20 — Workers and parallelism

One line: One JavaScript agent never runs two things at once; parallelism in Node needs a second agent, a worker or a process, and data crosses between them by copy, by transfer, or through shared memory that needs Atomics.

The chapter starts from the rule the rest of the library relies on: one agent runs one piece of JavaScript at a time. Worker threads add a second agent and real parallelism. Their messages are copied by structured clone or moved by transfer, and the third page shows which is which. Shared memory comes next, with the lost update it allows and the Atomics that prevent it. Child processes close the chapter: a separate program with its own exit status and signals.

The pages

Page	Level	In one line	Status
One thread per agent	101	Inside one agent, JavaScript never runs two things at once: two async tasks interleave only at await (a1 b1 a2 b2 a3 b3), and two 200 ms loops started together under Promise.all still take 400 ms.	stub
Worker threads	201	A Worker is a second agent with its own thread, event loop and globals — a worker that sets globalThis.counter leaves the main thread's copy at 0 — so four 500 ms loops on four workers finish in about the time of one.	stub
postMessage	201	postMessage sends a structured clone, not the object: a Point arrives as a plain { x: 3, y: 4 } without its methods, a function throws DataCloneError, and an ArrayBuffer in the transfer list moves — the sender's copy drops to byteLength 0.	stub
SharedArrayBuffer and Atomics	301	Two workers each doing shared[0]++ ten million times on one SharedArrayBuffer lose updates — the total falls short of 20,000,000 — because ++ is a read, an add and a write; Atomics.add does all three as one step and lands exactly.	stub
Child processes	201	exec hands its string to /bin/sh, so $((6 * 7)) becomes 42; spawn runs the program directly and passes the text through untouched; and a child killed by a signal reports code null and signal 'SIGTERM', where a shell would say 143.	stub

Boundaries

The event loop inside one agent is in Async and the event loop; Buffer and streams are in The Node.js runtime; ArrayBuffer and typed arrays are in Arrays and collections.