Steve Breese

Node.js Tutorial: Promisify your Node.js Callback Functions

Below is a quick tutorial on how to use promisify() to convert a callback-based function to a Promise-based one. Promises offer more flexibility to developers than callbacks and help you write cleaner, easy-to-read code.

1. Identify a Node.js callback function that should be converted to a promise. In this tutorial, we will convert a simple function that takes an image path as its only parameter and returns the images width and height in a JavaScript object.

   const sizeOf = require('image-size');
   sizeOf(path.join(form.uploadDir, file.name), function (err, dimensions) {
       console.log(dimensions.width, dimensions.height);
       if (err) {
            console.log(`[Error]: ${err}`);
       }
   });
                           

2. Add the Node.js Util module to the top of your JS file.

   const util = require('util');
   
   var sizeOf = require('image-size');
   sizeOf(path.join(form.uploadDir, file.name), function (err, dimensions) {
       console.log(dimensions.width, dimensions.height);
       if (err) {
            console.log(`[Error]: ${err}`);
       }
   });
   

3. Wrap the original callback function (in this case, the image-size require statement returns a single function) with the promisify utility, which converts it to a function that returns a promise. Delete the old require statement.

   const util = require('util');
   const sizeOfAsync = util.promisify(require('image-size'));
   
   var sizeOf = require('image-size');
   sizeOf(path.join(form.uploadDir, file.name), function (err, dimensions) {
       console.log(dimensions.width, dimensions.height);
       if (err) {
            console.log(`[Error]: ${err}`);
       }
   });
   

4. Replace your callback function call with the new promisified function and delete the ES5 function expression or ES6 arrow syntax.

   const util = require('util');
   const sizeOfAsync = util.promisify(require('image-size'));
   
   sizeOfAsync(path.join(form.uploadDir, file.name))
   sizeOf(path.join(form.uploadDir, file.name), function (err, dimensions) {
       console.log(dimensions.width, dimensions.height);
       if (err) {
            console.log(`[Error]: ${err}`);
       }
   });
   

5. Add the Promise's then statement and move the callback's return argument and its code from the callback body to the then body.

   const util = require('util');
   const sizeOfAsync = util.promisify(require('image-size'));
   
   sizeOfAsync(path.join(form.uploadDir, file.name))
       .then(dimensions => {
           console.log("Image dimensions:", dimensions.width, dimensions.height)
       })
    (err, dimensions) {
       console.log("Image dimensions:", dimensions.width, dimensions.height);
       if (err) {
            console.log(`[Error]: ${err}`);
       }
   });
   

6. Lastly, add the Promise's catch statement and move the callback's error argument and its code from the callback body to the catch body.

   const util = require('util');
   const sizeOfAsync = util.promisify(require('image-size'));
   
   sizeOfAsync(path.join(form.uploadDir, file.name))
       .then(dimensions => {
           console.log("Image dimensions:", dimensions.width, dimensions.height)
       })
       .catch(err => console.error(`[Error]: ${err}`));
       if (err) {
            console.log(`[Error]: ${err}`);
       }
   

7. After the conversion, the resulting Promise code should look like the following:

const util = require('util');
const sizeOfAsync = util.promisify(require('image-size'));

sizeOfAsync(path.join(form.uploadDir, file.name))
    .then(dimensions => {
        console.log("Image dimensions:", dimensions.width, dimensions.height)
    })
    .catch(err => console.error(`[Error]: ${err}`));

With these changes, we have converted a callback function to a promise and demonstrate how to use the new promise. Now go ahead and replace all your Node.js callback functions with promises to have cleaner and easier to read code.