Promisifying Callback-Based APIs in JavaScript

Promisifying Callback-Based APIs in JavaScript

Many older APIs (Node.js fs, crypto, browser APIs) are callback-based. You can convert them to promises.

Manual Promisification

function readFileP(path) {
  return new Promise((resolve, reject) => {
    fs.readFile(path, "utf8", (err, data) => {
      if (err) reject(err);
      else resolve(data);
    });
  });
}

readFileP("file.txt")
  .then((data) => console.log(data))
  .catch((err) => console.error(err));
``
### Generic promisify Function

```js
function promisify(fn) {
  return (...args) => {
    return new Promise((resolve, reject) => {
      fn(...args, (err, result) => {
        if (err) reject(err);
        else resolve(result);
      });
    });
  };
}

const readFileP = promisify(fs.readFile);
const writeFileP = promisify(fs.writeFile);
``
### Node.js util.promisify

```js
const { promisify } = require("util");
const fs = require("fs");

const readFileP = promisify(fs.readFile);
readFileP("file.txt", "utf8")
  .then((data) => console.log(data));
``
### Node.js Built-in Promise APIs

Many Node.js APIs now have promise-based versions:

```js
const fs = require("fs/promises");
// or
const fs = require("fs").promises;

async function main() {
  const data = await fs.readFile("file.txt", "utf8");
  console.log(data);
}
``
No need to promisify. Use `fs/promises` directly.

### Promisifying setTimeout

```js
function delay(ms) {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

async function main() {
  console.log("wait");
  await delay(1000);
  console.log("done");
}
``
Node.js provides `timers/promises`:

```js
const { setTimeout: setTimeoutP } = require("timers/promises");
await setTimeoutP(1000);
``
### Promisifying Events

```js
const { once } = require("events");
await once(server, "listen");
``
### The Takeaway

Promisify callback APIs with `new Promise` (manual), a generic `promisify` function, or `util.promisify` (Node.js). Many Node.js APIs now have built-in promise versions (`fs/promises`, `timers/promises`). Promisify `setTimeout` with `delay` and events with `events.once`. Use async/await with promisified APIs for clean code.