A custom implementation of Javascript promises from scratch. The purpose of this implementation was to deeply understand how promises allow asynchronous operations using setTimeout() Web API & how a data structure like a queue is maintained.
A promise represents the eventual result of an asynchronous operation. The primary way of interacting with a promise is through its then method, which registers callbacks to receive either a promise’s eventual value or the reason why the promise cannot be fulfilled.
const Promnom = require("./index");
const fs = require("fs");
const path = require("path");
const readFile = (filename, enc) =>
new Promnom((resolve, reject) => {
fs.readFile(filename, enc, (err, value) => {
if (err) {
return reject(err);
}
resolve(value);
});
});
const delay = (time, value) =>
new Promnom((resolve) => {
setTimeout(() => {
resolve(value);
}, time);
});
readFile(path.join(__dirname, "index.js"), "utf-8")
.then((text) => {
console.log(`${text.length} characters read`);
return delay(2000, text.replace(/[aeiou]/g, ""));
})
.then((newText) => {
console.log(newText.slice(0, 200));
})
.catch((err) => {
console.log("An error occurred");
console.error(err);
})
.finally(() => {
console.log("Promise done");
});
/*
Output:
3472 characters read
cnst stts = rqr("./stts");
// Chck f prms hs thn mthd
cnst sThnbl = (mybPrms) =>
mybPrms && typf mybPrms.thn === "fnctn";
// Glbl clss
clss Prmnm {
cnstrctr(cmpt) {
ths._stt = stts.PENDING
Promise done
*/