Functions in JavaScript — Interview Questions & Answers

Ques: What is a Function in JavaScript?

Ans: A function is a reusable block of code designed to perform a specific task. It executes when called or invoked.

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

Ques: What are the different ways to define a function in JavaScript?

1. Function Declaration
2. Function Expression
3. Arrow Function
4. Constructor Function
5. IIFE (Immediately Invoked Function Expression)

Ques: What is a Function Declaration?

Ans: Defined using the function keyword, it can be hoisted.

function add(a, b) {
  return a + b;
}

Ques: What is a Function Expression?

Ans: A function assigned to a variable — not hoisted.

const multiply = function(a, b) {
  return a * b;
};

Ques: What is the difference between Function Declaration and Expression?

Feature	Declaration	Expression
Hoisting	Yes	No
Syntax	Named	Can be anonymous
Use case	Define functions early	Dynamic assignment

Ques: What is an Anonymous Function?

Ans: A function without a name, often used in callbacks or IIFEs.

setTimeout(function() {
  console.log("Anonymous!");
}, 1000);

Ques: What is an IIFE (Immediately Invoked Function Expression)?

Ans: A function that executes immediately after creation.

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

Ques: What are Parameters and Arguments?

• Parameters → variables listed in function definition.
• Arguments → actual values passed when calling the function.

function greet(name) { // parameter
  console.log("Hi, " + name);
}
greet("Riya"); // argument

Ques: What are Default Parameters?

Ans: Used to assign a default value when no argument is provided.

function greet(name = "Guest") {
  console.log("Hi, " + name);
}
greet(); // Hi, Guest

Ques: What is the Rest Parameter (...)?

Ans: Collects multiple arguments into an array.

function sum(...nums) {
  return nums.reduce((a, b) => a + b);
}
sum(1, 2, 3); // 6

Ques: What is the Spread Operator (...) in functions?

Ans: Used to spread array elements as separate arguments.

const nums = [1, 2, 3];
console.log(Math.max(...nums)); // 3

Que: What is Function Invocation?

Ans: Executing a function using ().

function sayHi() { console.log("Hi!"); }
sayHi(); // invoked

Ques: What are the different ways to invoke a function?

1. Direct invocation
2. Method invocation
3. Constructor invocation (new)
4. Indirect invocation (call, apply, bind)

Ques: What is Direct Invocation?

Ans: Calling a function directly by name.

function test() { console.log(this); }
test(); // this → window / undefined (strict)

Ques: What is Method Invocation?

Ans: When a function is called as a property of an object.

const obj = {
  name: "Dev",
  greet() { console.log("Hi " + this.name); }
};
obj.greet(); // Hi Dev

Ques: What are Arrow Functions?

Ans: Introduced in ES6 — shorter syntax, no own this, arguments, or super.

const add = (a, b) => a + b;

Ques: What is Lexical this in Arrow Functions?

Ans: Arrow functions don’t bind this — they inherit from their parent scope.

function Counter() {
  this.count = 0;
  setInterval(() => console.log(++this.count), 1000);
}
new Counter(); // works correctly

Ques: What are Higher-Order Functions?

Ans: Functions that take other functions as arguments or return them.

function calculate(fn, x, y) {
  return fn(x, y);
}
calculate((a,b)=>a+b, 2, 3); // 5

Ques: What are Callback Functions?

Ans: Functions passed as arguments to other functions.

function greet(callback) {
  console.log("Hello!");
  callback();
}
greet(() => console.log("Callback executed"));

Ques: What are Async Functions?

Ans: Functions that use async/await to handle asynchronous code.

async function fetchData() {
  const res = await fetch("<https://api.example.com>");
  return res.json();
}

Ques: What is a Generator Function?

Ans: A function that can pause execution using yield.

function* gen() {
  yield 1;
  yield 2;
}
const g = gen();
console.log(g.next().value); // 1

Ques: What is Function Scope?

Ans: Variables declared inside a function are local to that function.

function demo() {
  let x = 10;
  console.log(x);
}
console.log(x); //  ReferenceError

Ques: What is Function Hoisting?

Ans: Function declarations are hoisted — you can call them before defining.

greet();
function greet() {
  console.log("Hoisted!");
}

Ques: What are Closures?

Ans: A closure is when an inner function remembers variables from its outer function even after it has returned.

function outer() {
  let count = 0;
  return function inner() {
    count++;
    console.log(count);
  };
}
const counter = outer();
counter(); // 1
counter(); // 2

Ques: Why are Closures useful?

• Data privacy
• State management
• Function factories

Ques: What is the this keyword?

Ans: Refers to the object that owns the function being executed.

Ques: What is this in regular functions vs arrow functions?

Function Type	this Value
Regular	Dynamic (depends on call site)
Arrow	Lexical (from outer scope)

Ques: Explain Call, Apply, and Bind.

• call() → invoke immediately, pass args individually
• apply() → invoke immediately, pass args as array
• bind() → returns a new bound function

function show(city) {
  console.log(this.name, city);
}
const obj = { name: "Alex" };
show.call(obj, "Delhi");
show.apply(obj, ["Goa"]);
const bound = show.bind(obj, "Mumbai");
bound();

Ques: What is Recursion?

Ans: When a function calls itself until a condition is met.

function factorial(n) {
  return n <= 1 ? 1 : n * factorial(n - 1);
}

Ques: What is Tail Recursion?

Ans: A recursion where the recursive call is the last statement. Optimizes stack usage.

Ques: What are Pure Functions?

Ans: Functions that always produce the same output for the same input and have no side effects.

function add(a, b) {
  return a + b;
}

Ques: What are Side Effects in functions?

When a function modifies something outside its scope (e.g., global variables, DOM).

let count = 0;
function increment() {
  count++; // side effect
}

Ques: How can you avoid side effects?

• Use pure functions
• Avoid mutating global data
• Use immutable data patterns

Ques: What is Function Composition?

Ans: Combining multiple functions to produce a new one.

const compose = (f, g) => x => f(g(x));
const add = x => x + 2;
const double = x => x * 2;
console.log(compose(add, double)(3)); // 8

Ques: What are Function Constructors?

Ans: Using new Function() to create functions dynamically (not recommended).

const add = new Function('a', 'b', 'return a + b');

Ques: What is Currying in JavaScript?

Ans: Breaking down a function with multiple arguments into a sequence of single-argument functions.

function curry(a) {
  return b => a + b;
}
curry(5)(3); // 8

Ques: What are First-Class Functions?

Ans: Functions treated like variables — can be passed, returned, or stored.

Ques: Difference between Named and Anonymous Function Expressions?

• Named: helps in debugging.
• Anonymous: used for inline callbacks.

const named = function test() {};
const anon = function() {};

Ques: Why are Functions called Objects in JavaScript?

Ans: Because they can have properties and methods (like .name, .length, .call()).

function f(a,b){}
console.log(f.length); // 2