JavaScript ES6+ Features You Must Know for Interviews in 2025

JavaScript’s come a long way, and if you’re aiming for a JavaScript developer job in Singapore, it’s no longer enough to just "know the basics."
Whether you’re applying to fast-paced tech startups at LaunchPad One-North or eyeing a role at big players like Shopee and Sea Group, understanding modern JavaScript ES6 features gives you a real advantage.

Technical interviews today aren’t about textbook answers — they’re about showing you can think in clean, scalable JavaScript. Concepts like destructuring assignment, async/await, and choosing between let vs const aren’t just nice-to-know skills anymore — they're expected as part of strong JavaScript interview preparation.

This guide breaks down the most important ES6+ JavaScript features that show up over and over again in real-world interviews — explained simply, with examples you’ll actually remember.
If you’re serious about standing out and landing that next big opportunity, let’s dive right into the world of JavaScript ES6 interview success.

Table of Contents

1. Variable Declarations: var, let, and const
2. Arrow Functions
3. Template Literals
4. Destructuring Assignment
5. Spread and Rest Operators
6. Default Parameters
7. Enhanced Object Literals
8. Promises and Async/Await
9. Classes and Inheritance
10. Modules: import and export
11. Other Important ES6+ Features
12. Bonus: Emerging Features from ES2020–2025

1. Variable Declarations: var, let, and const

One of the foundational changes introduced in ES6 was the new ways to declare variables using let and const instead of the traditional var. Understanding these distinctions is crucial for writing predictable, bug-free JavaScript code.

In modern JavaScript, the evolution from var to let and const is vital for writing cleaner code, especially in interviews for Java developers. These changes help developers avoid common pitfalls and ensure more predictable variable management. If you’re preparing for Java Developer Jobs, understanding variable scoping is crucial

1.1 var

var declarations are function-scoped, meaning a variable declared with var is accessible throughout the enclosing function. If declared outside a function, it becomes globally scoped. It also supports hoisting, but the variable is initialized with undefined.

function example()  
example();

1.2 let

let is block-scoped, meaning it is only accessible within the nearest pair of curly braces ( ). It also supports hoisting but does not initialize the variable, leading to a ReferenceError if accessed before declaration (Temporal Dead Zone).

1.3 const

const behaves similarly to let with block-scope behavior. However, variables declared with const must be initialized at the time of declaration and cannot be reassigned.

const pi = 3.14;
pi = 3.1415; // TypeError

Pro Tip: Always default to const unless you specifically need to reassign a variable with let.

Common Mistake: Developers often think const makes an object immutable. Only the reference is immutable, not the contents. You can still modify the properties of a const object.

const user =  ;
user.name = "Bob"; // This is allowed

2. Arrow Functions

Arrow functions, introduced in ES6, offer a concise syntax for writing function expressions. They are not just syntactic sugar — they have important behavioral differences, especially regarding the this keyword.

2.1 Syntax of Arrow Functions

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

If the function body contains only a single expression, the braces and return statement can be omitted. For more complex bodies, use braces and a return keyword.

const multiply = (a, b) =>  ;

2.2 Behavior of this

Arrow functions do not bind their own this. Instead, they inherit this from the surrounding (lexical) scope. This makes them ideal for callbacks and event listeners in many cases.

function Counter()  , 1000);
}
new Counter();

2.3 When Not to Use Arrow Functions

• When you need dynamic binding of this (e.g., in object methods)
• When creating constructor functions (arrow functions cannot be used as constructors)

Pro Tip: Use arrow functions for non-method callbacks like map, filter, or forEach — they result in cleaner, more maintainable code.

Common Mistake: Accidentally using arrow functions for object methods where this needs dynamic binding can lead to unexpected behavior.

const user =  `) // 'this' is not the user object!
};
user.greet(); // Output: Hello, undefined

3. Template Literals

Template literals are one of the most loved ES6 features that simplify string operations significantly. Instead of tedious concatenations, you can now create strings that are dynamic, readable, and multi-line.

3.1 Syntax

Enclosed by backticks (“ ` “), template literals allow embedded expressions inside $ .

const name = "Alice";
console.log(`Hello, $ !`);

3.2 Multi-line Strings

Template literals automatically handle multi-line strings without needing escape characters like \n.

const message = `Hello,
This is a
multi-line string.`;
console.log(message);

3.3 Expression Interpolation

Besides variables, you can insert any valid JavaScript expression inside template literals.

const a = 5;
const b = 10;
console.log(`Sum of a and b is $ `);

Pro Tip: Template literals are particularly powerful in generating dynamic HTML structures when working with frontend JavaScript frameworks.

Common Mistake: Forgetting to use backticks (“ ` “) and mistakenly using single or double quotes (” or ‘) results in a syntax error when trying to interpolate variables.

const name = "Alice";
console.log("Hello, $ "); // Incorrect! Won't interpolate.

4. Destructuring Assignment

Destructuring allows you to extract values from arrays and objects, making your code shorter and more readable. It’s frequently tested in programming interviews, particularly for developers applying for Java Developer Jobs.

4.1 Array Destructuring

Extract array elements into variables based on position.

const numbers = [10, 20, 30];
const [first, second, third] = numbers;
console.log(first, second, third); // 10 20 30

4.2 Object Destructuring

Extract properties based on their key names.

const user =  ;
const   = user;
console.log(name, age); // Alice 25

4.3 Nested Destructuring

Access deeply nested properties easily.

const person =  
};
const   } = person;
console.log(city); // New York

Pro Tip:

Use default values during destructuring to handle cases where the property might be undefined.

const   = user;
console.log(role); // Guest

Common Mistake:

Forgetting to use the correct key name when destructuring objects results in undefined values.

const   = user;
console.log(username); // undefined

—

5. Spread and Rest Operators

The spread (...) and rest (...) operators are powerful tools for working with arrays, objects, and function parameters. They provide flexibility and make the code cleaner and more expressive.

5.1 Spread Operator

Expands elements of an array or object into individual elements.

const arr1 = [1, 2, 3];
const arr2 = [...arr1, 4, 5];
console.log(arr2); // [1, 2, 3, 4, 5]

const obj1 =  ;
const obj2 =  ;
console.log(obj2); //  

5.2 Rest Operator

Collects remaining elements into an array or object.

const [first, ...others] = [1, 2, 3, 4];
console.log(first); // 1
console.log(others); // [2, 3, 4]

function sum(...nums)  
console.log(sum(1, 2, 3)); // 6

Pro Tip:

Use the spread operator to clone arrays and objects without affecting the original data — especially critical in functional programming and React state management.

Common Mistake:

Confusing spread and rest syntax: both use ..., but spread is used in function calls or literals, while rest is used in function parameters and destructuring assignments.

—

6. Default Parameters

Default parameters allow you to specify default values for function arguments if no value or undefined is passed.

function greet(name = "Guest")  !`;
}
console.log(greet()); // Hello, Guest!
console.log(greet("Alice")); // Hello, Alice!

6.1 Why Default Parameters Matter

• Eliminate redundant checks inside functions
• Improve readability and code safety
• Help avoid undefined values when parameters are optional

6.2 Default Parameters with Destructuring

Combine default parameters with destructuring for maximum power.

function createUser(  =  )   is $  years old.`;
}
console.log(createUser( )); // Bob is 18 years old
console.log(createUser()); // Anonymous is 18 years old

Pro Tip:

Always list parameters with defaults at the end of the parameter list to avoid unexpected behavior.

Common Mistake:

Misunderstanding default parameters when an argument is passed explicitly as null — only undefined triggers the default value, not null.

function greet(name = "Guest")  
greet(undefined); // Guest
greet(null); // null (NOT Guest)

7. Enhanced Object Literals

ES6 introduced several enhancements to object literals that make creating and working with objects faster, cleaner, and more powerful. This is crucial for writing modern, maintainable JavaScript code, especially during interviews where clean syntax is evaluated.

7.1 Property Shorthand

If the key and the variable name are the same, you can use shorthand syntax.

const name = "Alice";
const age = 25;
const user =  ;
console.log(user); //  

7.2 Method Shorthand

You can define functions inside objects without the function keyword.

const user =  
};
console.log(user.greet());

7.3 Computed Property Names

You can dynamically compute property keys inside objects using square brackets.

const prop = "id";
const user =  ;
console.log(user); //  

Pro Tip:

Use computed property names for dynamic keys in APIs or configuration objects, which is common in real-world development.

Common Mistake:

Forgetting to wrap dynamic keys in square brackets results in literal property names instead of evaluated expressions.

—

8. Promises and Async/Await

Promises and async/await revolutionized asynchronous programming in JavaScript, replacing callback hell with cleaner, more manageable code structures.

8.1 Promises

A Promise represents the eventual completion or failure of an asynchronous operation.

const promise = new Promise((resolve, reject) =>  );

promise
  .then(result => console.log(result))
  .catch(error => console.error(error));

8.2 Async/Await

Async functions allow you to write asynchronous code that looks synchronous using await inside an async function.

async function fetchData()   catch (error)  
}
fetchData();

8.3 Promise Chaining vs Async/Await

• Promise chaining is better when handling multiple independent asynchronous tasks.
• Async/Await shines when tasks depend sequentially on previous results.

Pro Tip:

In interviews, be prepared to explain how Promises work internally, the three states (pending, fulfilled, rejected), and how await pauses execution without blocking the thread.

Common Mistake:

Forgetting to wrap await calls inside a try/catch block can cause unhandled promise rejections.

—

9. Classes and Inheritance

ES6 classes provide a cleaner syntax for creating constructor functions and handling inheritance. Although classes are syntactic sugar over JavaScript’s prototype system, they make code more readable and object-oriented.

Classes and inheritance were introduced to JavaScript with ES6 to bring better structure to code. Understanding these concepts is important in technical interviews. These topics are closely related to concepts seen in advance Java interview questions and answers.

9.1 Defining a Class

class Person  
  greet()  !`;
  }
}
const person1 = new Person("Alice");
console.log(person1.greet());

9.2 Inheritance with extends

Subclasses are created using the extends keyword, allowing child classes to inherit and override methods from parent classes.

class Animal  
  speak()   makes a noise.`);
  }
}

class Dog extends Animal   barks.`);
  }
}

const dog = new Dog("Buddy");
dog.speak(); // Buddy barks.

9.3 The super Keyword

super() is used to call the constructor or methods of the parent class inside a subclass.

class Cat extends Animal  
}

Pro Tip:

Interviewers love to ask about how inheritance works under the hood — be ready to explain the prototype chain even when using class syntax.

Common Mistake:

Forgetting to call super() inside the subclass constructor before using this causes a ReferenceError.

class Car extends Vehicle  
}

10. Modules: import and export

Before ES6, JavaScript lacked a native module system. Developers relied on external solutions like CommonJS and AMD. ES6 modules finally introduced a standardized way to organize code into reusable pieces, improving code maintainability and scalability.

Developers interviewing for Java jobs will likely be asked about modules, especially when working with modern JavaScript applications. Familiarity with ES6 modules is essential, and if you’re interested in work-from-home Java developer jobs, it’s even more crucial regarding collaboration and module sharing.

10.1 Exporting

You can export functions, objects, or primitives from a module using either named or default exports.

// utils.js
export const add = (a, b) => a + b;
export const subtract = (a, b) => a - b;

10.2 Importing

Import code from other modules to reuse across different files.

import   from './utils.js';
import greet from './greet.js'; // For default exports

Pro Tip:

Use named exports when exporting multiple things from a module; use default exports when the module only exports one main thing.

Common Mistake:

Mixing up named and default imports — named exports must be imported with the same names, default exports can have any name.

—

11. Other Important ES6+ Features

From Symbols to Maps and Sets, there are other ES6+ features that every modern JavaScript developer should understand. These features often come up in programming interview questions for advanced Java roles. You can also explore the Top Programming Languages to Learn in 2025 for a deeper understanding of how JavaScript compares to other languages in the modern tech stack.

11.1 Symbol

Symbols are unique, immutable data types often used to create unique object property keys.

const sym1 = Symbol('description');
const sym2 = Symbol('description');
console.log(sym1 === sym2); // false

11.2 Sets and Maps

• Set: A collection of unique values.
• Map: A collection of key-value pairs, with any data type as keys.

const set = new Set([1, 2, 3, 2]);
console.log(set); //  

const map = new Map();
map.set('name', 'Alice');
console.log(map.get('name')); // Alice

11.3 Optional Chaining (?.)

Safely access deeply nested properties without worrying about undefined values.

const user =   };
console.log(user?.profile?.email); // test@example.com

11.4 Nullish Coalescing (??)

Provide a fallback only for null or undefined values.

const name = null ?? "Guest";
console.log(name); // Guest

—

12. Bonus: Emerging Features from ES2020–2025

As JavaScript continues to evolve, staying updated with the latest features will make you a valuable asset in any development team.

12.1 Dynamic Imports

Load JavaScript modules dynamically and on-demand.

import('./module.js').then(module =>  );

12.2 Promise.allSettled()

Wait for all promises to either resolve or reject without short-circuiting.

Promise.allSettled([
  Promise.resolve(1),
  Promise.reject('Error')
]).then(results => console.log(results));

12.3 Private Class Fields (#)

class Person  
}
const p = new Person();
console.log(p.getAge()); // 30

12.4 Top-Level Await

const response = await fetch('/api/data');
const data = await response.json();
console.log(data);

Pro Tip:
Learning Top-Level Await, Private Fields, and Dynamic Imports will give you an edge during senior-level interviews.

Conclusion

Mastering JavaScript ES6+ features is a must for any developer aiming to stand out in interviews and real-world projects. Whether you are managing asynchronous code, structuring clean objects, or utilizing modern syntax like optional chaining, being confident with these tools will make you a stronger and more efficient developer. Stay curious, keep practicing, and embrace modern JavaScript!

👉 Ready to test your skills?
Check Top 100 JavaScript Interview Questions and Answers (2025) and boost your preparation today!

Frequently Asked Questions (FAQ)

1. What are the top JavaScript ES6 features for interviews?

The most important ES6 features for interviews include arrow functions, let/const, promises, destructuring assignment, classes, modules (import/export), and template literals.

2. How important are Promises and Async/Await for JavaScript interviews?

Very important! Asynchronous programming concepts like Promises and Async/Await are heavily tested, especially for backend or full-stack JavaScript roles.

3. What is the difference between var, let, and const?

var is function-scoped and hoisted with undefined initialization. let and const are block-scoped, and const must be initialized and cannot be reassigned.

4. Why use optional chaining in JavaScript?

Optional chaining allows you to safely access nested object properties without throwing errors if a property is undefined. It improves code safety and readability.

5. Is learning ES2020+ features important for interviews?

Yes! Modern companies love candidates who are up-to-date with emerging features like Top-Level Await, Promise.allSettled(), and Private Class Fields. It shows adaptability and advanced knowledge.