Closures in JavaScript

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Questions and Answers

What is a closure in JavaScript responsible for?

Executing commands outside of function scope.

Returning undefined values.

Creating new global variables.

Accessing variables in its parent scope. (correct)

What does lexical scope refer to in JavaScript?

Utilizing external libraries to enhance functionality.

The ability to create global variables within functions.

The location of variables and functions in the source code. (correct)

Dynamic access to variables based on runtime conditions.

Which keyword is used to create an object from a constructor function in JavaScript?

new (correct)

init

function

define

How do closures enhance data encapsulation in JavaScript?

By preventing direct access to sensitive information. Signup and view all the answers

What structure represents an HTML document in JavaScript?

Tree structure Signup and view all the answers

What is the DOM used for in web development?

Manipulating HTML code dynamically. Signup and view all the answers

Which of the following best describes a node in the context of the DOM?

Any element represented in the hierarchical tree. Signup and view all the answers

What method can be utilized to create a unique identifier using closures and IIFEs?

Using a variable to count calls to a function. Signup and view all the answers

What does the method document.getElementById('elementId') return?

The DOM element with the specified ID. Signup and view all the answers

What is the purpose of the getElementsByTagName() method?

To retrieve all elements that match a given tag name. Signup and view all the answers

How can you convert an HTMLCollection to an array?

Using the Array.from() method. Signup and view all the answers

Which selector would you use to select the first
element in a document?

document.querySelector('h1') Signup and view all the answers

What does the asterisk (*) selector represent in querySelector?

All elements of any type. Signup and view all the answers

What will happen if you call document.getElementsByTagName('p').innerHTML?

It will throw an error because HTMLCollection does not have innerHTML. Signup and view all the answers

Which property is used to determine the type of a node in the DOM?

nodeType Signup and view all the answers

Which method would you use to select the first element that matches a CSS selector?

document.querySelector() Signup and view all the answers

Study Notes

Closure in JavaScript

A closure is created when a function is defined inside another function.
It retains access to variables in its parent function's scope, even after the outer function has finished executing.
This enables functions to "remember" their creation environment, including enclosing variables.
Lexical scope defines the accessibility of variables and functions based on their location in the source code. A closure maintains a reference to its lexical environment, capturing the state of the outer function at its creation time.

Example of Closure

A createCounter() function creates a closure.
It maintains an inner function that increments and returns the counter variable. This inner function has access to the outer function's variable count.
Each call to createCounter() generates a new closure instance, maintaining its own count and returning unique values.

Closure for Private Members

Closures can be used to create private data members within constructor functions.
This ensures data encapsulation. This approach protects sensitive data from direct access.

Unique Identifier Function Using Closures

Closures and immediately invoked function expressions (IIFEs) can produce a unique identifier on each call.
The IIFE sets up a counter variable.
The returned function increments the counter to generate a successive identifier each time called.

Document Object Model (DOM)

In JavaScript, the DOM is a programming interface for dynamically manipulating HTML.
It presents a structured tree representation of an HTML document.
Each component of the tree structure is referred to as a node. Nodes encompass elements, text, comments, and more.

DOM Node Types

The Document Object Model (DOM) tree comprises various node types. Each node contains a property nodeType that signifies the node type.
ELEMENT_NODE: Represents HTML elements like , <div>, etc.
TEXT_NODE: Contains the actual text content within elements.
COMMENT_NODE: Represents HTML comments (e.g., ).
DOCUMENT_NODE: Represents the document root.
DOCUMENT_TYPE_NODE: Denotes the document type declaration (e.g., <!DOCTYPE html>).

Accessing DOM Elements

document.getElementById('elementId'): Retrieves a DOM element using its ID.
document.getElementsByTagName('tagName'): Returns a collection of all elements with the specified tag name. The Array.from() function converts the HTMLCollection to an array.
document.querySelector('selector'): Selects the first matching element based on a CSS selector.
document.querySelectorAll('selector'): Returns all matching elements based on a CSS selector, similar to querySelectorAll. This is helpful for processing many matching elements based on a given CSS selector. These methods are very helpful and commonly used for DOM interaction and manipulation.

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Description

Explore the concept of closures in JavaScript, where functions defined within other functions can access variables from their parent function's scope. Understand how closures retain their lexical environment and learn through examples like creating a counter. This quiz examines how closures can be used for creating private members in JavaScript.