Node.js Design Patterns - Some learning notes

Random stuff I'm learning while reading the book. Raw notes.

Promises & Async

• .then() returns another promise immediately. All promises in the chain are created right away, but execution happens in sequence and always async:

fetch('user.json')           // Promise A
.then(res => res.json())     // Promise B - created now, runs later
.then(user => fetch(user.photo)) // Promise C - same
.then(photo => displayPhoto())   // Promise D - same

• .catch() can transform errors into success:

Promise.reject('error')
.catch(err => 'recovered')  // Error -> Success
.then(value => ...)        // Chain continues normally

• Promise.resolve() - Provides initial chain link for building promise chains

Async/Await Insights

• Async functions always return a promise synchronously

const data = await someAsyncFunc()
// data is immediately an unfulfilled promise

Practical Patterns

• It's normal to "promisify" callback-based functions to use promises instead
• Queues are used for limiting concurrent tasks - too many parallel processes can saturate the system
• Pub/sub is one way to implement event-driven architecture

Useful Libraries

• Async: Most installed NPM package. Wraps control flow patterns into reusable functions
• p-map: Like Promise.all() but with controlled concurrency and error handling

pMap(sites, mapper, {concurrency: 2});

Notes will be updated as I continue reading...

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