'use strict';

const fs = require('node:fs');

class Thenable {

constructor() {

this.next = null;

this.fn = null;

}

then(fn) {

this.fn = fn;

const next = new Thenable();

this.next = next;

return next;

}

resolve(value) {

const fn = this.fn;

if (fn) {

const next = fn(value);

if (next) {

next.then((value) => {

this.next.resolve(value);

});

}

}

}

}

// Usage

const readFile = (filename) => {

const thenable = new Thenable();

fs.readFile(filename, 'utf8', (err, data) => {

if (err) throw err;

thenable.resolve(data);

});

return thenable;

};

const main = async () => {

const file1 = await readFile('9-thenable.js');

console.dir({ length: file1.length });

};

main();