How to detect changes in the contents of a file, using Regex and Chokidar

Question

I would like to run a JS function when a specific section of a file changes. The contents that are being watched should be selected by Regex.

As far as I have seen, libraries like Chokidar can only detect changes to a file directly and do not provide information about the contents of the file before and after the changes.

Answer 1

I ended up creating a slightly adjusted version of Prashant Pokhriyal's answer.

const fs = require('fs');
const chokidar = require('chokidar');

const watchForFileContentChanges = (path, regex, callback) => {
    const regexFileContents = new Map();

    const registerFileContentMatches = (filePath) => {
        const currentFileContent = fs.readFileSync(filePath, 'utf8');
        const currentFileMatches = currentFileContent.match(regex);
        if (currentFileMatches === null) return;
        regexFileContents.set(filePath, currentFileMatches);
    }

    const handleContentChanges = (filePath) => {
        const previousFileMatches = regexFileContents.get(filePath);
        const hasFileBeenReadBefore = previousFileMatches !== undefined;

        const currentFileContent = fs.readFileSync(filePath, 'utf8');
        const currentFileMatches = currentFileContent.match(regex);

        if (hasFileBeenReadBefore === false) {
            regexFileContents.set(filePath, currentFileMatches);
            if (currentFileMatches === null) return;
            callback();
            return;
        }

        const haveFileMatchesChanged = JSON.stringify(previousFileMatches) !== JSON.stringify(currentFileMatches);

        if (haveFileMatchesChanged === false) return;

        regexFileContents.set(filePath, currentFileMatches);
        callback();
    }

    const handleFileRemoval = (filePath) => {
        regexFileContents.delete(filePath);
    }

    chokidar.watch(path)
        .on('add', registerFileContentMatches)
        .on('change', handleContentChanges)
        .on('unlink', handleFileRemoval);
}

watchForFileContentChanges('output/*.txt', /foo/g, () => {
    console.log('foo changed');
});