15 JavaScript ES6 Interview Questions and Answers

JavaScript ES6, also known as ECMAScript 2015, introduced significant improvements and new features to the JavaScript language, making it more powerful and easier to work with. These enhancements include new syntax for writing cleaner and more maintainable code, as well as advanced functionalities that streamline complex operations. Mastery of ES6 is essential for modern web development, as it is widely adopted in both front-end and back-end environments.

This article provides a curated selection of interview questions focused on JavaScript ES6. Reviewing these questions will help you deepen your understanding of ES6 features and prepare you to demonstrate your proficiency during technical interviews.

JavaScript ES6 Interview Questions and Answers

1. How do you declare variables using let and const, and what are the main differences between them?

In JavaScript ES6, variables can be declared using let and const. Both are block-scoped, unlike var, which is function-scoped. The main differences are:

• let allows reassignment of values.
• const does not allow reassignment; however, objects or arrays declared with const can have their contents modified.

Example:

// Using let
let x = 10;
x = 20; // Allowed

// Using const
const y = 30;
y = 40; // Error

// Modifying an object declared with const
const obj = { a: 1 };
obj.a = 2; // Allowed

2. Explain how arrow functions differ from traditional functions in terms of syntax and behavior.

Arrow functions in JavaScript ES6 offer a concise syntax and differ from traditional functions in key ways:

• Syntax: Arrow functions use the => syntax, making the code more readable.
• this Binding: Arrow functions lexically bind this, using it from the surrounding context.
• Implicit Return: They allow for implicit return if the function body is a single expression.

Example:

// Traditional Function
function traditionalFunction() {
    console.log(this);
}

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

// Example of `this` binding
const obj = {
    traditional: traditionalFunction,
    arrow: arrowFunction
};

obj.traditional(); // Logs the obj context
obj.arrow(); // Logs the global context (or undefined in strict mode)

3. How would you use template literals to create a multi-line string and embed expressions within it?

Template literals, introduced in ES6, simplify string creation and allow embedding expressions. They are enclosed by backticks ( ). Example: <pre>{{EJS45}}</pre> <h4>4. How do you set default values for function parameters, and why is this useful?</h4> Default parameters in JavaScript ES6 allow setting default values for function parameters, making functions more robust and reducing the need for additional checks. Example: <pre>{{EJS46}}</pre> <h4>5. Explain how the spread operator can be used to combine arrays or objects.</h4> The spread operator, denoted by three dots (...), expands elements of arrays or objects into a new array or object, simplifying merging or copying. Example for arrays: <pre>{{EJS47}}</pre> Example for objects: <pre>{{EJS48}}</pre> <h4>6. How do rest parameters work in functions, and when would you use them?</h4> Rest parameters in JavaScript ES6 enable functions to accept an indefinite number of arguments as an array, making it easier to handle variable arguments. Example: <pre>{{EJS49}}</pre> <h4>7. Describe how you would use destructuring assignment to extract values from an array and an object.</h4> Destructuring assignment in JavaScript ES6 allows unpacking values from arrays or properties from objects into distinct variables, enhancing code readability. For arrays: <pre>{{EJS50}}</pre> For objects: <pre>{{EJS51}}</pre> <h4>8. How do you define a class in ES6, and what are the benefits of using classes?</h4> In ES6, a class is defined using the keyword, providing a way to create objects with shared properties and methods. Classes support inheritance, promoting code reuse. Example: <pre>{{EJS52}}</pre> <h4>9. Explain how inheritance works with classes and how you would use the keyword.</h4> Inheritance in JavaScript ES6 allows one class to inherit properties and methods from another using the keyword. The keyword calls the parent class's constructor and methods. Example: <pre>{{EJS53}}</pre> <h4>10. How do you import and export functionalities between different JavaScript files using ES6 modules?</h4> In JavaScript ES6, modules allow you to break down your code into smaller, reusable pieces. You can export functions, objects, or primitive values from one module and import them into another. There are two main types of exports: <b>named exports</b> and <b>default exports</b>. Example: File: .js <pre>{{EJS54}}</pre> File: .js <pre>{{EJS55}}</pre> <h4>11. Describe how promises work and provide an example of how you would use them to handle asynchronous operations.</h4> <b>Promises</b> in JavaScript ES6 represent the eventual completion (or failure) of an asynchronous operation and its resulting value. A promise can be in one of three states: pending, fulfilled, or rejected. Example: <pre>{{EJS56}}</pre> <h4>12. How do and improve the readability and management of asynchronous code?</h4> and in JavaScript ES6 handle asynchronous operations more efficiently. They allow developers to write asynchronous code that looks and behaves like synchronous code. Example: <pre>{{EJS57}}</pre> <h4>13. How does the Map object differ from a regular JavaScript object, and when would you use it?</h4> The Map object in JavaScript ES6 provides a more efficient way to handle key-value pairs compared to regular objects. Here are the main differences: <ul> <li><b>Key Types:</b> In a Map, keys can be of any type, including objects, functions, and primitives.</li> <li><b>Iteration Order:</b> Maps maintain the insertion order of keys.</li> <li><b>Performance:</b> Maps are optimized for frequent additions and removals of key-value pairs.</li> <li><b>Size Property:</b> Maps have a size property that returns the number of key-value pairs.</li> </ul> Example: <pre>{{EJS58}}</pre> <h4>14. Describe how the Set object works and provide an example of its usage.</h4> The Set object in JavaScript ES6 allows you to store unique values of any type, ensuring the uniqueness of elements. Example: <pre>{{EJS59}}</pre> <h4>15. How do you implement a custom iterator using .iterator?</h4> In JavaScript ES6, an iterator is an object that defines a sequence and potentially a return value upon its termination. The .iterator` is a built-in symbol that specifies the default iterator for an object.

Example of a custom iterator:

class CustomIterable {
    constructor(data) {
        this.data = data;
    }

    [Symbol.iterator]() {
        let index = 0;
        let data = this.data;

        return {
            next() {
                if (index < data.length) {
                    return { value: data[index++], done: false };
                } else {
                    return { done: true };
                }
            }
        };
    }
}

const iterable = new CustomIterable([1, 2, 3, 4, 5]);

for (const value of iterable) {
    console.log(value);
}