By the end of this, developers should be able to:
- Explain the value of using promises instead of callback interfaces.
- Read Node documentation that uses callbacks and translate that into implementations using promises.
- Convert Node scripts using callbacks into scripts using promises.
- Fork and clone this repository.
- Install dependencies with
npm install.
Callback drawbacks:
- Callbacks can be messy when they're nested: "callback hell". See
lib/copy-json.js. - Each callback will have to handle it's own errors if necessary.
- In complex programs, it will be hard to tell in what order callbacks fire.
Pros for Promises:
- Promises, like callbacks, make asynchronicity explict.
- Promises, unlike callbacks, have a predictable order of execution.
- Promises are easier to read than callbacks.
- Promises can simplify error handling.
Take a few minutes to read the following API documentation for the native Promises.
Note function signatures and argument types as you read. What kind of object does a promise take when it is constructed?
// remember that callback is something you write, in this case to perform some
// processing on parsed JSON
let readJSON = function (filename, callback){
fs.readFile(filename, 'utf8', function (err, res){
if (err) return callback(err); // what's going on here?
callback(null, JSON.parse(res)); // what if JSON.parse errors out?
});
};What are some weaknesses in this code? And the following?
let readJSON = function (filename, callback){ // 👀 here
fs.readFile(filename, 'utf8', function (err, res){
if (err) return callback(err); // pass the error from readFile
try {
res = JSON.parse(res);
} catch (ex) {
return callback(ex); // pass the error from JSON.parse
}
callback(null, res); // don't pass the error, since we should have caught it
});
};What about this instead?
let readJSON = function (filename){ // 👀 here
return new Promise((resolve, reject) => {
fs.readFile(filename, 'utf8', (err, res) => {
if (err) {
reject(err);
} else {
resolve(res);
}
});
}).then((res) => {
return JSON.parse(res)
}).catch((err) => {
console.log(err);
});
};That's too verbose. This is better:
let readJSON = function (filename){
return new Promise((resolve, reject) => {
fs.readFile(filename, 'utf8', (err, res) => {
if (err) {
reject(err);
} else {
resolve(res);
}
});
}).then(JSON.parse).catch(console.log); // what can we surmise about .then?
};- Promise - JavaScript | MDN
- Promises
- Promisees · Courtesy of ponyfoo.com
- wbinnssmith/awesome-promises: A curated list of useful resources for JavaScript Promises
- How to escape Promise Hell — Medium
Source code distributed under the MIT license. Text and other assets copyright General Assembly, Inc., all rights reserved.
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