🔥 JavaScript Mastery Course (2026) | Modern JavaScript ES6+ from Beginner to Advanced - JavaScript Substring Secrets: includes, startsWith, & endsWith Explained (#89)

Stop writing complex loops just to find a word in a sentence! JavaScript provides three highly readable methods to check for substrings: .includes(), .startsWith(), and .endsWith(). In this tutorial, we explain how these methods return true or false and how to handle case sensitivity.👉 We’ll build a real-world Email Validator and a Product Search Filter to show these methods in action. What you will learn: ✔️ The Big Three: How to check for content at the beginning, end, or anywhere in a string. ✔️ The Case Trap: Why "Hello" and "hello" aren't the same and how to fix it with .toLowerCase(). ✔️ Optional Positioning: Using the second argument to choose where the search starts. ✔️ Practical Case Study 1: Creating a function to validate emails without complex Regex. ✔️ Practical Case Study 2: Building a dynamic search filter for an array of product objects. ✔️ Logic Chaining: Using the ! (NOT) operator to create strict validation rules. 🕛 Timelines: 0:00 Introduction to Boolean String Methods 0:35 Master the .includes() Method 2:45 Case Sensitivity & Case-Insensitive Workarounds 4:15 The .startsWith() Method: Checking the Beginning 6:03 The .endsWith() Method: Checking the Tail 8:20 Practical Project 1: Building an Email Validator 10:00 Logical Refactoring: Cleaning up "if" statements 13:35 Practical Project 2: Filtering Products by Name 15:35 Combining .filter() with .includes() 19:25 Summary and Best Practices 💡 The "2026 Best Practice" Tip As noted in the lecture, these methods are built-in "sugar" for .indexOf(). While .indexOf() !== -1 still works, using .includes() makes your code much easier for other developers to read. In 2026, clean code is fast code—always prioritize readability when performance is comparable! 💡 Peer Tip: Remember that these methods work on Strings, but .includes() also works on Arrays! If you have a list of banned usernames, you can check bannedUsers.includes(userInput) just as easily as checking a single string. Expert Guide Question: If you were building a search bar that shows results as the user types, would you use .includes() or .startsWith() to provide the most helpful experience? Why?