JavaScript Functions Interview Questions

Functions are core building blocks in JavaScript Programming. In JavaScript, functions are very flexible and that’s why it may be sometimes difficult to grasp in the beginning. Here’s a collection of common interview questions related to JavaScript functions, scopes, and related topics, along with their answers.

1. What is a function in JavaScript?

A function is a reusable block of code designed to perform a particular task. Functions can take parameters and return a value.

2. What is the difference between function declarations and function expressions?

Function Declaration: Defined with the function keyword and hoisted, allowing it to be called before its definition.

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

Function Expression: Defined as part of an expression, can be anonymous, and is not hoisted.

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

3. What is an arrow function?

Arrow functions provide a shorter syntax and lexically bind the this value (it retains the value of this from the enclosing context).

const greet = () => {
    console.log("Hello!");
};

4. What are higher-order functions?

Higher-order functions are functions that can take other functions as arguments or return them as output. For example

function applyFunction(fn, value) {
    return fn(value);
}

5. What is scope in JavaScript?

Scope determines the accessibility of variables in different parts of the code. JavaScript has two main types of scope: global scope and local scope (which includes function scope and block scope).

6. What is the difference between global scope and local scope?

• Global Scope: Variables declared outside any function are in the global scope and can be accessed from anywhere in the code.
• Local Scope: Variables declared within a function (or block) are in local scope and can only be accessed within that function (or block).

7. What is block scope?

Block scope is created by curly braces {}. Variables declared with let and const are block-scoped, which means that they only exist within that block.

if (true) {
    let x = 10;
}
console.log(x); // ReferenceError: x is not defined

8. What is the scope chain?

The scope chain is a hierarchy of scopes that JavaScript uses to resolve variable names. When looking up a variable, JavaScript first checks the local scope, then moves up to the outer scopes until it finds the variable or reaches the global scope.

9. What is a closure in JavaScript?

Closures are created when a function is defined inside another function. A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope.

function outerFunction() {
    let outerVar = 'I am from outer scope';
    return function innerFunction() {
        console.log(outerVar);
    };
}
const innerFunc = outerFunction();
innerFunc(); // "I am from outer scope"

10. How are closures used in JavaScript?

Closures are commonly used for:

• Data encapsulation: Keeping variables private.
• Creating factory functions that return functions with specific configurations.
• Maintaining state in asynchronous programming.

11. Can you give an example of a closure?

Here’s an example of a simple counter

function createCounter() {
    let count = 0; // Private variable
    return {
        increment: function() {
            count++;
            return count;
        },
        decrement: function() {
            count--;
            return count;
        },
        getCount: function() {
            return count;
        }
    };
}

const counter = createCounter();
console.log(counter.increment()); // 1
console.log(counter.increment()); // 2
console.log(counter.getCount());  // 2

12. What happens to the variables in a closure when the outer function has finished executing?

The variables remain in memory and are accessible to the inner function, allowing the inner function to continue accessing those variables even after the outer function has completed execution.