replica set members? → majority
16. What is MongoDB's WiredTiger storage engine's default compression algorithm for documents? → Snappy
17. Where does Node.js operate? → Server
18. Use of Node.js is ideal in _______ → All of the above
19. Which Angular module must be imported to enable routing in an Angular application? → RouterModule
20. In a MEAN stack app, which Angular decorator is used to define a component's metadata such as its template and selector? → @Component
21. What does the 'E' stand for in the MEAN stack? → Express.js
22. In Angular, what is the correct way to prevent memory leaks when subscribing to an Observable inside a component? → Always use the async pipe in templates or unsubscribe in ngOnDestroy
23. In Node.js _________ is the APIs of NOde.JS → Asynchronous
24. What is a key characteristic of async generators in Node.js? → They yield values lazily over time and can be iterated with `for await...of`
25. What is the difference between `@Input()` and `@Output()` decorators in Angular? → @Input() receives data from parent; @Output() emits events to the parent with EventEmitter
26. In Node.js, what is the correct order of phases in the event loop that handles I/O callbacks, timers, and setImmediate? → timers → pending callbacks → idle/prepare → poll → check → close callbacks
27. Which Angular module must be imported to use reactive forms? → ReactiveFormsModule
28. What is the default behavior of MongoDB's WiredTiger storage engine regarding data compression? → Uses snappy compression for collections and zlib for indexes by default
29. What Angular feature allows you to share data and logic across multiple unrelated components? → Services with dependency injection
30. What happens when an unhandled rejection occurs in a Promise in Node.js v15+? → The process exits with a non-zero exit code