Can anyone please explain event delegation in JavaScript and how is it useful?
Javascript – DOM Event delegation
dom-eventsevent handlingevent-delegationjavascript
Related Solutions
[Edited 2021-10-16 to reflect latest best-practices for producing RFC4122-complaint UUIDs]
Most readers here will want to use the uuid
module. It is well-tested and supported.
The crypto.randomUUID()
function is an emerging standard that is supported in Node.js
and an increasing number of browsers.
If neither of those work for you, there is this method (based on the original answer to this question):
function uuidv4() {
return ([1e7]+-1e3+-4e3+-8e3+-1e11).replace(/[018]/g, c =>
(c ^ crypto.getRandomValues(new Uint8Array(1))[0] & 15 >> c / 4).toString(16)
);
}
console.log(uuidv4());
Note: The use of any UUID generator that relies on Math.random() is strongly discouraged (including snippets featured in previous versions of this answer) for reasons best-explained here. TL;DR: Math.random()-based solutions do not provide good uniqueness guarantees.
A closure is a pairing of:
- A function, and
- A reference to that function's outer scope (lexical environment)
A lexical environment is part of every execution context (stack frame) and is a map between identifiers (ie. local variable names) and values.
Every function in JavaScript maintains a reference to its outer lexical environment. This reference is used to configure the execution context created when a function is invoked. This reference enables code inside the function to "see" variables declared outside the function, regardless of when and where the function is called.
If a function was called by a function, which in turn was called by another function, then a chain of references to outer lexical environments is created. This chain is called the scope chain.
In the following code, inner
forms a closure with the lexical environment of the execution context created when foo
is invoked, closing over variable secret
:
function foo() {
const secret = Math.trunc(Math.random()*100)
return function inner() {
console.log(`The secret number is ${secret}.`)
}
}
const f = foo() // `secret` is not directly accessible from outside `foo`
f() // The only way to retrieve `secret`, is to invoke `f`
In other words: in JavaScript, functions carry a reference to a private "box of state", to which only they (and any other functions declared within the same lexical environment) have access. This box of the state is invisible to the caller of the function, delivering an excellent mechanism for data-hiding and encapsulation.
And remember: functions in JavaScript can be passed around like variables (first-class functions), meaning these pairings of functionality and state can be passed around your program: similar to how you might pass an instance of a class around in C++.
If JavaScript did not have closures, then more states would have to be passed between functions explicitly, making parameter lists longer and code noisier.
So, if you want a function to always have access to a private piece of state, you can use a closure.
...and frequently we do want to associate the state with a function. For example, in Java or C++, when you add a private instance variable and a method to a class, you are associating state with functionality.
In C and most other common languages, after a function returns, all the local variables are no longer accessible because the stack-frame is destroyed. In JavaScript, if you declare a function within another function, then the local variables of the outer function can remain accessible after returning from it. In this way, in the code above, secret
remains available to the function object inner
, after it has been returned from foo
.
Uses of Closures
Closures are useful whenever you need a private state associated with a function. This is a very common scenario - and remember: JavaScript did not have a class syntax until 2015, and it still does not have a private field syntax. Closures meet this need.
Private Instance Variables
In the following code, the function toString
closes over the details of the car.
function Car(manufacturer, model, year, color) {
return {
toString() {
return `${manufacturer} ${model} (${year}, ${color})`
}
}
}
const car = new Car('Aston Martin','V8 Vantage','2012','Quantum Silver')
console.log(car.toString())
Functional Programming
In the following code, the function inner
closes over both fn
and args
.
function curry(fn) {
const args = []
return function inner(arg) {
if(args.length === fn.length) return fn(...args)
args.push(arg)
return inner
}
}
function add(a, b) {
return a + b
}
const curriedAdd = curry(add)
console.log(curriedAdd(2)(3)()) // 5
Event-Oriented Programming
In the following code, function onClick
closes over variable BACKGROUND_COLOR
.
const $ = document.querySelector.bind(document)
const BACKGROUND_COLOR = 'rgba(200,200,242,1)'
function onClick() {
$('body').style.background = BACKGROUND_COLOR
}
$('button').addEventListener('click', onClick)
<button>Set background color</button>
Modularization
In the following example, all the implementation details are hidden inside an immediately executed function expression. The functions tick
and toString
close over the private state and functions they need to complete their work. Closures have enabled us to modularise and encapsulate our code.
let namespace = {};
(function foo(n) {
let numbers = []
function format(n) {
return Math.trunc(n)
}
function tick() {
numbers.push(Math.random() * 100)
}
function toString() {
return numbers.map(format)
}
n.counter = {
tick,
toString
}
}(namespace))
const counter = namespace.counter
counter.tick()
counter.tick()
console.log(counter.toString())
Examples
Example 1
This example shows that the local variables are not copied in the closure: the closure maintains a reference to the original variables themselves. It is as though the stack-frame stays alive in memory even after the outer function exits.
function foo() {
let x = 42
let inner = function() { console.log(x) }
x = x+1
return inner
}
var f = foo()
f() // logs 43
Example 2
In the following code, three methods log
, increment
, and update
all close over the same lexical environment.
And every time createObject
is called, a new execution context (stack frame) is created and a completely new variable x
, and a new set of functions (log
etc.) are created, that close over this new variable.
function createObject() {
let x = 42;
return {
log() { console.log(x) },
increment() { x++ },
update(value) { x = value }
}
}
const o = createObject()
o.increment()
o.log() // 43
o.update(5)
o.log() // 5
const p = createObject()
p.log() // 42
Example 3
If you are using variables declared using var
, be careful you understand which variable you are closing over. Variables declared using var
are hoisted. This is much less of a problem in modern JavaScript due to the introduction of let
and const
.
In the following code, each time around the loop, a new function inner
is created, which closes over i
. But because var i
is hoisted outside the loop, all of these inner functions close over the same variable, meaning that the final value of i
(3) is printed, three times.
function foo() {
var result = []
for (var i = 0; i < 3; i++) {
result.push(function inner() { console.log(i) } )
}
return result
}
const result = foo()
// The following will print `3`, three times...
for (var i = 0; i < 3; i++) {
result[i]()
}
Final points:
- Whenever a function is declared in JavaScript closure is created.
- Returning a
function
from inside another function is the classic example of closure, because the state inside the outer function is implicitly available to the returned inner function, even after the outer function has completed execution. - Whenever you use
eval()
inside a function, a closure is used. The text youeval
can reference local variables of the function, and in the non-strict mode, you can even create new local variables by usingeval('var foo = …')
. - When you use
new Function(…)
(the Function constructor) inside a function, it does not close over its lexical environment: it closes over the global context instead. The new function cannot reference the local variables of the outer function. - A closure in JavaScript is like keeping a reference (NOT a copy) to the scope at the point of function declaration, which in turn keeps a reference to its outer scope, and so on, all the way to the global object at the top of the scope chain.
- A closure is created when a function is declared; this closure is used to configure the execution context when the function is invoked.
- A new set of local variables is created every time a function is called.
Links
- Douglas Crockford's simulated private attributes and private methods for an object, using closures.
- A great explanation of how closures can cause memory leaks in IE if you are not careful.
- MDN documentation on JavaScript Closures.
Related Topic
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Best Answer
DOM event delegation is a mechanism of responding to ui-events via a single common parent rather than each child, through the magic of event "bubbling" (aka event propagation).
When an event is triggered on an element, the following occurs:
Event bubbling provides the foundation for event delegation in browsers. Now you can bind an event handler to a single parent element, and that handler will get executed whenever the event occurs on any of its child nodes (and any of their children in turn). This is event delegation. Here's an example of it in practice:
With that example if you were to click on any of the child
<li>
nodes, you would see an alert of"click!"
, even though there is no click handler bound to the<li>
you clicked on. If we boundonclick="..."
to each<li>
you would get the same effect.So what's the benefit?
Imagine you now have a need to dynamically add new
<li>
items to the above list via DOM manipulation:Without using event delegation you would have to "rebind" the
"onclick"
event handler to the new<li>
element, in order for it to act the same way as its siblings. With event delegation you don't need to do anything. Just add the new<li>
to the list and you're done.This is absolutely fantastic for web apps with event handlers bound to many elements, where new elements are dynamically created and/or removed in the DOM. With event delegation the number of event bindings can be drastically decreased by moving them to a common parent element, and code that dynamically creates new elements on the fly can be decoupled from the logic of binding their event handlers.
Another benefit to event delegation is that the total memory footprint used by event listeners goes down (since the number of event bindings go down). It may not make much of a difference to small pages that unload often (i.e. user's navigate to different pages often). But for long-lived applications it can be significant. There are some really difficult-to-track-down situations when elements removed from the DOM still claim memory (i.e. they leak), and often this leaked memory is tied to an event binding. With event delegation you're free to destroy child elements without risk of forgetting to "unbind" their event listeners (since the listener is on the ancestor). These types of memory leaks can then be contained (if not eliminated, which is freaking hard to do sometimes. IE I'm looking at you).
Here are some better concrete code examples of event delegation:
focus
andblur
events (which do not bubble)