@atomiq/promisify

@atomiq/promisify

npm package

npm install @atomiq/promisify

Convert idiomatic async functions that expect callbacks to EcmaScript 2015 promises.

promisify is a very lightweight implementation that wraps callback-style async functions with native io.js promises.

Promisify an object or function and it will walk over all member properites and up prototype chains to ensure all callback-style async functions are converted to promises. The easiest thing is to just promisify modules when loading them.

promisify looks for functions that have one of the following names as the last parameter: callback, cb, done, callback_, cb_. It recognizes both standard functional declarations as well as ES6 fat arrow functions.

Under the hood, promisify uses denodeify to create the promise wrapper over individual async functions.

This package uses the new scoped package support available with npm versions greater than 2.7.0. If you're not familiar with using scoped packages, see this page.

Example: promisify a loaded module

    const promisify = require('@atomiq/promisify');

    const fs = promisify(require('fs'));
    
    let f = require('path').join(__dirname, './file.txt');
    
    fs.readFile(f, 'utf8')
        .then(data => {
          // do something with data
        })
        .catch(err => {
          // handle error
        });

  });

By promisifying fs, you can use the above style instead of the typical callback style shown below:

    var fs = require('fs');
    
    var f = require('path').join(__dirname, './file.txt');
    
    fs.readFile(f, 'utf8', function(err, data) {
      if (err) {
        // handle error
      } else {
        do something with data
      }
    }

Example: promisify a specific function

    var asyncFunc = function(callback) {
      // call back with result asynchronously
      setImmediate(() => callback(null, true));
    }

    // or, with with io.js fat arrow support (`node --harmony_arrow_functions`)
    let asyncFunc = callback => {
      // call back with result asynchronously
      setImmediate(() => callback(null, true));
    };

    let p = promisify(asyncFunc);

    p().then(result => {
      // do something with result
    }).catch(err => {
      // handle error
    });

More examples

See tests for more examples.

npm test

A bit more information

For trivial examples, promises might not seem to offer much of a difference over standard callback-style async functions. However, one of the nice features of promises is that promise handlers can be chained, avoiding the "Pyramid of Doom" of nested callbacks in more complicated control flow situations. This makes control flow logic easier to write and easier to follow. Compare the following example:

Idiomatic Node async callbacks

    asyncFunc(function(err, data) {
      if (err) { /* handle error */ }
      anotherAsyncFunc(function(err, data) {
        if (err) { /* handle error */ }
        andAnotherAsyncFunc(function(err, data) {
          if (err) { /* handle error */ }
          // finish processing data
        });
      });
    });

Promise version

    asyncFunc(function(data) {
      // process data
      return anotherAsyncFunc();
    })
    .then(function(data) {
      // process data
      return andAnotherAsyncfunc();
    })
    .then(function(data) {
      // finish processing data
    })
    .catch(function(err) {
      // handle error
    });