main.js

import {EventEmitter} from 'events';
import {readFile} from 'fs';

class FindRegex extends EventEmitter {
    constructor(regex) {
        super()
        this.regex = regex
        this.files = []
    }

    addFile(file) {
        this.files.push(file)
        return this
    }

    find() {
        process.nextTick(() => this.emit('start', this.files));

        for (const file of this.files) {
            readFile(file, 'utf8', (err, content) => {
                if (err) {
                    return this.emit('error', err)
                }
                this.emit('fileread', file)
                const match = content.match(this.regex)
                if (match) {
                    match.forEach(elem => this.emit('found', file, elem))
                }
            })
        }

        return this
    }
}

new FindRegex(/hello \w+/)
    .addFile('./data/fileA.txt')
    .addFile('./data/fileB.json')
    .find()
    .on('found', (file, match) => console.log(`Matched "${match}"`))
    .on('start', files => console.log(`The process starts with ${files}`))
    .on('error', console.error);