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?

<p>By preventing direct access to sensitive information. (C)</p> Signup and view all the answers

What structure represents an HTML document in JavaScript?

<p>Tree structure (B)</p> Signup and view all the answers

What is the DOM used for in web development?

<p>Manipulating HTML code dynamically. (C)</p> Signup and view all the answers

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

<p>Any element represented in the hierarchical tree. (C)</p> Signup and view all the answers

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

<p>Using a variable to count calls to a function. (A)</p> Signup and view all the answers

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

<p>The DOM element with the specified ID. (C)</p> Signup and view all the answers

What is the purpose of the getElementsByTagName() method?

<p>To retrieve all elements that match a given tag name. (D)</p> Signup and view all the answers

How can you convert an HTMLCollection to an array?

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

Which selector would you use to select the first

element in a document?

<p>document.querySelector('h1') (B)</p> Signup and view all the answers

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

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

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

<p>It will throw an error because HTMLCollection does not have innerHTML. (A)</p> Signup and view all the answers

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

<p>nodeType (C)</p> Signup and view all the answers

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

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

Flashcards

DOM Tree

A JavaScript object that represents the structure of an HTML document. It's like a tree with nodes representing HTML elements, text, comments, etc.

Node Type

A property of a DOM node that indicates its type. It helps you identify whether a node is an element, text, comment, or something else.

document.getElementById('elementId')

A method of the Document object that allows you to retrieve a specific element using its ID attribute.

document.getElementsByTagName('tagName')

A method of the Document object that allows you to retrieve all elements with a matching tag name. It returns an array-like object called HTMLCollection.

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Array.from(HTMLCollection)

A method that converts an HTMLCollection to an array, making it easier to work with.

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document.querySelector('CSS Selector')

A method of the Document object that allows you to select the first element that matches one or more CSS selectors.

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  • (asterisk)

A CSS selector that matches all elements regardless of their type.

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Type Selector (e.g., 'h1', 'p')

A CSS selector that selects elements by their tag name.

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What is a JavaScript Closure?

A closure in JavaScript is created when a function is defined within another function. It has access to variables in its parent scope, even after the outer function has finished executing. A closure allows a function to remember the environment in which it was created, including the variables that were in scope at that time.

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What is Lexical Scope?

Lexical Scope defines the accessibility of variables and functions based on their position in the code. It determines which variables are visible within a particular scope.

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How does a closure maintain its environment?

Closures maintain a reference to their lexical environment, capturing the state of the outer function at the time of its creation. This allows them to access and manipulate variables from the parent function even after it has finished executing.

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What is a constructor function in JavaScript?

A constructor function in JavaScript is used to create objects. It is defined like a regular function but is invoked with the new keyword to generate an object instance.

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How are objects created using constructor functions?

Using the new keyword when calling a constructor function creates a new object instance. This object inherits properties and methods defined within the constructor function.

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What does the this keyword represent in a constructor function?

The this keyword within a constructor function refers to the newly created object instance. It's used to attach properties and methods to this specific object.

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How can closures create private members in JavaScript?

Closures can be used to create private members within constructor functions. These members are accessible only within the constructor function and its inner functions, providing data encapsulation and preventing direct access to sensitive information.

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What is the Document Object Model (DOM)?

The Document Object Model (DOM) represents an HTML document as a tree structure, where elements like <p>, <h1>, <div> etc., are nodes in the tree. It provides a programmatic interface to access and manipulate HTML content.

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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 <p>, <div>, etc.
  • TEXT_NODE: Contains the actual text content within elements.
  • COMMENT_NODE: Represents HTML comments (e.g., <!-- comment -->).
  • 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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Related Documents

Closure and DOM in JavaScript PDF

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.

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