Introduction
Node is a programming language where everything could run in an asynchronous way. Below you could find some examples and the typical things of asynchronous working.
Syntax
doSomething([args], function([argsCB]) {
/* do something when done */
});
doSomething([args], ([argsCB]) => {
/* do something when done */
});Examples
Callback functions in JavaScript
Callback functions are common in JavaScript. Callback functions are possible in JavaScript because functions are first-class citizens.
First-class citizens mean the language supports passing functions as arguments to other functions, returning them as the values from other functions, and assigning them to variables or storing them in data structures.
Synchronous callbacks
Callback functions can be synchronous or asynchronous. Since Asynchronous callback functions may be more complex here is a simple example of an asynchronous callback function.
1. // a function that uses a callback named `cb` as a parameter
2. function getSyncMessage(cb) {
3. cb("Hello World!");
4. }
5.
6. console.log("Before getSyncMessage call");
7. // calling a function and sending in a callback function as an argument.
8. getSyncMessage(function(message) {
9. console.log(message);
10. });
11. console.log("After getSyncMessage call");The output for the above code is:
> Before getSyncMessage call
> Hello World!
> After getSyncMessage call
First, we will step through how the above code is executed. This is more for those who do not already understand the concept of callbacks if you do already understand it feel free to skip this paragraph.
First, the code is parsed and then the first interesting thing to happen is line 6 is executed which outputs Before getSyncMessage call to the console.
Then line 8 is executed which calls the function getSyncMessage sending in an anonymous function as an argument for the parameter named cb in the getSyncMessage function.
Execution is now done inside the getSyncMessage function on line 3 which executes the function cb which was just passed in, this call sends an argument string "Hello World" for the param named message in the passed in an anonymous function.
Execution then goes to line 9 which logs Hello World! to the console.
Then the execution goes through the process of exiting the callstack hitting line 10 then line 4 then finally back to line 11.
Some information to know about callbacks in general:
- The function you send into a function as a callback may be called zero times, once, or multiple times. It all depends on implementation.
- The callback function may be called synchronously or asynchronously and possibly both synchronously and asynchronously.
- Just like normal functions, the names you give parameters to your function are not important but the order is. So for example on line 8, the parameter message could have been named statement, msg, or if you're being nonsensical something like a jellybean. So you should know what parameters are sent into your callback so you can get them in the right order with proper names.
Asynchronous callbacks.
One thing to note about JavaScript is it is synchronous by default, but there are APIs have given in the environment (browser, Node.js, etc.) that could make it asynchronous.
Some common things that are asynchronous in JavaScript environments that accept callbacks:
• Events• setTimeout• setInterval• the fetch API• Promises
Also, any function that uses one of the above functions may be wrapped with a function that takes a callback and the callback would then be an asynchronous callback (although wrapping a promise with a function that takes a callback would likely be considered an anti-pattern as there are more preferred ways to handle promises).
So given that information, we can construct an asynchronous function similar to the above synchronous one.
1. // a function that uses a callback named `cb` as a parameter
2. function getAsyncMessage(cb) {
3. setTimeout(function () { cb("Hello World!") }, 1000);
4. }
5.
6. console.log("Before getSyncMessage call");
7. // calling a function and sending in a callback function as an argument.
8. getAsyncMessage(function(message) {
9. console.log(message);
10. });
11. console.log("After getSyncMessage call");Which prints the following to the console:
> Before getSyncMessage call > After getSyncMessage call > // pauses for 1000 ms with no output > Hello World!
Line execution goes to line 6 logs "Before getSyncMessage call".
Then execution goes to line 8 calling getAsyncMessage with a callback for the param cb.
Line 3 is then executed which calls setTimeout with a callback as the first argument and the number 300 as the second argument.
setTimeout does whatever it does and holds on to that callback so that it can call it later in 1000 milliseconds, but following setting up the timeout and before it pauses the 1000 milliseconds it hands execution back to where it left off so it goes to line 4, then line 11, and then pauses for 1 second and setTimeout then calls its callback function which takes execution back to line 3 where getAsyncMessages callback is called with value "Hello World" for its parameter message which is then logged to the console on line 9.
Callback functions in Node.js
NodeJS has asynchronous callbacks and commonly supplies two parameters to your functions sometimes conventionally called err and data. An example of reading a file text.
const fs = require("fs");
fs.readFile("./test.txt", "utf8", function(err, data) {
if(err) {
// handle the error
} else {
// process the file text given with data
}
});This is an example of a callback that is called a single time.
It's good practice to handle the error somehow even if you are just logging it or throwing it. The else is not necessary if you throw or return and can be removed to decrease indentation so long as you stop the execution of the current function in the if by doing something like throwing or returning.
Though it may be common to see err, data it may not always be the case that your callbacks will use that pattern.
Another example callback comes from the express library (express 4.x):
// this code snippet was on http://expressjs.com/en/4x/api.html
const express = require('express');
const app = express();
// this app.get method takes a url route to watch for and a callback
// to call whenever that route is requested by a user.
app.get('/', function(req, res){
res.send('hello world');
});
app.listen(3000);
This example shows a callback that is called multiple times. The callback is provided with two objects as params named here as req and res these names correspond to request and response respectively, and they provide ways to view the request coming in and set up the response that will be sent to the user.
As you can see there are various ways a callback can be used to execute sync and async code in JavaScript and callbacks are very ubiquitous throughout JavaScript.
Code example
Question: What is the output of the code below and why?
setTimeout(function() {
console.log("A");
}, 1000);
setTimeout(function() {
console.log("B");
}, 0);
getDataFromDatabase(function(err, data) {
console.log("C");
setTimeout(function() {
console.log("D");
}, 1000);
});
console.log("E");
Output: This is known for sure: E B A D. C is unknown when it will be logged.
Explanation: The compiler will not stop on the setTimeout and the getDataFromDatabase methods.
So the first line he will log is E. The callback functions (first argument of setTimeout) will run after the set timeout on an asynchronous way!
More details:
1. E has no setTimeout
2. B has a set timeout of 0 milliseconds
3. A has a set timeout of 1000 milliseconds
4. D must request a database after it must D wait 1000 milliseconds so it comes after A.
5. C is unknown because it is unknown when the data of the database is requested. It could be before or after A.
Async error handling
Try catch
Errors must always be handled. If you are using synchronous programming you could use a try-catch. But this does not work if you work asynchronously!
Example:
try {
setTimeout(function() {
throw new Error("I'm an uncaught error and will stop the server!");
}, 100);
} catch (ex) {
console.error("This error will not be work in an asynchronous situation: " + ex);
}Async errors will only be handled inside the callback function!
Callback hell
Callback hell (also a pyramid of doom or boomerang effect) arises when you nest too many callback functions inside a callback function.
Here is an example to read a file (in ES6).
const fs = require('fs');
let filename = `${__dirname}/myfile.txt`;fs.exists(filename, exists => {
if (exists) {
fs.stat(filename, (err, stats) => {
if (err) { throw err; }
if (stats.isFile()) {
fs.readFile(filename, null, (err, data) => {
if (err) { throw err; }
console.log(data);
});
} else {
throw new Error("This location contains not a file");
}
});
} else {
throw new Error("404: file not found");
}
});How to avoid "Callback Hell"
It is recommended to nest no more than 2 callback functions. This will help you maintain code readability and will be much easier to maintain in the future. If you have a need to nest more than 2 callbacks, try to make use of distributed events instead.
There also exists a library called async that helps manage callbacks and their execution available on npm. It increases the readability of callback code and gives you more control over your callback code flow, including allowing you to run them in parallel or in series.
There are four solutions to callback hell:
- Write comments
- Split functions into smaller functions
- Using Promises
- Using Async/await
