1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
|
{% callout %}
`Worker` support was added in Bun v0.7.0.
{% /callout %}
[`Worker`](https://developer.mozilla.org/en-US/docs/Web/API/Worker) lets you start and communicate with a new JavaScript instance running on a separate thread while sharing I/O resources with the main thread.
Bun implements a minimal version of the [Web Workers API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API) with extensions that make it work better for server-side use cases. Like the rest of Bun, `Worker` in Bun support CommonJS, ES Modules, TypeScript, JSX, TSX and more out of the box. No extra build steps are necessary.
## Creating a `Worker`
Like in browsers, [`Worker`](https://developer.mozilla.org/en-US/docs/Web/API/Worker) is a global. Use it to create a new worker thread.
From the main thread:
```js#Main_thread
const workerURL = new URL("worker.ts", import.meta.url).href;
const worker = new Worker(workerURL);
worker.postMessage("hello");
worker.onmessage = event => {
console.log(event.data);
};
```
Worker thread:
```ts#worker.ts_(Worker_thread)
self.onmessage = (event: MessageEvent) => {
console.log(event.data);
postMessage("world");
};
```
You can use `import`/`export` syntax in your worker code. Unlike in browsers, there's no need to specify `{type: "module"}` to use ES Modules.
To simplify error handling, the initial script to load is resolved at the time `new Worker(url)` is called.
```js
const worker = new Worker("/not-found.js");
// throws an error immediately
```
The specifier passed to `Worker` is resolved relative to the project root (like typing `bun ./path/to/file.js`).
### `"open"`
The `"open"` event is emitted when a worker is created and ready to receive messages. This can be used to send an initial message to a worker once it's ready. (This event does not exist in browsers.)
```ts
const worker = new Worker(new URL("worker.ts", import.meta.url).href);
worker.addEventListener("open", () => {
console.log("worker is ready");
});
```
Messages are automatically enqueued until the worker is ready, so there is no need to wait for the `"open"` event to send messages.
## Messages with `postMessage`
To send messages, use [`worker.postMessage`](https://developer.mozilla.org/en-US/docs/Web/API/Worker/postMessage) and [`self.postMessage`](https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage). This leverages the [HTML Structured Clone Algorithm](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm).
```js
// On the worker thread, `postMessage` is automatically "routed" to the parent thread.
postMessage({ hello: "world" });
// On the main thread
worker.postMessage({ hello: "world" });
```
To receive messages, use the [`message` event handler](https://developer.mozilla.org/en-US/docs/Web/API/Worker/message_event) on the worker and main thread.
```js
// Worker thread:
self.addEventListener("message", event => {
console.log(event.data);
});
// or use the setter:
// self.onmessage = fn
// if on the main thread
worker.addEventListener("message", event => {
console.log(event.data);
});
// or use the setter:
// worker.onmessage = fn
```
## Terminating a worker
A `Worker` instance terminates automatically once it's event loop has no work left to do. Attaching a `"message"` listener on the global or any `MessagePort`s will keep the event loop alive. To forcefully terminate a `Worker`, call `worker.terminate()`.
```ts
const worker = new Worker(new URL("worker.ts", import.meta.url).href);
// ...some time later
worker.terminate();
```
This will cause the worker's to exit as soon as possible.
### `process.exit()`
A worker can terminate itself with `process.exit()`. This does not terminate the main process. Like in Node.js, `process.on('beforeExit', callback)` and `process.on('exit', callback)` are emitted on the worker thread (and not on the main thread), and the exit code is passed to the `"close"` event.
### `"close"`
The `"close"` event is emitted when a worker has been terminated. It can take some time for the worker to actually terminate, so this event is emitted when the worker has been marked as terminated. The `CloseEvent` will contain the exit code passed to `process.exit()`, or 0 if closed for other reasons.
```ts
const worker = new Worker(new URL("worker.ts", import.meta.url).href);
worker.addEventListener("close", event => {
console.log("worker is being closed");
});
```
This event does not exist in browsers.
## Managing lifetime
By default, an active `Worker` will keep the main (spawning) process alive, so async tasks like `setTimeout` and promises will keep the process alive. Attaching `message` listeners will also keep the `Worker` alive.
### `worker.unref()`
To stop a running worker from keeping the process alive, call `worker.unref()`. This decouples the lifetime of the worker to the lifetime of the main process, and is equivlent to what Node.js' `worker_threads` does.
```ts
const worker = new Worker(new URL("worker.ts", import.meta.url).href);
worker.unref();
```
Note: `worker.unref()` is not available in browers.
### `worker.ref()`
To keep the process alive until the `Worker` terminates, call `worker.ref()`. A ref'd worker is the default behavior, and still needs something going on in the event loop (such as a `"message"` listener) for the worker to continue running.
```ts
const worker = new Worker(new URL("worker.ts", import.meta.url).href);
worker.unref();
// later...
worker.ref();
```
Alternatively, you can also pass an `options` object to `Worker`:
```ts
const worker = new Worker(new URL("worker.ts", import.meta.url).href, {
ref: false,
});
```
Note: `worker.ref()` is not available in browers.
## Memory usage with `smol`
JavaScript instances can use a lot of memory. Bun's `Worker` supports a `smol` mode that reduces memory usage, at a cost of performance. To enable `smol` mode, pass `smol: true` to the `options` object in the `Worker` constructor.
```js
const worker = new Worker("./i-am-smol.ts", {
smol: true,
});
```
{% details summary="What does `smol` mode actually do?" %}
Setting `smol: true` sets `JSC::HeapSize` to be `Small` instead of the default `Large`.
{% /details %}
|
