How to find file extension change with node.js? - node.js

Question

How do you detect a file extension change and save the new extension or full file path to a variable? This is a unique specific question.
Code I have so far dont mind if you dont use it:
const puppeteer = require('C:/Users/user1/Desktop/puppeteer_automation/node_modules/puppeteer');
(async () => {
var filename = "C:/Users/user1/Downloads/file.crdownload";
var downloadanduploadpath = "C:/Users/user1/Downloads";
var fs = require('fs');
var event1 = "change";
var currentstat = fs.stat();
const WATCH_TARGET = filename;
fs.watch(WATCH_TARGET, function(event1, downloadanduploadpath) {
console.log('File "' + filename + '" was changed: ' + eventType);
});
/*
fs.watch(downloadanduploadpath, (event1, filename) => {
console.log('event is: ' + event1);
if (filename) {
console.log('filename provided: ' + filename);
} else {
console.log('filename not provided');
}
});*/
//console.log(lastdownloadedimage);
})();
Error:
(node:9876) UnhandledPromiseRejectionWarning: TypeError
[ERR_INVALID_CALLBACK]: Callback must be a function. Received
undefined

Answer 1

As described in the docs,
The listener callback gets two arguments (eventType, filename). eventType is either 'rename' or 'change', and filename is the name of the file which triggered the event.
you can use the eventType argument to determine whether the filename has been changed.
UPDATE: I updated my example code to observe the directory instead of the file itself, in order to get the new filename in the filename argument
Example:
let fs = require('fs');
(async () => {
const WATCH_DIR = "C:/Users/user1/Downloads";
let target_file = "file.crdownload";
let renameTriggered = false;
fs.watch(WATCH_DIR, function(eventType, filename) {
if(eventType == 'rename') {
// Check if the target filename was changed (in the first event
// the old filename disappears, which marks the beginning of a renaming process)
if(filename == target_file) {
// Toggle renaming status
renameTriggered = true;
}
// The second event captures the new filename, which completes the process
else if(renameTriggered) {
// Toggle renaming status
renameTriggered = false;
console.log('File "' + target_file + '" was renamed to: "' + filename + '"');
// Update target filename
target_file = filename;
}
}
});
})();
Observing the directory however will trigger the callback on any filename changes that occur (includes even deleting and creating files). Therefore we check if the eventType is equal to 'rename'.
From the perspective of fs.watch() the renaming takes place in two steps: In the fisrt step the 'disappearing' of the old filename is detected which triggers the event passing the old filename as the argument filename. In the second step the 'appearance' of the new filename is detected which triggers the rename event again, this time with the new filename as the second argument, which is what we are looking for.

Answer 2

Your code example, is calling fs.watch() wrong. You don't pass arguments to the callback yourself. You just name parameters.
This program below all by itself works on Windows. I pass it a directory and watch for changes to any of the files in that directory. You can also pass it the filename for an existing file and watch for changes to just that file:
const fs = require('fs');
fs.watch("./watcherDir", function(eventType, filename) {
console.log(`eventType=${eventType}, filename=${filename}`);
});
eventType and filename are passed by fs.watch() to the callback, they are not something you pass. You just create the name of the arguments that you can then use as function arguments inside the callback.
When I rename a file in that directory from example.json to example.xxx, I get these two callbacks:
eventType=rename, filename=example.json
eventType=rename, filename=example.xxx
The filenames are, of course, relative to the directory I passed to fs.watch().
Note: As documented, this feature does not work the same on all platforms so for any further debugging we would need to know what platform you're running it on.