Node.js でサブプロセス起動して終了する

**[parent.js]**
```js
const cp = require("child_process")

const sp = cp.fork("./child.js", [], { detached: true })

sp.send("AAA")

sp.on("message", value => {
	console.log(value)

	sp.disconnect()
	sp.unref()
})

console.log("PID", process.pid)
```

**[child.js]**
```js
const fs = require("fs")

const log = data => fs.appendFileSync("log.txt", String(data) + "\n")

let n = 0
const t = setInterval(() => {
	log(n++)
	if(n > 3) clearInterval(t)
}, 2000)

process.on("message", value => {
	log("message: " + value)
	process.send("BBB")
})

log("[start] pid: " + process.pid)
```

`parent.js` のログ

```
PID 9840
BBB
```

`child.js` のログ

```
[start] pid: 9820
message: AAA
0
1
2
3
```