🔥 JavaScript Interview Series(16): ES6+ Features Every Interviewer Expects You to Know

Modern JavaScript interviews often dive deep into ES6+ (ECMAScript 2015 and beyond). Mastering these features not only shows your technical fluency but also demonstrates your ability to write clean, efficient, and scalable code. Below are 10 must-know ES6+ interview questions that test both your understanding and practical coding ability.

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1. (Interview Question 1) What are the key differences between var, let, and const?

Focus Area: Variable declarations and scoping
Standard Answer:

• var is function-scoped and can be redeclared or hoisted.
• let and const are block-scoped, meaning they exist only within {} blocks.
• const cannot be reassigned, while let can.
• Variables declared with let or const are not hoisted in the same way as var (they are in the temporal dead zone until initialization).

Example:

{
  var x = 1;
  let y = 2;
  const z = 3;
}
console.log(x); // 1
console.log(y); // ReferenceError

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. What is the temporal dead zone in JavaScript?
2. Can const be used with objects?
3. How does hoisting behave differently for var, let, and const?

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2. (Interview Question 2) Explain arrow functions and how they differ from regular functions.

Focus Area: Lexical this binding and concise syntax
Standard Answer:
Arrow functions do not have their own this or arguments object. Instead, they inherit this from the enclosing lexical scope. They also cannot be used as constructors.

Example:

function Regular() { this.value = 10; }
const Arrow = () => { this.value = 10; };

new Regular(); // Works
new Arrow();   // TypeError

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. When should you avoid arrow functions?
2. How do arrow functions behave in event listeners?
3. What happens if you use this inside an arrow function defined in a class?

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3. (Interview Question 3) What is destructuring in ES6 and why is it useful?

Focus Area: Object and array unpacking
Standard Answer:
Destructuring allows extracting values from arrays or properties from objects into distinct variables in a more concise way.

Example:

const [a, b] = [1, 2];
const { name, age } = { name: 'John', age: 30 };

This makes code cleaner and avoids repetitive property access.

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. Can you set default values when destructuring?
2. How do you rename variables during destructuring?
3. How can you destructure nested objects?

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4. (Interview Question 4) How does the spread operator (...) work?

Focus Area: Copying and merging arrays/objects
Standard Answer:
The spread operator expands iterable elements like arrays or objects.

• For arrays: it copies or merges elements.
• For objects: it creates shallow copies or merges multiple objects.

Example:

const arr1 = [1, 2];
const arr2 = [...arr1, 3]; // [1, 2, 3]

const obj1 = { a: 1 };
const obj2 = { ...obj1, b: 2 }; // { a: 1, b: 2 }

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How does the spread operator differ from Object.assign()?
2. What are its limitations with deep copies?
3. Can you use the spread operator on strings or maps?

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5. (Interview Question 5) What are template literals and why are they useful?

Focus Area: String interpolation and multiline strings
Standard Answer:
Template literals allow embedding variables and expressions inside strings using backticks (``). They also support multiline strings and expression evaluation.

Example:

`javascript
const name = "Alex";
console.log(`Hello, ${name}! Today is ${new Date().toDateString()}.`);
`

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. Can template literals execute functions inside expressions?
2. What are tagged template literals?
3. How do template literals handle escaping backticks?

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6. (Interview Question 6) What are default parameters in ES6 functions?

Focus Area: Function parameter handling
Standard Answer:
Default parameters allow setting initial values for function arguments. If a value is not provided, the default is used.

Example:

`javascript
function greet(name = "Guest") {
return `Hello, ${name}`;
}
greet(); // "Hello, Guest"
`

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. Can default parameters reference other parameters?
2. How do default parameters work with undefined values?
3. How do default parameters differ from using || in assignments?

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7. (Interview Question 7) What are Promises and how do they work?

Focus Area: Asynchronous programming
Standard Answer:
Promises represent a value that may be available now, later, or never. They simplify handling async operations compared to callbacks by chaining .then() and handling errors with .catch().

Example:

`javascript
fetch('/data.json')
.then(response => response.json())
.then(data => console.log(data))
.catch(err => console.error(err));
`

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How do you handle multiple Promises simultaneously?
2. What is the difference between Promise.all and Promise.race?
3. How does async/await simplify Promise usage?

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8. (Interview Question 8) Explain async and await.

Focus Area: Simplified asynchronous flow
Standard Answer:
async marks a function as asynchronous, automatically returning a Promise. await pauses execution until a Promise resolves, allowing synchronous-like async code.

Example:

`javascript
async function fetchData() {
const response = await fetch('/api');
const data = await response.json();
console.log(data);
}
`

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How does error handling work with async/await?
2. What happens if you forget to use await?
3. Can await be used outside of an async function?

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9. (Interview Question 9) What is the difference between for...of and for...in?

Focus Area: Iterating over collections
Standard Answer:

• for...in iterates over enumerable property names of an object.
• for...of iterates over values of iterable objects (like arrays, strings, maps).

Example:

`javascript
for (let key in {a: 1, b: 2}) console.log(key); // a, b
for (let val of [10, 20]) console.log(val); // 10, 20
`

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. Can you use for...of on objects directly?
2. What happens if you modify the iterable during iteration?
3. How does Object.keys() differ from for...in?

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10. (Interview Question 10) What are ES6 modules and how do they differ from CommonJS?

Focus Area: Modular JavaScript architecture
Standard Answer:
ES6 modules use import and export syntax and are statically analyzed, while CommonJS (require, module.exports) is dynamically loaded at runtime.

Example:

`javascript
// module.js
export const PI = 3.14;

// main.js
import { PI } from './module.js';
`

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. Why are ES modules preferred for tree-shaking?
2. Can ES6 modules run natively in browsers?
3. How do you handle default vs named exports?