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Streams process data in CHUNKS instead of loading everything into memory. Use them for large files: read a piece, process it, move on. Memory can stay bounded independently of the total file size when each stage respects backpressure and does not accumulate chunks. Ignoring write() returning false or collecting every chunk can make memory grow with the input. highWaterMark is a buffering threshold, not a hard memory limit.
The 4 types: Readable (a source: file read, HTTP request), Writable (a destination: file write, HTTP response), Duplex (both: TCP socket), Transform (a Duplex that MODIFIES data passing through: gzip, encryption).
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