fix: Improved understanding of setImmediate and setTimeout output

[legendAhsan]

Description

In the understanding-processnexttick.md file we have code example output for setImmediate and setTimeout. While that output holds true for ES modules case and have indeterministic behaviour in CommonJS case. I mentioned it to improve the understanding, incase a person tries to run the code and get confused.

Validation

Check List

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• [] I've covered new added functionality with unit tests if necessary. - Not required

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[legendAhsan]

@mikeesto Can you look into this PR?

[programORdie2]

Looks good!

[aduh95]

I tried to reproduce with the following script:

import assert from 'node:assert'
import {spawn} from 'node:child_process'

let _1223344 = 0;
let _1224433 = 0;
let other = 0

for (let i = 0; i < 999; i++) {
  const cp = spawn(process.execPath, ['--input-type', process.argv[2], '-e', `console.log('Hello => number 1');

setImmediate(() => {
  console.log('Running before the timeout => number 3');
  console.log('Running setImmediate in the check phase => number 3');
});

setTimeout(() => {
  console.log('The timeout running last => number 4');
  console.log('Running setTimeout in the timers phase => number 4');
}, 0);

process.nextTick(() => {
  console.log('Running at next tick => number 2');
  console.log('Running process.nextTick in the nextTick queue => number 2');
});`]);
  cp.stdout.toArray().then(stdout => {
    stdout = Buffer.concat(stdout).toString();
    const numbers = stdout.trim().split('\n').map(line => Number(line.at(-1)));
    try {
      assert.deepStrictEqual(numbers, [1, 2, 2, 3, 3, 4, 4]);
      _1223344++;
      return;
    } catch {}
    try {
      assert.deepStrictEqual(numbers, [1, 2, 2, 4, 4, 3, 3]);
      _1224433++;
      return;
    } catch {}
    console.log({ stdout, numbers });
    other++;
  });
}

process.on('exit', () => {
  console.log({ _1223344, _1224433, other });
});

Here are the results:

$ node repro.mjs commonjs
{ _1223344: 472, _1224433: 527, other: 0 }
$ node repro.mjs module  
{ _1223344: 311, _1224433: 688, other: 0 }

As you can see, the proportions are different in ESM vs CJS, but it seems very wrong to state that ESM output is guarenteed.

[legendAhsan]

@aduh95

import assert from 'node:assert'

let order1223344 = 0;
let order1224433 = 0;
let other = 0
const totalRuns = 999

function runTest() {
  return new Promise((resolve) => {
    const output = [];
    
    console.log('Hello => number 1');
    output.push(1);
    
    setImmediate(() => {
      console.log('Running before the timeout => number 3');
      console.log('Running setImmediate in the check phase => number 3');
      output.push(3);
      output.push(3);
      if (output.length === 7) resolve(output);
    });
    
    setTimeout(() => {
      console.log('The timeout running last => number 4');
      console.log('Running setTimeout in the timers phase => number 4');
      output.push(4);
      output.push(4);
      if (output.length === 7) resolve(output);
    }, 1);
    
    process.nextTick(() => {
      console.log('Running at next tick => number 2');
      console.log('Running process.nextTick in the nextTick queue => number 2');
      output.push(2);
      output.push(2);
    });
  });
}

for (let i = 0; i < totalRuns; i++) {
  runTest().then(numbers => {
    try {
      assert.deepStrictEqual(numbers, [1, 2, 2, 3, 3, 4, 4]);
      order1223344++;
    } catch {
      try {
        assert.deepStrictEqual(numbers, [1, 2, 2, 4, 4, 3, 3]);
        order1224433++;
      } catch {
        console.log({ numbers });
        other++;
      }
    }
    
    if (i === totalRuns -1) {
      console.log({ order1223344, order1224433, other });
    }
  });
}

I feel that the above output you got is because Spawning childs may run like a CJS script while in module it is asynchronous.
Put the above code in module file like common.mjs and you will see that it will only increment 1223344 variable.

[aduh95]

I feel that the above output you got is because Spawning childs may run like a CJS script while in module it is asynchronous.
Put the above code in module file like common.mjs and you will see that it will only increment 1223344 variable.

It seems to me it has nothing to do with ESM or CJS, rather how busy is the event loop. Anyway, it's good to know that sometimes it's deterministic I guess, but as I've shown in #6994 (review), it's not always the case.