Compatibility Guide

Common ES6 Features Not Supported in Older Browsers

Selman Sahbudak
8 min read

Writing modern JavaScript is great — it's cleaner, shorter, and more powerful. But not every browser understands the latest ES6 syntax. Features like let, const, arrow functions, and Promise can easily break older browsers like Internet Explorer or outdated Android WebViews. In this article, we'll explore the most common ES6 features that cause compatibility issues, why they matter, and how you can make your code run everywhere.

⚠️ Legacy Browser Alert

If your website needs to support Internet Explorer 11, older Android browsers (pre-2018), or corporate environments with strict browser policies, the ES6 features below will cause JavaScript errors.

Solution: Convert your ES6 code to ES5 using our free online transpiler.

🏹 1. Arrow Functions

Arrow functions are one of the most popular ES6 features, providing a shorter syntax for function expressions. However, they're completely unsupported in IE11 and older browsers.

❌ ES6 (Not Supported in IE11)

// Arrow functions
const add = (a, b) => a + b;

const users = ['Alice', 'Bob', 'Charlie'];
const upperNames = users.map(name => name.toUpperCase());

// Arrow function with this binding
const obj = {
  name: 'Example',
  greet: () => {
    console.log(`Hello, ${this.name}`);
  }
};

✅ ES5 (Works Everywhere)

// Regular functions
var add = function(a, b) { 
  return a + b; 
};

var users = ['Alice', 'Bob', 'Charlie'];
var upperNames = users.map(function(name) {
  return name.toUpperCase();
});

// Regular function with proper this binding
var obj = {
  name: 'Example',
  greet: function() {
    console.log('Hello, ' + this.name);
  }
};

⚠️ Behavior Note: Arrow functions behave differently with this binding — especially around scope — so always test carefully after conversion.

📦 2. Let & Const Variables

let and const provide block-scoped variables, which is much safer than var. Unfortunately, they don't exist in ES5, so all variables must use var.

❌ ES6 (Block-scoped)

// Block-scoped variables
const API_URL = 'https://api.example.com';
let count = 0;

function processItems(items) {
  for (let i = 0; i < items.length; i++) {
    const item = items[i];
    let result = processItem(item);
    count++;
  }
  // i and item are not accessible here
}

// const prevents reassignment
const config = { debug: true };
// config = {}; // TypeError!

✅ ES5 (Function-scoped)

// Function-scoped variables
var API_URL = 'https://api.example.com';
var count = 0;

function processItems(items) {
  for (var i = 0; i < items.length; i++) {
    var item = items[i];
    var result = processItem(item);
    count++;
  }
  // i and item are still accessible here!
}

// var allows reassignment
var config = { debug: true };
config = {}; // Works, but dangerous

⚠️ Important: Behavior is slightly different — especially around scope — so always test carefully when converting let/const to var.

💡 Pro Tip: Converting these manually is error-prone — automated tools like our ES6-to-ES5 converter handle it better and safer.

🎭 3. Template Literals

Template literals with backticks make string interpolation elegant and multiline strings easy. In ES5, you must use string concatenation instead.

❌ ES6 (Template Literals)

// Template literals with interpolation
const name = 'John';
const age = 30;
const message = `Hello, ${name}! 
You are ${age} years old.
Today is ${new Date().toDateString()}.`;

// Tagged template literals
function highlight(strings, ...values) {
  return strings.reduce((result, string, i) => 
    result + string + (values[i] ? `${values[i]}` : ''), 
  '');
}

const highlighted = highlight`Name: ${name}, Age: ${age}`;

✅ ES5 (String Concatenation)

// String concatenation
var name = 'John';
var age = 30;
var message = 'Hello, ' + name + '! \n' + 
              'You are ' + age + ' years old.\n' +
              'Today is ' + new Date().toDateString() + '.';

// Manual string building
function highlight(template, name, age) {
  return 'Name: ' + name + ', Age: ' + age + '';
}

var highlighted = highlight(null, name, age);

🔁 Converting templates manually is error-prone — automated tools handle string interpolation, escaping, and multiline conversions much better and safer.

🏗️ 4. Classes and Modules

ES6 classes provide a cleaner syntax for object-oriented programming, while modules enable better code organization. Both need significant transformation for ES5 compatibility.

❌ ES6 (Classes & Modules)

// ES6 Classes
class Vehicle {
  constructor(type) {
    this.type = type;
  }
  
  start() {
    console.log(`${this.type} started`);
  }
}

class Car extends Vehicle {
  constructor(brand) {
    super('car');
    this.brand = brand;
  }
  
  honk() {
    console.log(`${this.brand} car honks!`);
  }
}

// ES6 Modules
import { helper } from './utils.js';
export default Car;

✅ ES5 (Prototypes & Global/IIFE)

// ES5 Constructor Functions
function Vehicle(type) {
  this.type = type;
}

Vehicle.prototype.start = function() {
  console.log(this.type + ' started');
};

function Car(brand) {
  Vehicle.call(this, 'car');
  this.brand = brand;
}

// Inheritance setup
Car.prototype = Object.create(Vehicle.prototype);
Car.prototype.constructor = Car;

Car.prototype.honk = function() {
  console.log(this.brand + ' car honks!');
};

// Module pattern (IIFE)
window.Car = Car;

⏳ 5. Promises and Async/Await

Promises revolutionized asynchronous JavaScript, and async/await made it even cleaner. However, these don't exist in ES5 and require polyfills or callback-based alternatives.

❌ ES6 (Promises & Async/Await)

// Promises
function fetchUserData(id) {
  return fetch(`/api/users/${id}`)
    .then(response => response.json())
    .catch(error => {
      console.error('Error:', error);
      throw error;
    });
}

// Async/Await
async function getUserProfile(id) {
  try {
    const user = await fetchUserData(id);
    const posts = await fetch(`/api/users/${id}/posts`);
    return { user, posts: await posts.json() };
  } catch (error) {
    console.error('Failed to load profile:', error);
  }
}

✅ ES5 (Callbacks)

// Callback-based approach
function fetchUserData(id, callback, errorCallback) {
  var xhr = new XMLHttpRequest();
  xhr.open('GET', '/api/users/' + id);
  xhr.onload = function() {
    if (xhr.status === 200) {
      callback(JSON.parse(xhr.responseText));
    } else {
      errorCallback(new Error('Request failed'));
    }
  };
  xhr.onerror = errorCallback;
  xhr.send();
}

// Nested callbacks (callback hell)
function getUserProfile(id, callback, errorCallback) {
  fetchUserData(id, function(user) {
    fetchUserPosts(id, function(posts) {
      callback({ user: user, posts: posts });
    }, errorCallback);
  }, errorCallback);
}

🚨 Complex Conversion: Promises and async/await require significant restructuring. Consider using a Promise polyfill for better ES5 compatibility.

📊 Browser Support Summary

ES6 Feature IE11 Old Android Conversion Difficulty
Arrow Functions ❌ No ❌ No 🟢 Easy
let/const ❌ No ❌ No 🟡 Medium
Template Literals ❌ No ❌ No 🟢 Easy
Classes ❌ No ❌ No 🟡 Medium
Promises ❌ No ❌ No 🔴 Hard
Modules (import/export) ❌ No ❌ No 🔴 Hard

🛠️ Solution: ES6-to-ES5 Conversion Tool

Rather than manually converting each feature, you can use our free online ES6 to ES5 converter to automatically transform your modern JavaScript into legacy-compatible code.

🧰 Solution: Convert ES6 to ES5 Automatically

Instead of rewriting your code manually, use our simple browser-based converter like 👉 ES6-to-ES5 Converter.

**Benefits:**

  • ⚡ Converts instantly, right in your browser
  • 🔒 No data stored or uploaded
  • 🧩 Perfect for quick compatibility checks
  • 💻 Ideal for debugging older browser issues
  • 🔧 Includes minification options
  • 🚀 Instant results with copy functionality

This way, you can write modern JavaScript while ensuring your code runs everywhere — even in legacy systems.

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🎯 Best Practices for Legacy Support

✅ Do This

  • • Always test converted code in target browsers
  • • Use feature detection before polyfills
  • • Keep original ES6 code for development
  • • Document browser support requirements
  • • Use progressive enhancement strategies

❌ Avoid This

  • • Don't assume all ES6 converts perfectly
  • • Don't ignore scoping differences with let/const
  • • Don't forget about this binding in arrow functions
  • • Don't mix converted and unconverted code
  • • Don't skip testing in real legacy browsers

🔮 Looking Forward: When Can You Drop ES5?

The decision to stop supporting ES5 depends on your audience and requirements. Here's what to consider:

Browser Usage Statistics (2025)

Internet Explorer 11: < 1% globally
Chrome/Edge/Firefox (modern): > 95% globally
Corporate environments: Varies significantly

💡 Recommendation: For most new projects in 2025, you can safely use ES6+. For existing enterprise applications or maximum compatibility, continue using ES5 conversion.

🔍 Final Thoughts

ES6 brought a wave of improvements to JavaScript, making it cleaner and more powerful. But older browsers still lag behind. Understanding what's unsupported helps you decide when to transpile your code and how to maintain cross-browser compatibility.

In 2025, many developers still rely on ES5 builds for maximum coverage. The features we covered — arrow functions, let/const, template literals, classes, and promises — are the most common compatibility roadblocks you'll face.

The good news? You don't have to choose between modern development and legacy support. Write beautiful ES6 code, then use tools like ES6-to-ES5 Converter to make that process effortless.

Ready to Convert Your ES6 Code?

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