test_node_buffer.ts

import { spawn } from "child_process"

// 8192*2 (16384 aka 16KB) is the highWaterMark (aka buffer size?)
const node_process = spawn("node", ["-e", "process.stdout.write('a'.repeat(8192*2+1));setTimeout(()=>{console.log('done')},4000)"])

node_process.on('error', err => console.error(err))

let numFlushes = 0
node_process.stdout.on('data', (buffer: Buffer) => {
    numFlushes += 1
    const str = buffer.toString()
    console.log(`flush number: ${numFlushes}\
    Buffer length: ${buffer.length}`)
    if (buffer.length < 20) console.log(str)
})

/*
Expected result: 3 flushes, one when highWaterMark is reached, another with rest of stdout, final with done log
Acutal result: 2 flushes. All of stdout is recieved at once. 4 seconds later comes the done log.

Related docs: https://nodejs.org/docs/latest-v12.x/api/stream.html#stream_buffering

OUTPUT (courtesy of quokka)
flush number: 1    Buffer length: 16385
  at ​​​`flush number: ${ numFlushes }\ Buff...​​​ ​personal/test_node_buffer.ts:11:4

flush number: 2    Buffer length: 11
  at ​​​`flush number: ${ numFlushes }\ Buff...​​​ ​personal/test_node_buffer.ts:11:4

done

  at ​​​str​​​ ​personal/test_node_buffer.ts:13:8
*/