Node's Event-driven Architecture
- Callbacks
- Promises
- async/await
- The EventEmitter module
- Arguments
- errors
- order of listeners
- Practical Example: Task List Manager
Asynchrony
Callback
fs.readFile('/foo.txt', function(err, data) {
// If an error occurred, handle it (throw, propagate, etc)
if(err) {
console.log('Unknown Error');
return;
}
// Otherwise, log the file contents
console.log(data);
});
Callbacks !== Asynchrony
async.map(['file1','file2','file3'], fs.stat, function(err, results) {
// results is now an array of stats for each file
});
async.filter(['file1','file2','file3'], function(filePath, callback) {
fs.access(filePath, function(err) {
callback(null, !err)
});
}, function(err, results) {
// results now equals an array of the existing files
});
async.parallel([
function(callback) { ... },
function(callback) { ... }
], function(err, results) {
// optional callback
});
async.series([
function(callback) { ... },
function(callback) { ... }
]);
fs.readdir(source, function (err, files) {
if (err) {
console.log('Error finding files: ' + err)
} else {
files.forEach(function (filename, fileIndex) {
console.log(filename)
gm(source + filename).size(function (err, values) {
if (err) {
console.log('Error identifying file size: ' + err)
} else {
console.log(filename + ' : ' + values)
aspect = (values.width / values.height)
widths.forEach(function (width, widthIndex) {
height = Math.round(width / aspect)
console.log('resizing ' + filename + 'to ' + height + 'x' + height)
this.resize(width, height).write(dest + 'w' + width + '_' + filename, function(err) {
if (err) console.log('Error writing file: ' + err)
})
}.bind(this))
}
})
})
}
})
var form = document.querySelector('form')
form.onsubmit = function (submitEvent) {
var name = document.querySelector('input').value
request({
uri: "http://example.com/upload",
body: name,
method: "POST"
}, function (err, response, body) {
var statusMessage = document.querySelector('.status')
if (err) return statusMessage.value = err
statusMessage.value = body
})
}
How do I fix callback hell?
1. Keep your code shallow
document.querySelector('form').onsubmit = formSubmit
function formSubmit (submitEvent) {
var name = document.querySelector('input').value
request({
uri: "http://example.com/upload",
body: name,
method: "POST"
}, postResponse)
}
function postResponse (err, response, body) {
var statusMessage = document.querySelector('.status')
if (err) return statusMessage.value = err
statusMessage.value = body
}
2. Modularize
module.exports.submit = formSubmit
function formSubmit (submitEvent) {
var name = document.querySelector('input').value
request({
uri: "http://example.com/upload",
body: name,
method: "POST"
}, postResponse)
}
function postResponse (err, response, body) {
var statusMessage = document.querySelector('.status')
if (err) return statusMessage.value = err
statusMessage.value = body
}
var formUploader = require('formuploader')
document.querySelector('form').onsubmit = formUploader.submit
3. Handle every single error
var fs = require('fs')
fs.readFile('/Does/not/exist', handleFile)
function handleFile (error, file) {
if (error) return console.error('Uhoh, there was an error', error)
// otherwise, continue on and use `file` in your code
}
Promise
Async/Await
--harmony-async-await
--harmony_async_await
To work with async/await
> node --harmony-async-await app.js
EventEmitter
Practical Example: Task List Manager
Task List Manager
Available Commands
Node's Event-driven Architecture
By Shuhratbek Mamadaliyev
Node's Event-driven Architecture
- 949