Javascript · Question Set

Functions - Part 1 – Javascript interview questions

Functions - Part 1 interview questions for placements and exams.

Subject: JavascriptDifficulty: Mixed13 questions in this set

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Subject

Javascript

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Mixed

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Javascript – Functions - Part 1 questions (13)

Go through each question and its explanation. Use this set as a focused practice pack for Javascript.

Back to Javascript subject page

1. What is the difference between parameters and arguments?

Difficulty: EasyType: MCQTopic: JS Functions

Parameters are placeholders defined in a function declaration. Arguments are the real values passed when calling that function. In short, parameters are like empty boxes, and arguments are what you put inside them. If you pass fewer arguments than parameters, the missing ones become undefined. Matching them correctly ensures your function works as expected.

Example code

function add(a, b) { return a + b; }
console.log(add(2, 3)); // 2 and 3 are arguments; a and b are parameters

2. How does function hoisting work? Can you call a function before defining it?

Difficulty: MediumType: SubjectiveTopic: Scope Hoisting

Function declarations are hoisted to the top of their scope during compilation. That means you can call a declared function before it appears in your code. However, function expressions and arrow functions are not hoisted. They act like normal variables and exist only after assignment. Calling them early causes a reference error. Understanding hoisting helps you organize code properly and avoid unpredictable results.

Example code

sayHi(); // Works
function sayHi() {
  console.log('Hello');
}

// Not hoisted
// greet(); // Error
const greet = function() { console.log('Hi'); };

3. What happens if a function does not have a return statement?

Difficulty: EasyType: MCQTopic: JS Functions

Every JavaScript function returns something. If you do not write a return statement, it automatically returns undefined. This means the function completes its work but does not give any value back to the caller. Use a return statement when you want to send data out of a function. Remember, console.log only displays the result — it does not return it.

Example code

function add(a, b) {
  const sum = a + b;
}

console.log(add(2, 3)); // undefined

4. Which statement about arrow functions is true?

Difficulty: MediumType: MCQTopic: Arrow Functions

Arrow functions are short and modern alternatives to normal functions. They do not create their own 'this' value. Instead, they use the 'this' from the outer scope. This makes them perfect for callbacks where you want to use the same context. They cannot be used as constructors and do not have an arguments object. Use them for clean, compact logic, but avoid them in methods that rely on dynamic 'this'.

Example code

const person = {
  name: 'John',
  greet: () => console.log('Hello ' + this.name)
};

person.greet(); // 'Hello undefined' (no own this)

5. What is the output of function greet(name = 'Guest') { return 'Hi ' + name; } greet();

Difficulty: EasyType: MCQTopic: JS Functions

Default parameters allow a function to use fallback values when no argument is given. This avoids undefined values and makes code more reliable. Defaults are set from left to right, so earlier parameters can be used in later defaults. This feature was introduced in ES6 and is now common in modern JavaScript.

Example code

function greet(name = 'Guest') {
  return 'Hi ' + name;
}

console.log(greet()); // Hi Guest

6. What does IIFE stand for?

Difficulty: MediumType: MCQTopic: JS Modules

IIFE means a function that runs immediately after it is defined. It is wrapped in parentheses and then invoked right away. IIFEs create a private scope, keeping variables safe from the global scope. They are often used in modules and setup code. Think of an IIFE as a self-running function used to protect data or initialize logic once.

Example code

(function() {
  console.log('IIFE runs instantly');
})();

7. What is the purpose of an IIFE in JavaScript? Give a practical use case.

Difficulty: MediumType: SubjectiveTopic: JS Modules

An IIFE, or Immediately Invoked Function Expression, runs as soon as it is defined. It creates a private area where variables cannot be accessed from outside. Developers used IIFEs before ES6 modules to protect code and avoid global variable pollution. They are still useful for one-time setups, like initializing configurations or starting timers. Think of IIFE as a self-executing container that keeps your code safe and clean.

Example code

(function() {
  const secret = 'hidden';
  console.log('IIFE executed');
})();

// secret is not accessible outside
// console.log(secret); // Error

8. Which functions are hoisted in JavaScript?

Difficulty: MediumType: MCQTopic: Scope Hoisting

Hoisting means moving declarations to the top during compilation. Only function declarations are hoisted, not function expressions or arrow functions. That’s why you can call a declared function before writing it in code. This behavior helps JavaScript interpret functions flexibly but can also cause confusion if misused. Always declare clearly to keep your code readable.

Example code

greet(); // Works!
function greet() {
  console.log('Hello');
}

sayHi(); // Error
const sayHi = function() {
  console.log('Hi');
};

9. Explain function scope in JavaScript. How does it differ from block scope?

Difficulty: MediumType: SubjectiveTopic: Scope Hoisting

Function scope means variables declared inside a function can be used only inside that function. Before ES6, variables declared with var followed function scope. They were visible anywhere inside the function body. With ES6, let and const introduced block scope, which limits the variable to the nearest pair of curly braces. Block scope is tighter and safer because it avoids unwanted access outside its block. Use let or const for predictable, bug-free behavior.

Example code

function test() {
  if (true) {
    var a = 10;
    let b = 20;
  }
  console.log(a); // 10 (function scoped)
  // console.log(b); // Error (block scoped)
}

test();

10. Compare arrow functions and regular functions in JavaScript.

Difficulty: MediumType: SubjectiveTopic: Arrow Functions

Arrow functions and regular functions behave differently in how they handle 'this'. Arrow functions do not create their own 'this'; they use the one from the outer scope. Regular functions have their own 'this' and can be used as constructors. Arrow functions are shorter and great for small callbacks, but avoid them in object methods. In interviews, remember this rule: Arrow equals lexical this. Regular equals dynamic this. Use arrow for compact logic, and regular when context matters.

Example code

function normal() { console.log(this); }
const arrow = () => console.log(this);

const obj = {
  normal,
  arrow
};

obj.normal(); // obj context
obj.arrow();  // global or undefined (lexical this)

11. What is a callback function? Why is it important in JavaScript?

Difficulty: MediumType: SubjectiveTopic: JS Functions

A callback is a function you pass into another function to be executed later. It allows asynchronous actions, meaning your program can continue working while waiting for something to finish. Callbacks are essential in event handling, API calls, and timers. They prevent blocking and make JavaScript responsive. Before Promises and async await, callbacks were the main way to handle asynchronous behavior. Mastering callbacks builds a strong base for understanding Promises and async programming.

Example code

function fetchData(callback) {
  setTimeout(() => {
    console.log('Data ready');
    callback();
  }, 1000);
}

fetchData(() => console.log('Callback executed'));

12. How do you correctly declare a function in JavaScript?

Difficulty: EasyType: MCQTopic: JS Functions

In JavaScript, you declare a function using the keyword 'function', followed by a name and parentheses. Inside the curly braces, you write the code that will run when the function is called. Function declarations are hoisted, meaning you can call them even before they appear in the code. Functions are used to group related logic into one reusable block. This makes your program cleaner, easier to read, and more efficient to maintain.

Example code

function greet() {
  console.log('Hello World');
}

greet(); // Output: Hello World

13. Which of the following defines a function expression?

Difficulty: MediumType: MCQTopic: JS Functions

A function expression defines a function and assigns it to a variable. It can be named or anonymous and behaves like any other value in JavaScript. Function expressions are not hoisted, so you must define them before using them. They are widely used in callbacks, event listeners, and higher-order functions. Use const to prevent accidental reassignment and keep code predictable.

Example code

const sum = function(a, b) {
  return a + b;
};

console.log(sum(5, 3)); // 8

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Continue your preparation

For complete preparation, combine this set with full subject-wise practice for Javascript. You can also explore other subjects and sets from the links below.

Quick navigation:Javascript subject page•All subjects

Subject page

Javascript · Question Set

Functions - Part 1 – Javascript interview questions

Functions - Part 1 interview questions for placements and exams.

Subject: JavascriptDifficulty: Mixed13 questions in this set

Set overview

Questions

Included in this set

Subject

Javascript

Explore more sets

Difficulty

Mixed

Level of this set

Javascript – Functions - Part 1 questions (13)

Go through each question and its explanation. Use this set as a focused practice pack for Javascript.

Back to Javascript subject page

1. What is the difference between parameters and arguments?

Difficulty: EasyType: MCQTopic: JS Functions

Example code

function add(a, b) { return a + b; }
console.log(add(2, 3)); // 2 and 3 are arguments; a and b are parameters

2. How does function hoisting work? Can you call a function before defining it?

Difficulty: MediumType: SubjectiveTopic: Scope Hoisting

Example code

sayHi(); // Works
function sayHi() {
  console.log('Hello');
}

// Not hoisted
// greet(); // Error
const greet = function() { console.log('Hi'); };

3. What happens if a function does not have a return statement?

Difficulty: EasyType: MCQTopic: JS Functions

Example code

function add(a, b) {
  const sum = a + b;
}

console.log(add(2, 3)); // undefined

4. Which statement about arrow functions is true?

Difficulty: MediumType: MCQTopic: Arrow Functions

Example code

const person = {
  name: 'John',
  greet: () => console.log('Hello ' + this.name)
};

person.greet(); // 'Hello undefined' (no own this)

5. What is the output of function greet(name = 'Guest') { return 'Hi ' + name; } greet();

Difficulty: EasyType: MCQTopic: JS Functions

Example code

function greet(name = 'Guest') {
  return 'Hi ' + name;
}

console.log(greet()); // Hi Guest

6. What does IIFE stand for?

Difficulty: MediumType: MCQTopic: JS Modules

Example code

(function() {
  console.log('IIFE runs instantly');
})();

7. What is the purpose of an IIFE in JavaScript? Give a practical use case.

Difficulty: MediumType: SubjectiveTopic: JS Modules

Example code

(function() {
  const secret = 'hidden';
  console.log('IIFE executed');
})();

// secret is not accessible outside
// console.log(secret); // Error

8. Which functions are hoisted in JavaScript?

Difficulty: MediumType: MCQTopic: Scope Hoisting

Example code

greet(); // Works!
function greet() {
  console.log('Hello');
}

sayHi(); // Error
const sayHi = function() {
  console.log('Hi');
};

9. Explain function scope in JavaScript. How does it differ from block scope?

Difficulty: MediumType: SubjectiveTopic: Scope Hoisting

Example code

function test() {
  if (true) {
    var a = 10;
    let b = 20;
  }
  console.log(a); // 10 (function scoped)
  // console.log(b); // Error (block scoped)
}

test();

10. Compare arrow functions and regular functions in JavaScript.

Difficulty: MediumType: SubjectiveTopic: Arrow Functions

Example code

function normal() { console.log(this); }
const arrow = () => console.log(this);

const obj = {
  normal,
  arrow
};

obj.normal(); // obj context
obj.arrow();  // global or undefined (lexical this)

11. What is a callback function? Why is it important in JavaScript?

Difficulty: MediumType: SubjectiveTopic: JS Functions

Example code

function fetchData(callback) {
  setTimeout(() => {
    console.log('Data ready');
    callback();
  }, 1000);
}

fetchData(() => console.log('Callback executed'));

12. How do you correctly declare a function in JavaScript?

Difficulty: EasyType: MCQTopic: JS Functions

Example code

function greet() {
  console.log('Hello World');
}

greet(); // Output: Hello World

13. Which of the following defines a function expression?

Difficulty: MediumType: MCQTopic: JS Functions

Example code

const sum = function(a, b) {
  return a + b;
};

console.log(sum(5, 3)); // 8

More sets in Javascript

JavaScript Fundamentals(15+ Questions)
Operators and Functions(15+ Questions)
Arrays and Loops(15+ Questions)
JavaScript Core Interview Essentials(13+ Questions)
Closures and Asynchronous JavaScript(14+ Questions)
Objects & Prototypes(12+ Questions)
Objects and this Keyword(15+ Questions)
Strings and Template Literals(15+ Questions)
Prototypes and Inheritance(14+ Questions)
ES6+ Advanced Features(12+ Questions)
Performance Optimization and Advanced Techniques(10+ Questions)
Call, Apply, Bind and Advanced Functions(14+ Questions)
DOM Manipulation and Events(12+ Questions)

Explore related subjects

HTML CSS Typescript SQL & Databases React Node Js

Continue your preparation

For complete preparation, combine this set with full subject-wise practice for Javascript. You can also explore other subjects and sets from the links below.

Quick navigation:Javascript subject page•All subjects

Functions - Part 1 – Javascript interview questions

Set overview

1. What is the difference between parameters and arguments?

2. How does function hoisting work? Can you call a function before defining it?

3. What happens if a function does not have a return statement?

4. Which statement about arrow functions is true?

5. What is the output of function greet(name = 'Guest') { return 'Hi ' + name; } greet();

6. What does IIFE stand for?

7. What is the purpose of an IIFE in JavaScript? Give a practical use case.

8. Which functions are hoisted in JavaScript?

9. Explain function scope in JavaScript. How does it differ from block scope?

10. Compare arrow functions and regular functions in JavaScript.

11. What is a callback function? Why is it important in JavaScript?

12. How do you correctly declare a function in JavaScript?

13. Which of the following defines a function expression?

More sets in Javascript

Explore related subjects

Continue your preparation

Master Interviews Anywhere, Anytime

Functions - Part 1 – Javascript interview questions

Set overview

1. What is the difference between parameters and arguments?

2. How does function hoisting work? Can you call a function before defining it?

3. What happens if a function does not have a return statement?

4. Which statement about arrow functions is true?

5. What is the output of function greet(name = 'Guest') { return 'Hi ' + name; } greet();

6. What does IIFE stand for?

7. What is the purpose of an IIFE in JavaScript? Give a practical use case.

8. Which functions are hoisted in JavaScript?

9. Explain function scope in JavaScript. How does it differ from block scope?

10. Compare arrow functions and regular functions in JavaScript.

11. What is a callback function? Why is it important in JavaScript?

12. How do you correctly declare a function in JavaScript?

13. Which of the following defines a function expression?

More sets in Javascript

Explore related subjects

Continue your preparation

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Anywhere, Anytime