nodejs / nodejs/node

Strange behavior with `fs.watch` on file with JSON stringified -> first update isn't emitted if too fast after `fs.watch`

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confirmed-bug fs macos
Dominant language
JavaScript
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Description

Node.js Version

v18.17.1

NPM Version

9.6.7

Operating System

23.2.0 Darwin

Subsystem

fs

Description

I would like to watch changes on a file storing JSON stringified data but the watcher created with fs.watch is not emitted if the change happen to quickly after the call of fs.watch. The strange thing is that if the content is only a text (without JSON format) the watcher emit the event correctly...

Minimal Reproduction
const fs = require('fs');
const path = require('path');

const filePath = path.join(__dirname, 'test.json');
// Create the file if it doesn't exist
if (!fs.existsSync(filePath)) {
    fs.writeFileSync(filePath, "{ }");
}

fs.watch(filePath, (eventType, filename) => {
    console.log(`Event type is: ${eventType}`);
    if (filename) {
        console.log(`Filename provided: ${filename}`);
    } else {
        console.log('Filename not provided');
    }
});

fs.writeFileSync(filePath, "{ \"test\": \"test\", \"test2\": \"test2\" }"); // same with JSON.stringify(...)
// Listener above is called if we replace line above with:
// fs.writeFileSync(filePath, "very ok or not but I need a longer text to test something...");
Output
Expected
Event type is: change
Filename provided: test.json
Received

Nothing

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  • My issue follows the guidelines in the README file, and follows the 'How to ask a good question' guide at https://stackoverflow.com/help/how-to-ask

Contributor guide

Open the contributing guide

First steps

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  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the minimal reproduction using fs.watch and the immediate fs.writeFileSync call on Darwin. Compare the JSON-stringified write with the longer plain-text write and investigate the first-event timing behavior. Done means the expected change event is reliably emitted for the JSON write without breaking the documented fs.watch behavior.

Written by the indexing model from the issue text.

Assessment

Tech stack
javascript, node.js
Domain
operating-systems
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Quiet
Clarity
Mostly clear
Newbie friendliness
45/100

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