Top 10 JavaScript Coding Problems Asked in an Interview with Solutions – 2024

March 5, 2024 suresh

1. Closures and Variables

How does the concept of closures work in JavaScript, and what will be logged when invoking a function returned from another function?

function outer() {
  var x = 10;
  function inner() {
    console.log(x);
  }
  return inner;
}

var closureFunction = outer();
closureFunction(); // What will be logged?

Output:

The closure allows inner to retain access to the variable x even after outer has finished executing. It logs 10.

2. Object Property Assignment

In JavaScript, when two objects are assigned to each other, how does modifying one object affect the other? What will be logged in this scenario?

var obj1 = { key: 'value' };
var obj2 = { key: 'new value' };

obj2 = obj1;
obj2.key = 'updated value';

console.log(obj1.key); // What will be logged?

Output:

updated value

obj1 and obj2 reference the same object after the assignment. Modifying obj2 also affects obj1. It logs 'updated value'.

3. Promise Chaining

Explain the sequence of execution when working with promises and promise chaining. What will be logged in the given promise and then method?

const promise = new Promise((resolve) => {
  console.log('Promise');
  resolve();
});

promise.then(() => console.log('Resolved')); // What will be logged?

Output:

Promise
Resolved

The code logs ‘Promise’ when the Promise is created and ‘Resolved’ after the Promise is resolved.

4. Array Methods

When using array methods in JavaScript, specifically map, how can you transform each element of an array? What will be the output of mapping a given array?

const numbers = [1, 2, 3, 4];
const doubled = numbers.map(num => num * 2);

console.log(doubled); // What will be logged?

Output:

[2, 4, 6, 8]

The map method creates a new array by applying the provided function to each element of the original array. It logs [2, 4, 6, 8].

5. Prototype and Inheritance

How does prototype-based inheritance work in JavaScript, and what will be logged when accessing a property from an object created through inheritance?

function Parent() {
  this.parentProperty = 'Parent Property';
}

function Child() {
  this.childProperty = 'Child Property';
}

Child.prototype = new Parent();

var obj = new Child();
console.log(obj.parentProperty); // What will be logged?

Output:

Parent Property

The prototype chain is established, and obj inherits from both Child and Parent. It logs 'Parent Property'.

6. Async/Await

In JavaScript, when using async functions and await, what is the order of execution, and what will be logged during the execution of an asynchronous function?

function delay(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

async function asyncFunction() {
  console.log('Start');
  await delay(1000);
  console.log('End');
}

asyncFunction(); // What will be logged and in what order?

Output:

Start
[pauses for 1000 ms]
End

The code logs ‘Start’, pauses for 1000 milliseconds due to await delay(1000), and finally logs ‘End’.

7. Falsy Values

In JavaScript, when using loose equality (==), how are undefined and null compared? What will be the result of the given comparison?

console.log(undefined == null); // true or false?

Output:

true

The loose equality (==) operator performs type coercion. undefined and null are considered equal after type coercion, so it logs true.

8. String Operations

When working with strings in JavaScript, how can you extract a portion of a string, and what will be logged when using the slice method?

var str = 'Hello, World!';
console.log(str.slice(0, -1)); // What will be logged?

Output:

Hello, World

The slice method extracts a section of a string. str.slice(0, -1) extracts characters from the beginning to the second-to-last character. It logs 'Hello, World'.

9. Arrow Functions

Explain how arrow functions in JavaScript handle the this keyword. What will be logged when invoking an arrow function?

const arrowFunction = () => {
  console.log(this);
};

arrowFunction(); // What will be logged?

Output:

[global object] or undefined (in strict mode)

Arrow functions do not have their own this binding; they inherit this from the surrounding scope. It logs the global this (or undefined in strict mode).

10. Object Destructuring

In JavaScript, how does object destructuring work, and what will be logged when extracting values from a given object?

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

console.log(name, age); // What will be logged?

Output:

John 30

Object destructuring extracts values from an object and assigns them to variables. It logs ‘John 30’.

If you have any questions or doubt's regarding these please do a comment below.

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