JavaScript Closures
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Questions and Answers

What is a closure in JavaScript primarily used for?

  • Preventing variable hoisting within functions
  • Maintaining the accessibility of the outer function's variables after it finishes executing (correct)
  • Creating global variables accessible from anywhere
  • Automatically executing functions on page load
  • What keyword is used to create a new object from a constructor function in JavaScript?

  • new (correct)
  • function
  • create
  • object
  • How can closures be beneficial in constructor functions?

  • They simplify syntax for function calls
  • They enable the creation of private members, ensuring data encapsulation (correct)
  • They allow external modification of object properties
  • They increase the execution speed of functions
  • What does the Document Object Model (DOM) represent in web development?

    <p>A hierarchical tree structure that represents the HTML document (B)</p> Signup and view all the answers

    Which of the following best describes lexical scope in JavaScript?

    <p>The accessibility of variables based on their location in the code (B)</p> Signup and view all the answers

    What are nodes in the context of the Document Object Model?

    <p>The various components of the hierarchical tree representing the web page (C)</p> Signup and view all the answers

    What is an Immediately Invoked Function Expression (IIFE)?

    <p>A function defined and immediately executed at the same time (B)</p> Signup and view all the answers

    Which of the following is NOT a reason for using closures in programming?

    <p>To improve the readability of the code (D)</p> Signup and view all the answers

    What is the purpose of the nodeType property in the DOM?

    <p>To determine the type of node (B)</p> Signup and view all the answers

    Which method would you use to retrieve an element by its unique ID?

    <p>document.getElementById() (D)</p> Signup and view all the answers

    What does the getElementsByTagName() method return?

    <p>A HTMLCollection of elements (B)</p> Signup and view all the answers

    How do you convert an HTMLCollection to an array?

    <p>Using Array.from() (B)</p> Signup and view all the answers

    What does the querySelector() method do?

    <p>Finds and returns the first matching element based on CSS selectors (B)</p> Signup and view all the answers

    Which selector does '*' represent in querySelector()?

    <p>Selects all elements of any type (A)</p> Signup and view all the answers

    If you want to change the inner HTML of a paragraph element, which of the following would be correct?

    <p>document.querySelector('p').innerHTML = 'New Content'; (A), document.getElementsByTagName('p')[0].innerHTML = 'New Content'; (D)</p> Signup and view all the answers

    What does the method getElementsByClassName() return?

    <p>An HTMLCollection of elements that share a class name (C)</p> Signup and view all the answers

    Study Notes

    JavaScript Closures

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

    Closure Example

    • A function (createCounter()) creates another inner function that uses variables declared in the higher-level function.
    • The inner function is a closure.
    • The inner function can access and modify variables in the outer function's scope.

    Closure Use with Constructors

    • JavaScript uses functions, called constructor functions, to create objects.
    • A constructor function is defined, and the new keyword creates a new object from that function.
    • Inside the function, this is used to attach properties and methods to the new object.

    Example of Constructor Function

    • The code defines a function called Person that takes name and age as arguments.
    • Inside the function, this.name = name; and this.age = age; attach the name and age to the new object.
    • person1 and person2 are new objects created from the Person function.
    • console.log(person1.name) //Output: Alice
    • console.log(person2.age) //Output: 35

    Closures and Private Members

    • Using closures, private members can be created within constructor functions, enforcing data encapsulation and preventing direct access to sensitive information.
    • Private data is typically held within a const or let that is enclosed in the constructor's scope.
    • A getter function can be used to access private data, this prevents direct access.

    Unique Identifier Function

    • Closures and Immediately Invoked Function Expressions (IIFEs) can be used to create a unique identifier function.
    • The function generates a unique id each time it's called
    • The code maintains a counter within a closure that increments each time a unique id is needed.

    Document Object Model (DOM)

    • The DOM is a programming interface that makes it possible to access and manipulate HTML elements.
    • The browser creates a hierarchical tree representation of the HTML document when the page loads.
    • The components are called nodes.

    DOM Node Types

    • HTML elements, text, comments, are types of nodes.
    • Each node has a nodeType property indicating its type.

    Accessing DOM Elements

    • document.getElementById() takes an element ID to find that element within the HTML document.
    • document.getElementsByTagName() takes a tag name to find all the elements with that tag.
    • document.querySelector() selects the first element matching a CSS selector.
    • document.querySelectorAll() selects all elements matching a CSS selector.

    Additional DOM Selectors

    • Class Selectors allow you to select elements with a given class.
    • ID Selectors allows you to select elements based on their unique ID.
    • Attribute Selectors let you choose elements with specific attributes.

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    Description

    This quiz covers the concept of closures in JavaScript, focusing on how functions defined within other functions can access their parent scope. You'll explore lexical scope and the practical use of closures with constructor functions. Test your understanding of these critical concepts in JavaScript development.

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