What is backpressure in Node.js streams?

A tanker fills a small water tank faster than the tank can be emptied into the house. If nobody closes the tanker's valve, water overflows. Backpressure is the signal that says, "Tank is full, slow down." In streams, the "overflow" is memory: unwritten chunks pile up in RAM.

const fs = require("fs");
const src = fs.createReadStream("big-video.mp4");
const dst = fs.createWriteStream("copy.mp4");

src.on("data", (chunk) => {
  const ok = dst.write(chunk);       // false means the internal buffer is full
  if (!ok) {
    src.pause();                     // stop reading
    dst.once("drain", () => src.resume());  // continue when emptied
  }
});
src.on("end", () => dst.end());

How it works:

• write() returns false once the writable's internal buffer exceeds its highWaterMark (16 KB by default for most streams).
• The drain event fires when the buffer has emptied.

You rarely code this by hand. readable.pipe(writable) and stream.pipeline() handle pausing and resuming for you, as does for await over a stream. Backpressure is a main reason to use these instead of manual data handlers.

• Ignoring the return value of write() in a loop, which grows memory without limit.
• Writing a custom copy loop when pipeline would do.