🌐15 Tricky JavaScript Interview Questions 🧠 That Will Test Your Skills 💡

Master these tricky JavaScript interview questions to ace your next interview.

JavaScript Interview Questions

Introduction

JavaScript is one of the most widely used programming languages in web development. As the backbone of interactive web applications, it empowers developers to create dynamic, responsive, and user-friendly websites. For both aspiring and experienced JavaScript developers, job interviews represent a crucial milestone in their career journey. Whether you are a fresh graduate or a seasoned professional, preparing for JavaScript interviews is essential to showcase your expertise and stand out in a competitive job market.

In this article, we have compiled a comprehensive set of 15 tricky JavaScript interview questions that will challenge your grasp of various JavaScript concepts. Each question comes with detailed explanations and code examples to help you understand the underlying principles and reasoning behind the answers.

1. What will be the output of the following code?

var x = 1;
function foo() {
  x = 10;
  return;
  function x() {}
}
foo();
console.log(x); // Output: 1;

Explanation:

Explanation: This question tests your understanding of variable hoisting and function scope. In JavaScript, variable and function declarations are hoisted to the top of their respective scopes. The function x is hoisted to the top of the foo function scope, and it becomes a local variable. The assignment x = 10 inside the foo function modifies this local variable x, not the global one. Therefore, the output will be 1.

2. What will be the output of the following code?

var name = "Lokesh Prajapati";
(function() {
  console.log(name);
  var name = "Lokesh Prajapati";
})(); // Output: undefined;

Explanation:

This question tests your understanding of variable hoisting within functions. Even though the name variable is declared outside the function, the var name declaration inside the function is hoisted to the top of the function scope. However, the value of name is undefined at the point of the console.log call, because the assignment var name = "Jane Doe"; is not yet executed. The output will be undefined.

3. What will be the output of the following code?

var x = 1;
if (function test() {}) {
  x += typeof test;
}
console.log(x); // Output: 1undefined;

Explanation:

This question checks your understanding of function expressions and the typeof operator. Although there is a function expression inside the if statement, it evaluates to true, and the block is entered. However, f is not defined in the outer scope because it is a function expression and not a function declaration. Therefore, typeof f will be 'undefined', and the output will be 1undefined.

4. What will be the output of the following code?

function sayHelloWorld() {
  return sayGoodbyWorld();
  var sayGoodbyWorld = function() {
    return "Hello, World!";
  };
  function sayGoodbyWorld() {
    return "Goodbye, World!";
  }
}
console.log(sayHelloWorld());

Explanation:

This question tests your understanding of function hoisting and the order of function declarations and variable declarations. The function sayGoodbyWorld is hoisted to the top of the sayHelloWorldfunction scope, and the sayGoodbyWorld variable declaration is also hoisted, but its assignment is not. Therefore, the first sayGoodbyWorld function is used, and the output will be "Goodbye, World!".

5. What will be the output of the following code?

function Parent() {}
function Child() {}
Child.prototype = new Parent();
var obj = new Child();
console.log(obj instanceof Parent); // Output: true;

Explanation:

This question tests your knowledge of prototypes and inheritance in JavaScript. The code creates a Parent and a Child constructor function. The Child prototype is set to an instance of Parent, creating a prototype chain. When obj is created using the Child constructor, it inherits from Parent. So, the output will be true.

6. What will be the output of the following code?

var x = 10;
function testValue() {
  console.log(x);
  var x = 20;
}
testValue(); // Output: undefined;

Explanation:

This question checks your understanding of variable hoisting within functions. The var x declaration inside the testValue function is hoisted to the top of the function scope, but its assignment is not hoisted. So, at the time of console.log, x is undefined, and the output will be undefined.

7. What will be the output of the following code?

function Person(name) {
  this.name = name;
}

Person.prototype.greet = function() {
  return "Hello, my name is " + this.name;
};

var person1 = new Person("Lokesh Prajapati");
var person2 = new Person("Lucky");

console.log(person1.greet === person2.greet); // Output: true;

Explanation:

This question tests your knowledge of prototype-based inheritance and function reference equality. Both person1 and person2 are instances of the Person constructor, and they share the same greet method through the prototype chain. So, the output will be true because the greet function reference is the same for both instances.

8. What will be the output of the following code?

console.log([] == ![]); // Output: true;

Explanation:

This question checks your understanding of type coercion and truthy/falsy values in JavaScript. The array [] is truthy, and the ! operator negates it, making it falsy. When comparing falsy and truthy values with the abstract equality (==) operator, both operands are converted to numbers. Falsy values are converted to 0. So, the expression becomes 0 == 0, and the output will be true.

9. What will be the output of the following code?

function sayHi() {
  return hi;
  var hi = "Hello, World!";
}
console.log(sayHi()); // Output: undefined.

Explanation:

This question checks your understanding of variable hoisting and function return values. The var hi declaration is hoisted to the top of the sayHi function scope, but its assignment is not hoisted. At the time of the return statement, hi is undefined, and that is the value returned. The output will be undefined.

10. What will be the output of the following code?

var x = 5;
function outer() {
  var x = 10;
  function inner() {
    console.log(x);
  }
  return inner;
}
var finalResult = outer();
finalResult();

Explanation:

This question tests your understanding of closures. The outer function creates a closure over the x variable, and the inner function can access it. When the inner function is called, it logs the value of the variable x from its closure, which is 10. The output will be 10.

11. What will be the output of the following code?

function userData() {
  return userData;
}
console.log(typeof userData()); // Output: function;

Explanation:

This question tests your understanding of the return value of a function. The userData function returns itself (the function reference). Therefore, the result of calling typeof userData() will be 'function'.

12. What will be the output of the following code?

var x = 10;
function testNum() {
  console.log(x);
  if (true) {
    var x = 20;
  }
  console.log(x);
}
testNum();

Explanation:

This question checks your understanding of variable hoisting and variable scope within functions. Inside the testNum function, the var x declaration Inside the if block is hoisted to the top of the function scope but its assignment is not hoisted. So, at the first console.log, x is undefined. Then, x is assigned the value 20 within the if block, and at the second console.log, x will be 20. The output will be: 20.

13. What will be the output of the following code?

var a = [1, 2, 3];
var b = [1, 2, 3];
console.log(a == b); // Output: false;

Explanation:

This question tests your understanding of array comparison in JavaScript. Although a and b contain the same values, arrays are reference types in JavaScript. When comparing two arrays using the abstract equality operator (==), the comparison checks whether both operands refer to the same array object in memory, which is not the case here. So, the output will be false.

14. What will be the output of the following code?

var x = 5;
(function() {
  console.log(x);
  var x = 10;
})(); // Output: undefined;

Explanation:

This question checks your understanding of variable hoisting within functions. The var x declaration inside the immediately-invoked function expression (IIFE) is hoisted to the top of the function scope, but its assignment is not hoisted. So, at the time of console.log, x is undefined, and the output will be undefined.

15. What will be the output of the following code?

function a() {
  console.log(this);
}
var b = {
  foo: a
};
b.foo(); // Output: { foo: a };

Explanation:

This question tests your understanding of the value of this in JavaScript functions. In this case, the function a is called as a method of the object b, so the value of this inside a will refer to the object b. The output will be the object b itself.

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Conclusion

JavaScript is a versatile and widely used programming language, but it can also be tricky. This article explored a set of tricky JavaScript questions that can help developers master the language. By delving into these questions, we touched on critical aspects of JavaScript, such as variable hoisting, function scope, closures, prototypes, type coercion, and more.

Understanding these concepts not only prepares you for challenging interviews, but it also enhances your overall comprehension of JavaScript’s behavior and best practices. Mastering JavaScript requires continuous learning and practice, so be sure to keep exploring real-world projects and coding exercises. Additionally, always strive to write clean, efficient, and maintainable code. With a strong foundation and practical experience, debugging and understanding the intricacies of JavaScript will become easier.

As you continue your JavaScript journey, embrace the challenges and celebrate the breakthroughs. Becoming proficient in JavaScript opens doors to various opportunities in web development, front-end frameworks, back-end technologies, mobile app development, and even the burgeoning field of Internet of Things (IoT).

Stay curious, keep coding, and never stop honing your JavaScript expertise. The more you delve into its intricacies, the more you’ll appreciate its power in shaping the digital world. Happy coding!

We hope you enjoyed this article and found it helpful. If you have any questions, please feel free to leave a comment below.

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