100 JavaScript MCQ Questions and Answers

Correct Answer: C. A scripting language used to add interactivity to web pages. JavaScript is a scripting language that enables dynamic and interactive elements on websites.

Correct Answer: B. Brendan Eich. Brendan Eich created JavaScript while working at Netscape Communications.

Correct Answer: C. Defining the structure and content of a web page. Defining the structure and content of a web page is primarily done with HTML, not JavaScript.

Correct Answer: A. On the user’s web browser. Client-side JavaScript runs within the user’s web browser.

Correct Answer: C. ECMAScript is the standardized specification that JavaScript is based on. ECMAScript provides the standards that JavaScript implementations adhere to.

Correct Answer: C. Object. An object is a complex data type, not a primitive one.

Correct Answer: C. “object”. This is a quirk of JavaScript; typeof null returns “object”.

Correct Answer: B. A unique and immutable value. Symbols are primarily used as unique property keys in objects.

Correct Answer: D. Both B and C. Both let and const declare variables with block scope.

Correct Answer: A. let can be reassigned, while const cannot. let allows for reassignment of a variable, whereas const creates a read-only variable.

Correct Answer: C. It will throw a TypeError. Reassigning a const variable results in a TypeError.

Correct Answer: B. Implicitly converting one data type to another. Type coercion is JavaScript’s automatic conversion of values from one data type to another.

Correct Answer: C. “55”. The number 5 is coerced into a string, resulting in string concatenation.

Correct Answer: D. Both B and C. Both parseInt() and parseFloat() convert strings to numbers, with the latter handling decimals.

Correct Answer: B. Divides two numbers and returns the remainder. The modulus operator gives the remainder after division.

Correct Answer: A. true. The expression evaluates to true because 5 > 2 is true, and the || operator only needs one condition to be true.

Correct Answer: C. =. The single equals sign (=) is the assignment operator.

Correct Answer: C. Both value and type equality. The strict equality operator ensures that both the value and the data type are the same.

Correct Answer: A. == performs type coercion, while === does not. The double equals (==) allows for type coercion, while the triple equals (===) requires the types to be the same.

Correct Answer: B. Multiplication. Multiplication has higher precedence than addition.

Correct Answer: A. The order of operations when operators have the same precedence. Associativity defines the direction of operation when operators have equal precedence.

Correct Answer: C. To provide a shorthand way to write an if-else statement. The ternary operator offers a concise way to express conditional logic.

Correct Answer: B. A sequence of characters enclosed in single or double quotes. String literals are how you represent text within your code.

Correct Answer: B. length. The length property provides the number of characters in a string.

Correct Answer: B. Converts the string to uppercase. toUpperCase() transforms all characters in a string to uppercase.

Correct Answer: C. Both A and B. You can concatenate strings with either the + operator or the concat() method.

Correct Answer: D. All of the above. All three methods (substring(), slice(), and substr()) can be used to extract parts of a string, each with slightly different behaviors.

Correct Answer: C. A special value indicating an invalid number operation. NaN signifies that a mathematical operation did not result in a meaningful number.

Correct Answer: C. Math.round(). Math.round() rounds a number to the nearest integer.

Correct Answer: C. true and false. Booleans hold either true or false.

Correct Answer: B. Any value that evaluates to true in a boolean context. Truthy values are treated as true in conditions.

Correct Answer: D. All of the above. All listed values are considered falsy in JavaScript.

Correct Answer: C. Both A and B.

Correct Answer: B. 0. JavaScript arrays are zero-indexed.

Correct Answer: B. Adds an element to the end of the array. push() appends an element to the end.

Correct Answer: D. Removes the last element from the array. pop() removes and returns the last element.

Correct Answer: B. unshift(). unshift() adds elements to the front of an array.

Correct Answer: A. Add or remove elements at any position. splice() is versatile for adding and removing elements at specific indices.

Correct Answer: B. for...in loop. The for...in loop is used to loop through the keys of an object.

Correct Answer: A. Executes a provided function once for each array element. forEach() iterates over array elements and applies a function to each.

Correct Answer: C. Both A and B. Both the object literal ({}) and the Object() constructor create objects.

Correct Answer: A. It refers to the current object. this provides a way to access properties and methods within the object itself.

Correct Answer: A. Dot notation and bracket notation.

Correct Answer: A. Executes a block of code only if a specified condition is true. if statements control code execution based on a condition.

Correct Answer: B. To specify a block of code to execute if the if condition is false. The else block provides an alternative path when the if condition isn’t met.

Correct Answer: C. Only if the preceding if condition is false and the else if condition is true. else if provides additional conditional checks when the previous if is false.

Correct Answer: B. To provide an alternative to using multiple if-else if statements. switch statements offer a cleaner way to handle multiple possible values of a variable.

Correct Answer: A. Exits the entire switch statement. break ensures that only the matching case’s code is executed.

Correct Answer: B. The code execution continues to the next case, even if it doesn’t match. Without break, execution “falls through” to subsequent cases.

Correct Answer: A. To execute a block of code repeatedly for a specific number of times. for loops are designed for repeated execution based on a counter.

Correct Answer: B. while loop. A while loop continues as long as its condition remains true.

Correct Answer: A. A while loop checks the condition before executing the code block, while a do...while loop checks it after. A do...while loop guarantees at least one execution of its code block.

Correct Answer: C. for...of loop. The for...of loop provides a clean way to iterate through array values.

Correct Answer: B. Exits the entire loop immediately. break terminates the loop entirely.

Correct Answer: B. Skips the rest of the current iteration and continues with the next iteration. continue jumps to the next loop iteration.

Correct Answer: A. A block of code that can be reused multiple times. Functions encapsulate reusable blocks of code.

Correct Answer: B. function. The function keyword starts a function declaration.

Correct Answer: A. Values passed to a function when it is called. Parameters act as placeholders for values that a function will receive.

Correct Answer: C. To specify the value that the function should return. return sends a value back from the function.

Correct Answer: B. The accessibility of variables within and outside a function. Scope determines which variables a function can access.

Correct Answer: A. A shorthand way to write functions. Arrow functions provide a more concise syntax for function expressions.

Correct Answer: B. Regular functions have their own this binding, while arrow functions do not. Arrow functions inherit this from their surrounding context.

Correct Answer: B. To handle potential errors that may occur in a block of code. try...catch allows you to gracefully manage runtime errors.

Correct Answer: B. Generates a custom error. throw creates an exception that can be caught.

Correct Answer: B. Document Object Model. The DOM is a tree-like representation of an HTML document.

Correct Answer: B. Individual elements, attributes, and text within an HTML document. Nodes are the fundamental building blocks of the DOM tree.

Correct Answer: A. getElementById(). getElementById() retrieves a single element with a specific ID.

Correct Answer: B. A live HTMLCollection of elements with the specified tag name. getElementsByTagName() gives you a live collection of elements.

Correct Answer: B. querySelectorAll() returns a NodeList, while querySelector() returns a single element. querySelector() gets the first matching element, while querySelectorAll() gets all of them.

Correct Answer: C. Both A and B. Both innerHTML (which can include HTML tags) and textContent (plain text only) can modify element content.

Correct Answer: A. setAttribute(). setAttribute() updates or adds an attribute to an element.

Correct Answer: C. Both A and B. You can directly manipulate the style object or add/remove CSS classes.

Correct Answer: A. createElement(). createElement() generates a DOM element that you can then insert.

Correct Answer: B. Adds a new child node to the end of a parent node. appendChild() inserts a node as the last child of an element.

Correct Answer: A. A function that waits for a specific event to occur on an element. Event listeners are set up to react to user interactions or other occurrences.

Correct Answer: A. addEventListener(). addEventListener() is the standard way to register event handlers.

Correct Answer: D. resize. While resize is an event, it’s related to the window, not a typical element event like the others.

Correct Answer: D. The way events are passed from child elements to their parent elements. Event bubbling describes how an event propagates up the DOM tree from the target element to its ancestors.

Correct Answer: A. To store information about the event that occurred. The event object provides details like the event type, target element, and mouse coordinates.

Correct Answer: B. An object that serves as a template for other objects. Prototypes provide a mechanism for inheritance in JavaScript.

Correct Answer: C. To create a blueprint for objects. The class keyword provides a more structured way to define object blueprints (constructors) and their associated methods.

Correct Answer: A. A special method that is called when an object is created from a class. Constructors initialize the properties of newly created objects.

Correct Answer: A. The process of combining data and methods that operate on that data within a single unit (the object). Encapsulation bundles data and methods, promoting organization and modularity.

Correct Answer: B. The ability of objects to share behaviors and characteristics from other objects. Inheritance allows objects to inherit properties and methods, reducing code duplication.

Correct Answer: A. The ability of objects to take on many forms. Polymorphism allows objects of different classes to be treated as objects of a common type.

Correct Answer: A. A function that is passed as an argument to another function and is executed after the first function completes. Callbacks are a way to handle operations that take time to complete, like network requests.

Correct Answer: B. An object that represents the eventual result of an asynchronous operation. Promises provide a more structured way to handle asynchronous tasks than callbacks.

Correct Answer: A. Pending, Fulfilled, Rejected. A Promise can be pending (ongoing), fulfilled (successful), or rejected (failed).

Correct Answer: C. To simplify working with Promises. async/await makes asynchronous code look and behave a bit more like synchronous code.

Correct Answer: B. A lightweight data interchange format. JSON is a text-based format for representing data as key-value pairs.

Correct Answer: B. JSON.stringify(). JSON.stringify() serializes a JavaScript object into JSON format.

Correct Answer: A. A pattern used to match character combinations in strings. Regular expressions are powerful tools for text processing and validation.

Correct Answer: C. To check if a string matches a pattern. The test() method checks for matches between a regular expression and a string.

Correct Answer: A. A reusable block of code that can be imported into other files. Modules help organize code and promote reusability.

Correct Answer: B. A programming interface for interacting with the web browser. The BOM provides objects for controlling browser features like the history and window.

Correct Answer: B. window. The window object is the top-level object in the BOM.

Correct Answer: C. navigator. The navigator object gives details about the browser itself.

Correct Answer: D. All of the above. All three events are related to mouse clicks, but click represents a complete click action.

Correct Answer: B. mouseover. mouseover fires when the mouse enters the area of an element.

Correct Answer: A. setTimeout(). setTimeout() schedules a function to run once after a given delay.