why there's two threads in node when it's idle?

Question

why there's two threads in node when it's idle?

function test_sleep() {
    var sleep = require('sleep');
    sleep.sleep(5);
}

test_sleep();

I use pstree to show threads for node

 │                ├─bash─┬─node───2*[{node}]
 │                │      └─pstree

my node version is v0.11.3

I tried this code in v0.6.3,And there's only one thread. so what's the differece between this two versions?

multithreading
node.js

Answer 1

Node.js (or rather libuv) maintains a small pool of worker threads. Threads are fairly expensive to spin up and tear down, that's why they stay around.

-- Ben Noordhuis

These threads are used to conduct I/O operations without blocking the main event loop. See Basics of libuv.