NGNT JS - event-loop snippets

example-01.js

// example 01: microtask and timer queues

setTimeout(() => console.log("setTimeout 1"), 0);
setTimeout(() => {
  console.log("setTimeout 2");
  process.nextTick(() => console.log("setTimeout 2 nextTick"))
}, 0);
setTimeout(() => console.log("setTimeout 3"), 0);

process.nextTick(() => console.log("nextTick 1"));
process.nextTick(() => {
  console.log("nextTick 2");
  process.nextTick(() => console.log("nextTick 2 nextTick"));
});
process.nextTick(() => console.log("nextTick 3"));

Promise.resolve().then(() => console.log("Promise 1"));
Promise.resolve().then(() => {
  console.log("Promise 2");
  process.nextTick(() => console.log("Promise 2 nextTick"));
});
Promise.resolve().then(() => console.log("Promise 3"));

/*
Result:

nextTick 1
nextTick 2
nextTick 3
nextTick 2 nextTick
Promise 1
Promise 2
Promise 3
Promise 2 nextTick
setTimeout 1
setTimeout 2
setTimeout 3
setTimeout 2 nextTick
*/

example-02.js

// example 02: I/O queue and timer queue anomaly

const fs = require("fs");

setTimeout(() => console.log("setTimeout"), 0);
fs.readFile(__filename, () => console.log("readFile"));
// for (let i = 0; i < 1000000000; i++) {}

/*
Result:

setTimeout
readFile
// anomaly
readFile
setTimeout
*/

example-03.js

// example 03: I/O queue and I/O polling

const fs = require("fs");

fs.readFile(__filename, () => {
  console.log("readFile");
});
process.nextTick(() => console.log("nextTick"));
Promise.resolve().then(() => console.log("Promise"));
setTimeout(() => console.log("setTimeout"), 0);
setImmediate(() => console.log("setImmediate"));

for (let i = 0; i < 1000000000; i++) {}

/*
Result:

nextTick
Promise
setTimeout
setImmediate
readFile
*/

example-04.js

// example 04: check queue & timer queue anomaly

setTimeout(() => console.log("setTimeout"));
setImmediate(() => console.log("setImmediate"));

/*
setTimeout
setImmediate
// anomaly
setImmediate
setTimeout
*/

example-05.js

// example 05: Close queue in the end

const fs = require("fs");

const readableStream = fs.createReadStream(__filename);
readableStream.on("close", () => {
  console.log("readableStream");
});
readableStream.close();

setImmediate(() => console.log("setImmediate"));
setTimeout(() => console.log("setTimeout"), 0);
Promise.resolve().then(() => console.log("Promise"));
process.nextTick(() => console.log("nextTick"));

/*
Result:

nextTick
Promise
setTimeout
setImmediate
readableStream
*/