Promisification Quick Example

promisifying.js

// This example illustrates how a synchronous method from the "fs" module could be promisified.
// Note that Node provides a built-in promisified fs module itself out of the box, and this snippet is just for illustration purposes.


const fs = require("fs");

// Non-promisified (async using callbacks, which can lead to callback hell... 👿🔥)
fs.readFile("./package.json", function callback(err, data) {
  const package = JSON.parse(data.toString("utf8"));
  console.log(package.name);
});


// Here's where we promisify the fs.readFile() method used above:
async function fsReadFilePromisified(param) {
  return new Promise((resolve, reject) => {
    fs.readFile(param, (err, data) => {
      if (err) {
        reject(data);
      } else {
        resolve(data);
      }
    });
  });
}


// Using the promisified API using .then() syntax (async using .then(), old, not recommended)
fsReadFilePromisified("./package.json")
  .then((response) => {
    const package = JSON.parse(response.toString("utf8"));
    console.log(package.name);
  })
  .catch((err) => console.log(err));


// Using the promisified API using async/await syntax (async using async/await, modern, recommended)
// This is an IIFE in form of `(()=>{})()`.
// If you'd rather not use an IIFE and want to use top-level async/await, just turn
// this module into an ESM module (e.g., by changing the file extension from .js to .mjs).
(async () => {
  try {
    const resp = await fsReadFilePromisified("./package.json");
    const package = JSON.parse(resp.toString("utf8"));
    console.log(package.name);
  } catch (error) {
    console.error(error);
  }
})();