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Tricky Javascript Code Snippets Asked in the Interview (es6/es7/es8/es9)

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Updated Jan 07, 2024

In this article, we will cover a range of JavaScript interview questions, including those related to the latest versions of the language (ES6, ES7, ES8, and ES9).

We'll also compare the differences between ES5 and the newer versions of the language and provide examples of ES6 code snippets.

Additionally, we'll include multiple choice questions (MCQs) on ES6 and explore some of the trickiest code snippets that are commonly asked in JavaScript interviews.

Whether you're preparing for a job interview or simply want to brush up on your knowledge of the language, this article has something for you.

Javascript ES6 ES7 ES8

ECMAScript (ES) is a standardized version of JavaScript, a programming language that is widely used to create interactive web pages and applications. ECMAScript is a formal specification developed and maintained by the Ecma International organization.

JavaScript is a programming language that is commonly used to create interactive web pages and applications. It is based on the ECMAScript specification and is an implementation of it. JavaScript is a client-side scripting language, meaning it is executed by the user's web browser rather than a server. It is used to create dynamic and interactive elements on web pages, such as form validation, image sliders, and modal windows.

ECMAScript and JavaScript are closely related, and the terms are often used interchangeably. However, ECMAScript is the formal specification, and JavaScript is the implementation of that specification.

*Discover the answers by clicking on the questions.

1. Guess the output of this code?
let a = {};
let b = { key: "b" };
let c = { key: "c" };

a[b] = 123;
a[c] = 456;

console.log(a[b]);

The output of this code will be 456.

In this code, a is an empty object that is being assigned properties using the square bracket notation. The values of the properties are being set to the numbers 123 and 456. The keys of the properties are the objects b and c.

When the console.log statement is executed, it logs the value of the property of a whose key is the object b. In this case, the value of this property is 456, because the value of the property was last set to 456 when the object c was used as the key.

This behavior occurs because when objects are used as keys in an object, the object's default behavior is to convert the object to a string representation. In this case, both b and c are converted to the string [object Object], which means that they both end up being used as the same key in the a object. As a result, the value of the property that is set using the object c as the key overwrites the value of the property that was set using the object b as the key.

So the object a looks like -

{
    "[object Object]": 456
}

2. Guess the output of this code?
let obj1 = { key: "value" };
let obj2 = obj1;
let obj3 = obj2;

obj1.key = "new value";
obj2 = { key: "another value" };

console.log(obj1.key, obj2.key, obj3.key);

The output of this code will be `new value` `another value` `new value`.

In this code, we are declaring three variables obj1, obj2, and obj3, and assigning an object to each of them. Then, we are reassigning a new object to obj2 and modifying a property of obj1.

When the console.log statement is executed, it logs the values of the key property for each object. The value of the key property for obj1 is "new value", the value of the key property for obj2 is "another value", and the value of the key property for obj3 is "new value".

This is because when an object is assigned to a variable, the variable stores a reference to the object in memory rather than the object itself. Changing the value of a property of the object using one variable will affect the value of that property when accessed using a different variable that references the same object. However, reassigning a new object to a variable will change the reference stored in that variable, so the original object is no longer accessible using that variable.

In this case, the value of the key property for obj1 was changed to "new value" using the obj1 variable, which affected the value of the key property when accessed using the obj3 variable, because both variables reference the same object. However, the value of the key property for obj2 was not affected, because the obj2 variable was reassigned to reference a new object.

3. Guess the output of this JavaScript code?
const obj = {
  a: "foo",
  b: function () {
    console.log(this.a);
  },
};

const c = obj.b;

obj.b();
c();

Answer - foo, undefined

When the method obj.b is called directly on obj, the output will be "foo". This is because this refers to the object that the method is called on, and obj.a is equal to "foo".

When the variable c is assigned the value of obj.b, it is a reference to the function itself and not the object obj. When c is called, it is not called on an object, so this will not refer to obj and the value of this.a is undefined. As a result, the output when calling c() will be undefined.

4. Guess the output of this code?
const x = { foo: 1 };
const y = { foo: 2 };

function addFoo(obj) {
  return obj.foo + 1;
}

console.log(addFoo(x));
console.log(addFoo(y));

Answer - 2, 3

The addFoo function takes an object as an argument and returns the value of obj.foo + 1. When addFoo is called with x as the argument, the output will be 2, because x.foo is equal to 1. When addFoo is called with y as the argument, the output will be 3, because y.foo is equal to 2.

5. Guess the output of below JavaScript code?
const arr = [1, 2, 3, 4, 5];

for (var i = 0; i < arr.length; i++) {
  setTimeout(function () {
    console.log(i);
  }, 1000);
}

Answer - 5, 5, 5, 5, 5

The setTimeout function is called inside of a loop that iterates through the elements in the arr array. The setTimeout function will execute its callback function after a delay of 1000 milliseconds. However, by the time the delay has elapsed and the callback function is called, the loop will have already completed and the value of i will be 5. As a result, the output will be 5 printed five times.

6. Guess the output of this JavaScript code?
const arr = [1, 2, 3, 4, 5];

arr.forEach(function (element) {
  console.log(element);
});

Answer - 1, 2, 3, 4, 5

The forEach method is called on the arr array and a callback function is passed as an argument. The callback function will be executed for each element in the array, with the element passed as an argument to the callback. As a result, the output will be the elements of the array, 1, 2, 3, 4, and 5, printed on separate lines.

7. Guess the output of this code?
let x = 1;

if (function f() {}) {
  x += typeof f;
}

console.log(x);

Answer - 1undefined

The if statement is evaluating the function f as a boolean value. In JavaScript, functions are truthy values, so the condition will evaluate to true and the code block inside the if statement will be executed. The value of x is then incremented by the string "undefined", which is the result of calling typeof f.

8. Guess the output of this code?
let a = {
  x: 1,
  y: 2,
};
let b = a;
a.x = 5;
console.log(a);
console.log(b);

Answer - {x:5, y:2} {x:5, y:2}

JavaScript objects are passed by reference. So when we assigned a object to b. b also pointing to the same object in memory. When we change the value of a.x it also affects b.x

9. Guess the output of this code?
let x = [1, 2, 3];
let y = [1, 2, 3];
let z = y;

console.log(x == y);
console.log(x === y);
console.log(z == y);
console.log(z == x);

Answer -

false false true false

When comparing two objects with the == operator, it compares their references, not their values. In this case, x and y are different objects with the same values. z is assigned the value of y, so they refer to the same object. As a result, the first comparison returns false, the second comparison also returns false and the third comparison returns true. and the last comparison also returns false.

10. Guess the output of the below JavaScript code?
var x = 0;
for (let i = 0; i < 5; i++) {
  setTimeout(() => {
    x++;
    console.log(x);
  }, 1000);
}

Answer:

1 2 3 4 5

The for loop is iterating 5 times, and in each iteration, it is scheduling a function to be invoked after 1000 milliseconds (1 second) using setTimeout. This function increments the value of x and logs it.

But all the 5 functions invoked after 1000 milliseconds.

Since, javascript is single threaded and event loop queue all the functions in the event loop and execute them one by one.

But inside each setTimeout callback execution, x++ increments x value by 1.

It makes difference when position of x++ code changes wrt the setTimout callback.

So all the 5 callbacks logs the values in incremental way, which is 1 2 3 4 5.

11. Guess the output of this JavaScript code?
let a = { x: 1 };
let b = { x: 2 };
let c = { x: 3 };
let d = { x: 4 };
let e = { x: 5 };
let arr = [a, b, c, d, e];

arr.forEach((obj) => (obj.x = obj.x * 2));

console.log(a.x, b.x, c.x, d.x, e.x);

Answer:

2 4 6 8 10

The code is using the forEach method to iterate over an array of objects, and it is modifying the x property of each object by multiplying it by 2.

It's updating the original objects with x*2 values.

So, the output of the code is 2 4 6 8 10.

12. Guess the output of the JavaScript code?
let num = 0;

function test() {
  var num = 1;
  return num;
}

console.log(test());
console.log(num);

Answer: 1 0

The code defines a global variable num with the value of 0 and then a function test which declares a local variable num with the value of 1 and returns it.

When test() is called, it first declares a local variable num with the value of 1.

Then the function return statement logs 1 on the console.

After that, it logs the value of global variable num which is 0.

Because the global and local variables have different scope and different memory allocation.

13. Guess the output of the below JavaScript code?
let obj = { name: "John", age: 25 };
let newObj = { ...obj, age: 30 };

console.log(obj.age);
console.log(newObj.age);

Answer: 25 30

The code creates an object obj with properties name and age. Then it creates a new object newObj using the spread operator to copy the properties of obj and then it updates the age property to 30.

The spread operator ... creates a new object with properties copied from the original object.

So, the first console.log statement logs the value of age property of obj which is 25.

And, the second console.log statement logs the value of age property of newObj which is 30.

It doesn't affect the original object obj.

14. Guess the output of the below JavaScript code?
const add = (a = 1, b = 2) => a + b;
console.log(add());
console.log(add(5));
console.log(add(undefined, 10));

Answer:

3
7
11

The code defines a function add which takes two parameters a and b and returns the sum of both. It uses default parameters to assign default values 1 to a and 2 to b if they are not provided.

So, the first console.log statement logs the result of add() which is 1 + 2 = 3 as both the parameters are not provided and default values are used.

The second console.log statement logs the result of add(5) which is 5 + 2 = 7 as only the first parameter is provided and the default value of b is used.

The third console.log statement logs the result of add(undefined, 10) which is 1 + 10 = 11 as the first parameter is provided as undefined and it takes the default value 1 and the second parameter is provided as 10.

15. Guess the output of the below JavaScript code?
const name = "John";
const age = 25;

const user = { name, age };
console.log(user);

Answer:

{ name: "John", age: 25 }

The code defines two variables name and age with values "John" and 25 respectively.

Then, it uses object literal notation to create an object user with properties name and age and the values are assigned from the variables name and age respectively.

So, the console.log statement logs the user object which is { name: "John", age: 25 }.

In ES6+, you can use object literal notation to create objects with properties using the same name as the variables with the values assigned to them.

16. Guess the output of the below JavaScript code?
const arr = [1, 2, 3];
const [a, b, c] = arr;

console.log(a);
console.log(b);
console.log(c);

Answer:

1
2
3

The code defines an array arr with values [1, 2, 3].

Then, it uses destructuring assignment to extract the values from the array arr and assign them to variables a, b, and c respectively.

So, the first console.log statement logs the value of a which is 1.

The second console.log statement logs the value of b which is 2.

The third console.log statement logs the value of c which is 3.

In ES6+, you can use destructuring assignment to extract values from arrays and objects and assign them to variables in a concise way.

17. What is the output of the below JavaScript code and why?
console.log(typeof null);
console.log(typeof undefined);
console.log(null === undefined);
console.log(null == undefined);

Answer:
object
undefined
false
true

typeof null returns object which is an error in JavaScript. This is a historical bug in the language that cannot be fixed without breaking existing code. So, to check for null, you should use === null instead of typeof operator.

typeof undefined returns undefined.

null === undefined is false because null and undefined are two distinct types in JavaScript.

null == undefined is true because == is the loose equality operator in JavaScript, which performs type coercion before comparison. In this case, both null and undefined are coerced to undefined before comparison, and since they both have the same value, the comparison returns true. However, it is generally recommended to use === instead of == to avoid unexpected behavior due to type coercion.

18. Write a function in JavaScript that takes an array of numbers and returns the sum of all positive numbers in the array.

Answer:
function sumOfPositiveNumbers(numbers) {
  let sum = 0;
  for (let i = 0; i < numbers.length; i++) {
    if (numbers[i] > 0) {
      sum += numbers[i];
    }
  }
  return sum;
}

// Example usage:
const arr = [1, -2, 3, 4, -5, 6];
console.log(sumOfPositiveNumbers(arr)); // Output: 14

The sumOfPositiveNumbers function takes an array of numbers as its parameter and initializes a variable sum to 0. It then loops through each element of the array and checks if the number is greater than 0. If the number is positive, it adds the number to the sum. Finally, it returns the sum of all positive numbers in the array.

In the example usage, we pass an array [1, -2, 3, 4, -5, 6] to the sumOfPositiveNumbers function. The function returns the sum of all positive numbers in the array, which is 14.

19. Write a function in JavaScript that takes a string as input and returns a new string with all the vowels removed.

Answer:
function removeVowels(str) {
  const vowels = ["a", "e", "i", "o", "u", "A", "E", "I", "O", "U"];
  let newStr = "";
  for (let i = 0; i < str.length; i++) {
    if (!vowels.includes(str[i])) {
      newStr += str[i];
    }
  }
  return newStr;
}

// Example usage:
const str = "This is a test string with vowels";
console.log(removeVowels(str)); // Output: Ths s  tst strng wth vwls

The removeVowels function takes a string as its parameter and initializes an array vowels containing all the vowels. It then loops through each character of the input string and checks if the character is not present in the vowels array. If the character is not a vowel, it adds the character to the newStr string. Finally, it returns the newStr string with all the vowels removed.

In the example usage, we pass a string 'This is a test string with vowels' to the removeVowels function. The function returns a new string with all the vowels removed, which is 'Ths s tst strng wth vwls'.

20. Write a function in JavaScript that takes an array of strings as input and returns a new array with the strings sorted in alphabetical order.

Answer:
function sortStrings(arr) {
  return arr.sort();
}

// Example usage:
const strings = ["apple", "banana", "cherry", "date", "elderberry"];
console.log(sortStrings(strings)); // Output: ['apple', 'banana', 'cherry', 'date', 'elderberry']

The sortStrings function takes an array of strings as its parameter and uses the sort() method to sort the array in alphabetical order. The sort() method sorts the elements of an array in place and returns the sorted array. By default, the sort() method sorts the array elements in ascending order based on the Unicode values of the characters. Therefore, for strings, it sorts them in alphabetical order.

In the example usage, we pass an array of strings ['apple', 'banana', 'cherry', 'date', 'elderberry'] to the sortStrings function. The function returns a new array with the strings sorted in alphabetical order, which is ['apple', 'banana', 'cherry', 'date', 'elderberry'].

21. Write a function in JavaScript that checks if a string is a palindrome.

Answer:
function isPalindrome(str) {
  const reversedStr = str.split("").reverse().join("");
  return str === reversedStr;
}

// Example usage:
console.log(isPalindrome("racecar")); // Output: true
console.log(isPalindrome("hello")); // Output: false

The isPalindrome function takes a string as its parameter and first reverses the string using the split(), reverse(), and join() methods. It then checks if the reversed string is equal to the original string and returns true if they are equal and false otherwise.

In the example usage, we pass the strings racecar and hello to the isPalindrome function. The function returns true for the first string because it is a palindrome (i.e., the reversed string is equal to the original string), and false for the second string.

22. Write a function in JavaScript that finds the second highest number in an array of numbers.

Answer:
function findSecondHighest(arr) {
  const sortedArr = arr.sort((a, b) => b - a);
  return sortedArr[1];
}

// Example usage:
const numbers = [10, 5, 20, 15, 8];
console.log(findSecondHighest(numbers)); // Output: 15

The findSecondHighest function takes an array of numbers as its parameter and first sorts the array in descending order using the sort() method and a comparison function. It then returns the second element in the sorted array, which is the second highest number.

In the example usage, we pass the array [10, 5, 20, 15, 8] to the findSecondHighest function. The function returns 15, which is the second highest number in the array.

23. Write a function in JavaScript that removes duplicates from an array.

Answer:
function removeDuplicates(arr) {
  return [...new Set(arr)];
}

// Example usage:
const numbers = [1, 2, 3, 2, 1, 4, 5, 4];
console.log(removeDuplicates(numbers)); // Output: [1, 2, 3, 4, 5]

The removeDuplicates function takes an array as its parameter and first creates a Set object from the array using the Set() constructor. A Set is a collection of unique values, so all duplicates are automatically removed. We then use the spread syntax (...) to convert the Set back to an array.

In the example usage, we pass the array [1, 2, 3, 2, 1, 4, 5, 4] to the removeDuplicates function. The function returns a new array with duplicates removed, which is [1, 2, 3, 4, 5].

24. Write a function in JavaScript that returns the sum of two numbers without using the + operator.

Answer:
function add(a, b) {
  while (b !== 0) {
    let carry = a & b;
    a = a ^ b;
    b = carry << 1;
  }
  return a;
}

// Example usage:
console.log(add(5, 7)); // Output: 12

The add function takes two numbers a and b as its parameters and uses a bitwise approach to add them. It repeatedly performs a bitwise AND operation between a and b to get the carry bits and performs a bitwise XOR operation between a and b to get the sum bits. It then shifts the carry bits one position to the left and assigns the result to b. This process continues until b is equal to 0. The final value of a is the sum of the two numbers.

In the example usage, we pass the numbers 5 and 7 to the add function. The function returns 12, which is the sum of the two numbers.

25. Write a function in JavaScript that checks if a given string is a valid IPv4 address.

Answer:
function isValidIPv4(str) {
  const octets = str.split(".");
  if (octets.length !== 4) return false;
  for (let i = 0; i < octets.length; i++) {
    const octet = octets[i];
    if (isNaN(octet) || octet < 0 || octet > 255) return false;
    if (octet.length > 1 && octet[0] === "0") return false;
  }
  return true;
}

// Example usage:
console.log(isValidIPv4("192.168.0.1")); // Output: true
console.log(isValidIPv4("256.0.0.0")); // Output: false

The isValidIPv4 function takes a string as its parameter and checks if it is a valid IPv4 address.

An IPv4 address consists of four octets separated by periods, with each octet being a number between 0 and 255.

The function first splits the string into an array of octets using the split() method.

It then checks if the array has exactly four elements and if each element is a number between 0 and 255.

It also checks if each octet does not start with a 0 unless it is a single-digit octet.

If any of these conditions are not met, the function returns false. Otherwise, it returns true.

In the example usage, we pass the strings '192.168.0.1' and '256.0.0.0' to the isValidIPv4 function. The function returns true for the first string because it is a valid IPv4 address and false for the second string because it is not a valid IPv4 address.

26. Write a function in JavaScript that converts a given string to title case.

Answer:
function toTitleCase(str) {
  return str.replace(/\b\w+/g, function (word) {
    return word.charAt(0).toUpperCase() + word.slice(1).toLowerCase();
  });
}

// Example usage:
console.log(toTitleCase("the quick brown fox")); // Output: 'The Quick Brown Fox'

The toTitleCase function takes a string as its parameter and converts it to title case. Title case is a style of capitalization where the first letter of each word is capitalized, and the rest of the letters are in lowercase.

The function uses a regular expression to match the first letter of each word in the string and then uses the charAt() and slice() methods to capitalize the first letter and convert the rest of the letters to lowercase. It returns the modified string.

In the example usage, we pass the string 'the quick brown fox' to the toTitleCase function. The function returns 'The Quick Brown Fox', which is the string converted to title case.

27. Write a function in JavaScript that generates a random hexadecimal color code.

Answer:
function getRandomColor() {
  const hexChars = "0123456789ABCDEF";
  let color = "#";
  for (let i = 0; i < 6; i++) {
    color += hexChars[Math.floor(Math.random() * 16)];
  }
  return color;
}

// Example usage:
console.log(getRandomColor()); // Output: e.g. '#3D5A89'

The getRandomColor function generates a random hexadecimal color code. A hexadecimal color code is a six-digit code that represents a color by specifying the amount of red, green, and blue in it.

Each digit of the code can be any of the sixteen hexadecimal characters (0 to 9 and A to F).

The function generates a random color code by selecting six random hexadecimal characters from the hexChars string using the Math.random() function and concatenating them together with a # symbol.

It then returns the generated color code.

In the example usage, we call the getRandomColor function, which returns a randomly generated color code. The output can vary each time the function is called, but it should be a valid hexadecimal color code.

28. Guess the output of the following code:

let x = 5;
{
  let x = 10;
  console.log(x);
}
console.log(x);

The output of the code above is:

Answer:

10
5

This is because of the block scoping behavior of the let keyword in ES6.

When we declare a variable with let inside a block (in this case, the block is defined by the curly braces), the variable is only accessible inside that block and its sub-blocks.

In the code above, we define a variable x outside the block with the value of 5. Then we define another variable x inside the block with the value of 10.

The first console.log() statement inside the block will print 10, because x refers to the variable defined inside the block. The second console.log() statement outside the block will print 5, because it refers to the variable defined outside the block.

29. Guess the output of the following code:

const obj = { a: 1, b: 2, c: 3 };
const { a, b } = obj;
console.log(a, b);

The output of the code above is:

Answer:

1 2

This is because of the object destructuring syntax introduced in ES6.

We declare a constant variable obj with an object containing three properties. Then we use object destructuring to extract the properties a and b from the object and assign them to separate variables with the same names.

The console.log() statement then prints the values of the a and b variables, which are 1 and 2 respectively.

30. Guess the output of the following code:

const arr1 = [1, 2, 3];
const arr2 = [4, 5, 6];
const arr3 = [...arr1, ...arr2];
console.log(arr3);

The output of the code above is:

Answer:

[1, 2, 3, 4, 5, 6]

This is because of the spread syntax (...) introduced in ES6.

The ... operator can be used to spread the elements of an array or the properties of an object into a new array or object.

In the code above, we define two arrays arr1 and arr2. Then we create a new array arr3 by spreading the elements of arr1 and arr2 into it using the spread syntax.

The console.log() statement then prints the contents of arr3, which are the elements of arr1 followed by the elements of arr2.

31. What is the output of the following code?

const arr1 = [1, 2, 3];
const arr2 = [...arr1];

arr2.push(4);

console.log(arr1);
console.log(arr2);

The output of the code will be:

Answer:

[1, 2, 3]
[1, 2, 3, 4]

The code creates an array arr1 with the values [1, 2, 3]. It then creates a new array arr2 using the spread syntax (...) to spread the values of arr1 into a new array.

arr2.push(4) adds the value 4 to the end of arr2.

However, arr1 remains unchanged because arr2 is a new array with its own reference to the values of arr1. This is known as creating a shallow copy of the array.

Therefore, the first console.log(arr1) prints [1, 2, 3] and the second console.log(arr2) prints [1, 2, 3, 4].

32. What is the output of the following code?

const x = 10;

function foo() {
  console.log(x);
  const x = 20;
}

foo();

The output of the code will be:

Answer:

ReferenceError: Cannot access 'x' before initialization

The foo function attempts to log the value of x before it is initialized. This causes a ReferenceError to be thrown, as x is not yet defined in the function scope.

This happens because of variable hoisting in JavaScript. When a variable is declared with const or let, it is not hoisted to the top of the scope like variables declared with var are. Instead, they are only accessible after they are declared.

Therefore, when console.log(x) is called in the foo function, the local variable x has not yet been initialized, resulting in a ReferenceError.

33. What is the output of the following code?

const a = [1, 2, 3];
const b = a;

b.push(4);

console.log(a);
console.log(b);

The output of the code will be:

Answer:

[1, 2, 3, 4]
[1, 2, 3, 4]

The code creates an array a with the values [1, 2, 3]. It then creates a new variable b and assigns it to a, creating a reference to the same array.

b.push(4) adds the value 4 to the end of the array.

Since a and b reference the same array, both console.log(a) and console.log(b) will print [1, 2, 3, 4].

This is different from the previous example where ... spread operator was used, which created a new array with the same values as the original array instead of referencing the same array.

34. What is the output of the following code?

const companies = [
  { id: "1", name: "Facebook" },
  { id: "2", name: "Apple" },
  { id: "3", name: "Google" },
];

companies.sort((a, b) => (a.name > b.name ? -1 : 1));
console.log(companies);

The output of the code will be:

Answer:

[
    {id: "3", name:"Google"},
    {id: "1", name:"Facebook"},
    {id: "2", name:"Apple"}
]

The comparison function takes two parameters, "a" and "b", which represent two elements being compared in the array.

If the "name" property of "a" comes before the "name" property of "b" in alphabetical order, then the function returns -1, which means "a" should come before "b" in the sorted array.

Otherwise, if the "name" property of "a" comes after the "name" property of "b" in alphabetical order, then the function returns 1, which means "b" should come before "a" in the sorted array.

35. Guess the output of the following code that uses the typeof operator.

console.log(typeof 42);
console.log(typeof "Hello");
console.log(typeof true);
console.log(typeof [1, 2, 3]);
console.log(typeof { name: "John", age: 25 });
console.log(typeof null);
console.log(typeof undefined);
console.log(typeof function () {});
Answer:
bash
Copy code
number
string
boolean
object
object
object
undefined
function

The typeof operator in JavaScript is used to determine the type of a value or expression. Here's the breakdown of the output:

  • typeof 42 returns "number" because 42 is a numeric value.
  • typeof "Hello" returns "string" because "Hello" is a string.
  • typeof true returns "boolean" because true is a boolean value.
  • typeof [1, 2, 3] returns "object" because arrays are considered objects in JavaScript.
  • typeof { name: "John", age: 25 } returns "object" because objects are considered objects in JavaScript.
  • typeof null returns "object", which is a known quirk in JavaScript. null is considered an object type.
  • typeof undefined returns "undefined" because it is a special value in JavaScript representing an uninitialized variable.
  • typeof function() {} returns "function" because it is a function object.

36. Write a function in JavaScript to determine the type of a value.

console.log(getType(42));
console.log(getType("Hello"));
console.log(getType(true));
console.log(getType([1, 2, 3]));
console.log(getType({ name: "John", age: 25 }));
console.log(getType(null));
console.log(getType(undefined));
console.log(getType(function () {}));

//The function should print "array" for "[]" and "null" for "null" types.
Answer:
const getType = (val) => (val === null ? null : val?.constructor.name);

The output of the code will be:

number
string
boolean
Array
Object
null
undefined
Function

The function getType takes a value as an argument and returns its type. If the value is null, it returns null. Otherwise, it uses the constructor.name property to determine the type of the value.

  • getType(42) returns "number" because 42 is a numeric value.
  • getType("Hello") returns "string" because "Hello" is a string.
  • getType(true) returns "boolean" because true is a boolean value.
  • getType([1, 2, 3]) returns "Array" because arrays are considered objects in JavaScript, and the constructor name for an array is "Array".
  • getType({ name: "John", age: 25 }) returns "Object" because objects are considered objects in JavaScript, and the constructor name for an object is "Object".
  • getType(null) returns null because null is a special value in JavaScript.
  • getType(undefined) returns "undefined" because it is a special value in JavaScript representing an uninitialized variable.
  • getType(function() {}) returns "Function" because it is a function object, and the constructor name for a function is "Function".
  • The getType function can be used to dynamically determine the type of values in JavaScript.

37. Which of the following options accurately describes the output or error thrown by the code above?

function calculateSum() {
  console.log(result);
  var num1 = 5;
  let num2 = 10;
  const num3 = 15;
  var result = num1 + num2 + num3;
}

calculateSum();
  • A: 30
  • B: undefined
  • C: ReferenceError
  • D: TypeError

Answer -
B: undefined

In the code, the variable result is declared using the var keyword, but it is assigned a value after the console.log statement.

When JavaScript executes the function calculateSum(), it hoists the variable declaration of result to the top of the function scope. However, since the assignment of the value num1 + num2 + num3 comes after the console.log statement, the variable is undefined at the point of the console.log.

So, the code is effectively interpreted like this:

function calculateSum() {
  var result; // Variable declaration is hoisted to the top

  console.log(result); // undefined

  var num1 = 5;
  let num2 = 10;
  const num3 = 15;
  result = num1 + num2 + num3; // Assignment is performed here
}

calculateSum();

Since the variable result is hoisted, it exists in the function scope but does not have any assigned value until after the console.log statement. Therefore, when console.log(result) is executed, the variable result exists but is undefined.

38. What will be the output of the following code snippet?

let x = 10;

function updateX() {
  if (true) {
    let x = 20;
    console.log(x);
  }
  console.log(x);
}

updateX();
  • A: 20, 10
  • B: 20, 20
  • C: 10, 10
  • D: 10, 20

Answer -
B: 20, 10

In this code, the variable x is declared and assigned a value of 10 outside the updateX function.

Inside the function, a new block is created using an if statement. Within that block, a new variable x is declared and assigned a value of 20 using let. This creates a separate scope for the inner x, which is only accessible within the if block.

When the console.log statements are executed, the first one inside the if block will output 20, as it refers to the inner x variable. The second console.log statement outside the if block will output 10, as it refers to the outer x variable.

Therefore, the output will be 20, 10.

39. What will be the output of the following code snippet?

const person = {
  name: "John",
  age: 30,
};

Object.freeze(person);
person.age = 40;

console.log(person.age);
  • A: 30
  • B: 40
  • C: TypeError
  • D: ReferenceError

Answer -
A: 30

In this code, the Object.freeze() method is used to make

the person object immutable. This means that the properties of the object cannot be modified.

When attempting to assign a new value to person.age after freezing the object, it does not throw an error or modify the object. Instead, it silently fails in non-strict mode and throws a TypeError in strict mode.

Since the code is not running in strict mode, the assignment person.age = 40 does not have any effect. Therefore, when console.log(person.age) is executed, it will output the original value of 30.

Hence, the output will be 30.

40. What will be the output of the following code snippet?

function foo() {
  let x = 1;

  function bar() {
    let y = 2;
    console.log(x + y);
  }

  bar();
}

foo();
  • A: 1
  • B: 2
  • C: 3
  • D: ReferenceError

Answer -
C: 3

In this code, there are two nested functions: foo and bar. The variable x is declared and assigned a value of 1 within the foo function, while the variable y is declared and assigned a value of 2 within the bar function.

When the foo function is called, it invokes the bar function. Inside the bar function, the values of x and y are accessed and summed together using console.log(x + y).

Since x is accessible within the bar function due to lexical scoping, the value of x is 1. Similarly, the value of y is 2. Therefore, the output of console.log(x + y) will be 3.

Hence, the correct answer is C: 3.

41. What will be the output of the following code snippet?

let x = 10;

function outer() {
  console.log(x);

  if (false) {
    var x = 20;
  }
}

outer();
  • A: 10
  • B: 20
  • C: undefined
  • D: ReferenceError

Answer: C: undefined

In this code snippet, there's a variable hoisting issue with the var declaration inside the outer function. The variable x is declared using var within the outer function scope.

When the function outer() is called, the console.log(x) statement is executed. At this point, the variable x is hoisted to the top of the function scope and is initialized with undefined. This means that the local variable x inside the function is different from the global x.

The if (false) block will not be executed, so the assignment var x = 20; will not take place.

Thus, the console.log(x) statement inside the outer function will log the value of the locally hoisted variable x, which is undefined.

Hence, the correct answer is C: undefined.

42. What will be the output of the following code snippet?

const obj = {
  a: 1,
  b: 2,
  c: {
    a: 3,
    b: 4,
  },
};

const {
  a,
  b,
  c: { a: nestedA },
} = obj;

console.log(a, b, nestedA);
  • A: 1 2 3
  • B: 1 2 4
  • C: 3 2 1
  • D: SyntaxError

Answer: A: 1 2 3

This code snippet uses destructuring assignment to extract values from the obj object. It extracts the properties a, b, and the nested property a from the c object and assigns them to the corresponding variables a, b, and nestedA, respectively.

After destructuring, the variables will hold the following values:

  • a: 1 (value of obj.a)
  • b: 2 (value of obj.b)
  • nestedA: 3 (value of obj.c.a)

When console.log(a, b, nestedA) is executed, it will print 1 2 3, as the values of the variables match the above assignments.

Hence, the correct answer is A: 1 2 3.

43. What will be the output of the following code snippet?

function* generatorFunction() {
  yield 1;
  yield 2;
  return 3;
}

const generator = generatorFunction();

console.log(generator.next());
console.log(generator.next());
console.log(generator.next());
  • A: { value: 1, done: false }, { value: 2, done: false }, { value: 3, done: true }
  • B: { value: 1, done: false }, { value: 2, done: false }, { value: undefined, done: true }
  • C: { value: 1, done: true }, { value: 2, done: true }, { value: 3, done: true }
  • D: SyntaxError

Answer: A: { value: 1, done: false }, { value: 2, done: false }, { value: 3, done: true }

This code snippet demonstrates the use of a generator function. When a generator function is called, it returns an iterator object, which can be used to control the execution of the generator function.

The generatorFunction is a generator function that yields three values: 1, 2, and 3. The yield keyword pauses the execution and returns the yielded value. When the generator is finished, it returns the value using the return statement.

When the generator is executed step by step using generator.next(), it proceeds through the generator function's code, stopping at each yield statement.

  • The first call to generator.next() will return { value: 1, done: false }, indicating that the first value yielded is 1, and the generator is not yet done.
  • The second call to generator.next() will return { value: 2, done: false }, indicating that the second value yielded is 2, and the generator is not yet done.
  • The third call to generator.next() will return { value: 3, done: true }, indicating that the third value yielded is 3, and the generator is done (done is true).

After the generator is done, any further calls to generator.next() will keep returning { value: undefined, done: true }.

Hence, the correct answer is A: { value: 1, done: false }, { value: 2, done: false }, { value: 3, done: true }.

44. Write a function to make the following code snippet work?

console.log(sum(4, 6, 8, 10).value); //output - 28
console.log(sum(4)(6)(8)(10).value); //output - 28

Answer

function sum(...args) {
  const ans = args.reduce((a, b) => a + b, 0); //just to get sum of all the array elements

  const myFunc = (num) => {
    return sum(num, ...args);
  };

  myFunc.value = ans;

  return myFunc;
}

console.log(sum(4, 6, 8, 10).value); //output - 28
console.log(sum(4)(6)(8)(10).value); //output - 28

The sum function takes any number of arguments using the rest parameter ...args and calculates the sum of those arguments.

It defines a nested function called myFunc, which takes a new number num and returns a new instance of the sum function with the accumulated sum and the new number.

The current sum value is assigned to a property named value on myFunc.

The myFunc function is then returned, allowing you to chain multiple function calls together.

As a result, you can use this sum function to either pass all numbers at once or chain multiple function calls to add numbers incrementally, and it will provide the correct sum when you access the value property.

45. Understanding the `.sort()` Method Variations

const fruits = ["banana", "apple", "orange", "grape", "kiwi"];

// Task 1: Sort the array of fruits in alphabetical order (default behavior)
// Task 2: Sort the array of fruits in descending alphabetical order
// Task 3: Sort the array of fruits based on the length of the fruit names in ascending order
// Task 4: Sort the array of fruits in ascending order by the second character of each fruit name

Answer

Task 1: Sort in alphabetical order (default behavior)

const alphabeticalOrder = [...fruits].sort();
console.log(alphabeticalOrder); // Output: ['apple', 'banana', 'grape', 'kiwi', 'orange']

Task 2: Sort in descending alphabetical order

const descendingOrder = [...fruits].sort((a, b) => b.localeCompare(a));
console.log(descendingOrder); // Output: ['orange', 'kiwi', 'grape', 'banana', 'apple']

Task 3: Sort based on the length of the fruit names in ascending order

const sortByLength = [...fruits].sort((a, b) => a.length - b.length);
console.log(sortByLength); // Output: ['kiwi', 'apple', 'grape', 'banana', 'orange']

Task 4: Sort in ascending order by the second character of each fruit name

const sortBySecondChar = [...fruits].sort((a, b) => a[1].localeCompare(b[1]));
console.log(sortBySecondChar); // Output: ['banana', 'kiwi', 'apple', 'orange', 'grape']

Explanation:

  • Task 1 utilizes the default behavior of .sort() to arrange elements alphabetically.
  • Task 2 reverses the order by using localeCompare() with the reverse comparison.
  • Task 3 sorts the array by the length of the elements, ensuring ascending order.
  • Task 4 specifically compares the second character of each element for sorting.

These variations demonstrate different uses of the .sort() method by customizing the sorting logic through comparator functions, enabling diverse sorting patterns for arrays.

46. Explain the output of the following code and correct any errors.

let numbers = [1, 2, 3, 4, 5];
numbers = numbers.map((number) => number * 2);
console.log(numbers.reduce((total, num) => total + num));

Answer:

// Output: 30 (No errors in the code)

Explanation:

  1. Array Creation: The code starts by creating an array named numbers containing the values [1, 2, 3, 4, 5].

  2. Array Mapping: The map() method is used to create a new array by applying a function to each element of the original array. In this case, the function number => number * 2 doubles each number in the array. The new array becomes [2, 4, 6, 8, 10].

  3. Array Reduction: The reduce() method is used to reduce the array to a single value by applying a function against an accumulator and each element in the array (from left to right). The function (total, num) => total + num adds each number in the array to the total, starting with an initial total of 0.

  4. Output: The final console.log() statement outputs the result of the reduce() operation, which is 30 (the sum of all the doubled numbers in the array).

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