Are you ready to test your JavaScript skills and knowledge? JavaScript remains a cornerstone of web development, powering dynamic content on websites across the globe. Whether you’re a beginner looking to solidify your understanding or an experienced developer aiming to brush up on the nuances of the language, tackling a set of targeted questions can be an invaluable exercise. Today, we’re diving into a collection of 50 JavaScript questions that cover a broad range of topics—from basic syntax and operations to more complex concepts like promises, async/await, and event handling.
Let’s start with a teaser:
- Understanding
typeof: What do you thinkconsole.log(typeof null);will output? If you guessed"object", you’re right! This behavior, often considered a quirk of the language, is retained for compatibility reasons. - Creating Objects: JavaScript offers several ways to create objects, including using the
new Object(),Object.create(null), and{}syntaxes. Each method has its unique use cases and benefits. - Array Manipulations: Knowing how to work with arrays is fundamental. For instance,
arr.lengthhelps determine the size of an array, while methods likearr.unshift(element)andArray.prototype.map()allow for adding elements to the beginning and transforming each element in an array, respectively. - Operator Nuances: The
===operator checks for both value and type equality, which is crucial for writing predictable JavaScript code. Meanwhile, the seemingly simple addition of"5" + 3results in"53", showcasing JavaScript’s type coercion in action. - Exception Handling: The
try...catchstatement is vital for gracefully handling exceptions and ensuring your application can recover from unexpected errors. - Asynchronous JavaScript: Understanding promises and the
async/awaitsyntax is key to managing asynchronous operations, a common requirement in modern web applications. - The Power of Prototypes: Functions like
Array.prototype.splice()and creating custom promises withnew Promise(function(resolve, reject) {})demonstrate the flexibility and power of JavaScript’s prototype-based inheritance. - Type Coercion and Comparison: Questions around the behavior of
0.1 + 0.2and the use of thetypeofoperator highlight the importance of understanding JavaScript’s type system and coercion rules. - Advanced Topics: Delving into serialization with
JSON.stringify(), managing data withlocalStorage, and leveraging theSetobject to store unique values are just a few examples of the depth and breadth of JavaScript. - Fundamentals Revisited: Even seasoned developers must occasionally revisit the basics, such as the correct use of
document.querySelector()to select elements from the DOM or the nuances of adding comments in JavaScript code.
These questions barely scratch the surface but serve as a starting point for deeper exploration. Each topic invites you to dive into the intricacies of JavaScript, understand its behavior in different contexts, and apply best practices in your projects.
How many questions can you answer correctly? Challenge yourself, and remember, the journey to mastering JavaScript is ongoing. Embrace the learning process, and don’t hesitate to experiment, ask questions, and share your insights with the community.
Ready to take on the full challenge? Dive into these questions and more as you journey through the fascinating world of JavaScript. Happy coding!
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50 JavaScript Questions
🚀 Test your skills how many can you answer 🚀
What will be the output of the following JavaScript code?
console.log(typeof null);
A) “object”
B) “null”
C) “undefined”
D) “string”
Answer: A) “object”
Explanation: In JavaScript, typeof null is historically returned as “object”, which is considered a bug in the language. This behavior is retained for compatibility reasons.
Which of the following is a correct way to create an object in JavaScript?
A) var obj = new Object();
B) var obj = Object.create(null);
C) var obj = {};
D) All of the above
Answer: D) All of the above
Explanation: All options are valid ways to create an object in JavaScript. Option A uses the new keyword with Object constructor, option B uses the Object.create method, and option C uses object literal syntax.
How do you find the number of elements in an array named arr?
A) arr.length
B) arr.size
C) arr.count
D) arr.quantity
Answer: A) arr.length
Explanation: The .length property is used to find the number of elements in an array.
What does the === operator check?
A) Value equality
B) Type similarity
C) Both value equality and type equality
D) None of the above
Answer: C) Both value equality and type equality
Explanation: The === operator is the strict equality operator, which checks for both value and type equality, ensuring no type coercion occurs.
Which method can be used to round a number to the nearest integer?
A) Math.round()
B) Math.ceil()
C) Math.floor()
D) Math.trunc()
Answer: A) Math.round()
Explanation: Math.round() rounds a number to the nearest integer. Math.ceil() rounds up, Math.floor() rounds down, and Math.trunc() removes the decimal part without rounding.
What is the purpose of the Array.prototype.map() method?
A) To loop through an array
B) To modify each item in an array and create a new array with the modified items
C) To filter items in an array
D) To reduce an array to a single value
Answer: B) To modify each item in an array and create a new array with the modified items
Explanation: Array.prototype.map() creates a new array populated with the results of calling a provided function on every element in the calling array.
What will the following code output?
console.log(0.1 + 0.2 === 0.3);
A) true
B) false
C) undefined
D) Error
Answer: B) false
Explanation: Due to floating-point arithmetic and rounding errors, 0.1 + 0.2 does not exactly equal 0.3 in JavaScript.
Which statement is used to handle exceptions in JavaScript?
A) try…catch
B) if…else
C) for…catch
D) while…catch
Answer: A) try…catch
Explanation: The try…catch statement is used to catch exceptions and handle them gracefully within a block of code.
Which of the following is not a reserved word in JavaScript?
A) undefined
B) with
C) true
D) import
Answer: A) undefined
Explanation: undefined is a type in JavaScript, not a reserved word. with, true, and import are all reserved words.
How do you add an element at the beginning of an array named arr?
A) arr.push(element);
B) arr.unshift(element);
C) arr.pop(element);
D) arr.shift(element);
Answer: B) arr.unshift(element);
Explanation: arr.unshift(element) adds one or more elements to the beginning of an array and returns the new length of the array. arr.push(element) adds elements to the end, arr.pop() removes an element from the end, and arr.shift() removes the first element from an array.
What is the result of “5” + 3 in JavaScript?
A) “53”
B) 8
C) “8”
D) TypeError
Answer: A) “53”
Explanation: In JavaScript, using the + operator with a string and a number results in string concatenation, so the number is converted to a string and concatenated to the other string.
What keyword is used to declare a variable that cannot be reassigned?
A) var
B) let
C) const
D) immutable
Answer: C) const
Explanation: The const keyword declares a block-scoped variable that cannot be reassigned. let allows you to declare a variable that can be reassigned, and var declares a function-scoped or globally-scoped variable.
What does the splice method do to an array?
A) Copies a portion of the array into a new array
B) Changes the contents of an array by removing or replacing existing elements and/or adding new elements
C) Searches the array for specified items and returns their positions
D) All of the above
Answer: B) Changes the contents of an array by removing or replacing existing elements and/or adding new elements
Explanation: The splice method changes the original array by removing, replacing, or adding new elements at a specified index.
How do you create a Promise in JavaScript?
A) Promise.call(function)
B) new Promise(function(resolve, reject) {})
C) Promise.new(function)
D) createPromise(function)
Answer: B) new Promise(function(resolve, reject) {})
Explanation: A Promise is created using the new Promise constructor that takes a function as its argument. This function is called the executor and has two parameters, resolve and reject, which are functions used to resolve or reject the promise.
What is the purpose of the async keyword in JavaScript?
A) To pause the execution of a function
B) To declare a function as asynchronous
C) To iterate over arrays
D) To perform mathematical calculations
Answer: B) To declare a function as asynchronous
Explanation: The async keyword is used to declare a function as asynchronous, allowing it to implicitly return a promise and enabling the use of the await keyword within it.
Which of the following is true about the this keyword in JavaScript?
A) It always refers to the global object
B) It refers to the object from where it was called
C) It cannot be used inside functions
D) It refers to the current module
Answer: B) It refers to the object from where it was called
Explanation: In JavaScript, this refers to the object that is executing the current function. The value of this is determined by the invocation context of the function and may vary.
How can you convert a JSON string into a JavaScript object?
A) JSON.parse(jsonString)
B) JSON.stringify(jsonString)
C) JSON.toObject(jsonString)
D) JSON.fromString(jsonString)
Answer: A) JSON.parse(jsonString)
Explanation: JSON.parse() method parses a JSON string, constructing the JavaScript value or object described by the string.
Which of the following methods can be used to capture events in the capturing phase?
A) element.addEventListener(‘click’, function, true)
B) element.captureEvent(‘click’, function)
C) element.onclick(function, true)
D) element.addEventListener(‘click’, function, false)
Answer: A) element.addEventListener(‘click’, function, true)
Explanation: The addEventListener method can take an optional third argument to specify whether to use capturing or bubbling. Setting it to true uses capturing, while false (or omitting it) uses bubbling.
What will the following code return?
Boolean(“”);
A) true
B) false
C) null
D) undefined
Answer: B) false
Explanation: In JavaScript, converting an empty string (“”) to a boolean results in false. This is because an empty string is considered a falsy value.
How do you declare a class in JavaScript?
A) class MyClass {}
B) function MyClass() {}
C) create class MyClass {}
D) new Class(MyClass) {}
Answer: A) class MyClass {}
Explanation: The class syntax introduced in ES6 (ECMAScript 2015) provides a more clear and concise way to create classes in JavaScript. Option A is the correct syntax for declaring a class.
Which operator is used to check both the value and the type of a variable?
A) ==
B) !=
C) ===
D) !==
Answer: C) ===
Explanation: The strict equality operator (===) checks whether its two operands are equal, both in value and in type.
What method is used to merge two or more arrays in JavaScript?
A) Array.concat()
B) Array.join()
C) Array.merge()
D) Array.combine()
Answer: A) Array.concat()
Explanation: Array.concat() is used to merge two or more arrays. This method does not change the existing arrays but instead returns a new array.
Which of the following is not a valid way to define a function in JavaScript?
A) function myFunc() {}
B) var myFunc = function() {};
C) let myFunc = () => {};
D) let myFunc = function => {};
Answer: D) let myFunc = function => {};
Explanation: Option D is incorrect syntax for defining a function. The correct syntax for an arrow function is let myFunc = () => {};.
How can you stop the propagation of an event in JavaScript?
A) event.stopPropagation()
B) event.preventDefault()
C) event.stop()
D) event.pausePropagation()
Answer: A) event.stopPropagation()
Explanation: event.stopPropagation() is used to stop the propagation of an event, preventing it from bubbling up or down the DOM tree.
What will the following code output to the console?
console.log(“1” – – “1”);
A) 0
B) “11”
C) 2
D) “2”
Answer: C) 2
Explanation: In this case, the subtraction operator (-) converts the string operands into numbers and the double negatives cancel each other out, so it effectively becomes 1 + 1 which equals 2.
Which object is used to store and manipulate text strings with language-sensitive comparison capabilities?
A) String
B) Locale
C) Intl
D) RegExp
Answer: C) Intl
Explanation: The Intl object provides language sensitive string comparison, number formatting, and date and time formatting. The String object is used to represent and manipulate a sequence of characters, but does not have language-sensitive comparison capabilities by itself.
What is the output of the following code?
let x = 10;
const changeX = () => {
let x = 20;
};
changeX();
console.log(x);
A) 10
B) 20
C) undefined
D) Error
Answer: A) 10
Explanation: The variable x inside the changeX function is a local variable with the same name as the global x. The global x remains unchanged after the function call.
Which of the following is a way to declare a variable in JavaScript that ensures block-level scope?
A) var
B) let
C) const
D) Both B and C
Answer: D) Both B and C
Explanation: Both let and const provide block-level scope for variables, unlike var, which provides function-level scope.
Which method can be used to send a network request to a server and load information in JavaScript?
A) XMLHttpRequest
B) fetch()
C) axios()
D) Both A and B
Answer: D) Both A and B
Explanation: Both XMLHttpRequest and the fetch() API can be used to send network requests and load information from a server. axios() is also a popular choice, but it is a third-party library and not a built-in JavaScript method.
What is a JavaScript Promise?
A) A function that performs a task and returns a value
B) An object representing the eventual completion or failure of an asynchronous operation
C) A data structure that stores elements in a LIFO (Last In, First Out) manner
D) A callback function used to execute code after another function has completed
Answer: B) An object representing the eventual completion or failure of an asynchronous operation
Explanation: A Promise in JavaScript is an object that represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.
How can you convert the string ‘123’ to the number 123 in JavaScript?
A) parseInt(‘123’)
B) ‘123’.toNumber()
C) Number(‘123’)
D) Both A and C
Answer: D) Both A and C
Explanation: Both parseInt(‘123’) and Number(‘123’) are valid methods to convert the string ‘123’ into the number 123. parseInt parses a string and returns an integer, while Number can convert a string to a number.
What will the following code snippet output?
console.log([’10’,’10’,’10’,’10’].map(parseInt));
A) [10, 10, 10, 10]
B) [10, NaN, 2, 3]
C) [10, 10, 10]
D) NaN
Answer: B) [10, NaN, 2, 3]
Explanation: The map function passes three arguments to parseInt: the current element, the current index, and the entire array. parseInt takes the string to parse and the radix (base) as its arguments. Thus, parseInt(’10’, 0) returns 10, parseInt(’10’, 1) returns NaN (since base 1 is not valid), parseInt(’10’, 2) returns 2 (binary), and parseInt(’10’, 3) returns 3 (base 3).
Which of the following will correctly clone an array arr?
A) let clone = arr;
B) let clone = arr.slice(0);
C) let clone = […arr];
D) Both B and C
Answer: D) Both B and C
Explanation: Both arr.slice(0) and […arr] create a shallow copy of the array. Using let clone = arr; simply copies the reference to the array, not the array itself.
What is the output of the following code?
let msg = ‘Hello’;
let shout = msg.toUpperCase();
console.log(shout);
A) hello
B) HELLO
C) undefined
D) TypeError
Answer: B) HELLO
Explanation: The toUpperCase() method converts all the characters of a string to uppercase. Therefore, the output will be HELLO.
Which statement about the Set object in JavaScript is true?
A) A Set allows duplicate values.
B) A Set is an ordered collection of items.
C) A Set can contain different types of values.
D) A Set does not allow mathematical operations.
Answer: C) A Set can contain different types of values.
Explanation: A Set is a collection of values that can be of any type, and each value must be unique. The values in a Set are not in any particular order, and while Set itself does not allow mathematical operations, it provides methods to work with the collection of values.
How do you check if an object obj is an array in JavaScript?
A) obj.isArray()
B) Array.isArray(obj)
C) typeof obj === ‘array’
D) obj instanceof Array
Answer: B) Array.isArray(obj)
Explanation: Array.isArray(obj) is the correct method to determine if an object is an array. typeof will return ‘object’ for arrays, and while obj instanceof Array can sometimes be used, it’s not as reliable as Array.isArray for all cases.
What does the finally block in a try…catch statement do?
A) Executes code after the try block only if an error does not occur.
B) Executes code after the catch block regardless of the result.
C) Skips the catch block if no errors occur.
D) Is used to finalize the execution of procedures.
Answer: B) Executes code after the catch block regardless of the result.
Explanation: The finally block executes after the try and catch blocks have executed but before the statements following the try…catch statement. It executes regardless of whether an exception was thrown or caught.
What method can be used to remove whitespace from both ends of a string?
A) trim()
B) strip()
C) chop()
D) cut()
Answer: A) trim()
Explanation: The trim() method removes whitespace from both ends of a string.
What is the result of typeof Symbol(“id”) in JavaScript?
A) “symbol”
B) “string”
C) “object”
D) “undefined”
Answer: A) “symbol”
Explanation: The typeof operator returns “symbol” for symbols, which are a primitive data type introduced in ECMAScript 6.
Which event occurs when the user clicks on an HTML element?
A) onchange
B) onclick
C) onmouseover
D) onmouseclick
Answer: B) onclick
Explanation: The onclick event occurs when the user clicks on an element. It is a widely used event to trigger actions in web pages.
Which statement is used to execute a block of code a specific number of times?
A) for
B) while
C) do…while
D) All of the above
Answer: D) All of the above
Explanation: All three statements (for, while, and do…while) can be used to execute a block of code multiple times, with for being the most commonly used for a specific number of iterations, and while and do…while being more suited to conditions that are not fixed iterations.
What does the document.querySelector() method return?
A) All elements that match a specified CSS selector(s)
B) The first element that matches a specified CSS selector(s)
C) The last element that matches a specified CSS selector(s)
D) An array of elements that match a specified CSS selector(s)
Answer: B) The first element that matches a specified CSS selector(s)
Explanation: document.querySelector() returns the first element that matches a specified CSS selector(s). If no matches are found, it returns null.
How can you add a comment in JavaScript?
A) <!–This is a comment–>
B) /* This is a comment */
C) // This is a comment
D) Both B and C
Answer: D) Both B and C
Explanation: In JavaScript, // is used for single-line comments, and /* */ is used for multi-line comments.
Which of the following is a correct way to create a JavaScript Date object for January 1, 2020?
A) new Date(2020, 1, 1);
B) new Date(‘2020-01-01’);
C) new Date(2020, 0, 1);
D) Both B and C
Answer: D) Both B and C
Explanation: JavaScript counts months from 0 to 11, so January is 0. Thus, new Date(2020, 0, 1); correctly represents January 1, 2020. ISO date string format new Date(‘2020-01-01’); is also correct.
What is the correct way to check if a variable v is an instance of the class C?
A) v instanceof C
B) C.instanceof(v)
C) instanceof v(C)
D) v.instanceof C
Answer: A) v instanceof C
Explanation: The instanceof operator is used to test whether the prototype property of a constructor appears anywhere in the prototype chain of an object. The syntax is object instanceof constructor.
Which method would you use to serialize an object to a JSON string?
A) JSON.parse()
B) JSON.stringify()
C) Object.toJSON()
D) String.convert()
Answer: B) JSON.stringify()
Explanation: JSON.stringify() method converts a JavaScript object or value to a JSON string.
What will the following code output?
console.log(1 > 2 > 3);
A) true
B) false
C) undefined
D) TypeError
Answer: B) false
Explanation: The expression is evaluated from left to right as (1 > 2) > 3. Since 1 > 2 is false (which is equivalent to 0 in JavaScript when compared with numbers), the expression becomes 0 > 3, which is also false.
Which of the following correctly describes the use of the localStorage object in web development?
A) It stores data for one session only.
B) It stores data with no expiration date.
C) It sends data to the server for permanent storage.
D) It can store up to 50MB of data per domain.
Answer: B) It stores data with no expiration date.
Explanation: The localStorage web API allows storing of key/value pairs in a web browser with no expiration date. Data stored in localStorage persists even after the browser window is closed, unlike sessionStorage, which stores data for one session only.
What does the break statement do in JavaScript?
A) Pauses the execution of a loop
B) Exits the current loop or switch statement
C) Breaks the line of code execution
D) Terminates the script execution
Answer: B) Exits the current loop or switch statement
Explanation: The break statement is used to exit from a loop or a switch statement before it has finished executing all of its code.
Which method is used to remove the first element from an array and return that element?
A) pop()
B) shift()
C) unshift()
D) push()
Answer: B) shift()
Explanation: shift() removes the first element from an array and returns that removed element. This method changes the length of the array. pop(), in contrast, removes the last element from an array.