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Understand util.promisify in Node JS With Example


Below function returns a callback with an error and message.
const delay = (sec, cb) => {
  if (sec > 3) {
    cb(new Error(`${sec} seconds is too long. Try shorter time delay.`));
  } else {
    setTimeout(() => cb(null, `${sec} delay completed.`), sec)
  }
}

const cbfunc = (error, message) => {
  if (error) {
    console.error(error.message);
  } else {
    console.log(message);
  }
}

delay(4, cbfunc);
This is the way callbacks is used.
Now let's do the same thing with util.promisify
const { promisify } = require('util');

const delay = (sec, cb) => {
  if (sec > 3) {
    cb(new Error(`${sec} seconds is too long. Try shorter time delay.`));
  } else {
    setTimeout(() => cb(null, `${sec} delay completed.`), sec)
  }
}

const promiseDelay = promisify(delay); // passing function to promisify, it returns a promise

promiseDelay(2)
.then(console.log)
.catch(error => console.error(error));
The delay function remains the same way. The way the callback is used is modified with promisify.

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