All The JavaScript You Need To Know For React
Written by Massa Medi
If you've ever found yourself wondering "Do I know enough JavaScript to start building with React?"—or maybe you've already dipped your toes into React but find yourself floundering—you're in exactly the right place. Whether you're a self-taught coder sprinting through projects or the methodical type who wants a checklist of everything necessary, this article pulls back the curtain on every JavaScript concept you need to know before learning React. Our mission? To make your React journey as smooth and painless as humanly possible.
Welcome to Web Dev Simplified: Where the Web Gets Easy
Hey there! I’m Kyle from Web Dev Simplified, and my passion is simplifying web development so that you can get to building your dream projects—sooner rather than later.
Before we jump in, if you’re itching for a quick and complete answer, you’re in luck. In the description of the original video (and here, as a reference), you’ll find a link to the Table of Contents for my JavaScript Simplified course. Head over to the beginner section—every single video and topic there is foundational for React. And don’t skip the first section of the advanced course; those concepts are crucial prerequisites, too. This list is constantly updated to stay current, making it the ideal checklist before you start building with React.
Why JavaScript Basics Are Non-Negotiable
Many people get excited learning JavaScript: you master the basics, build fun projects, and before long, you’re diving into evermore complex applications. And while that momentum is fantastic, it can mean that the core fundamentals get glossed over. You might “get” how certain things work, but not grasp why they work that way. When you leap into React, this knowledge gap can become a stumbling block—because many things you take for granted in vanilla JavaScript show up in React’s own unique ways.
If you don’t fully understand why things behave as they do in JavaScript, expect confusion once you start tackling React projects.
The JavaScript Concepts Every Aspiring React Developer Must Know
1. Scoping: Where Do Your Variables Live?
Variable scoping determines where your variables exist—can they be seen from everywhere in your code, or only in certain blocks? Failing to understand this will quickly lead to bugs in React, where functions, components, and hooks often depend on precise scoping.
2. Callbacks & Passing Functions Around
You may think you understand callbacks—after all, you’ve probably passed functions to setTimeouts or event listeners. But here’s the rub: unless you can explain how and why functions are passed to other functions, and exactly what’s happening under the hood, you’re bound to run into tricky issues when using event handlers in React.
Hot tip: Misusing functions in event listeners is the most common mistake among new React developers!
3. Value vs. Reference: The Object Conundrum
It’s easy to assume that declaring const person = { name: 'Kyle' }; means the value is locked—but you can still do person.name = 'Sally';. Why? Because objects and arrays are passed by reference in JavaScript (their variables refer to a location in memory, not a fixed value). By contrast, primitive types like strings, numbers, and booleans are passed by value.
In React, this distinction is not just theoretical: hooks, state management, and component updates all hinge on understanding references versus values. If you skip this, expect elusive bugs that seem impossible to fix!
Want an in-depth dive? Check out my dedicated video on reference vs. value (linked in the original video’s cards and description).
4. Double vs. Triple Equals & Type Coercion
Ever wondered about == versus ===? The double equals operates with type coercion (i.e., it tries to convert types to match), whereas triple equals insists types are identical. Since React often relies on strict comparisons, knowing how and when JavaScript coerces types is key to reading and debugging React code.
5. Advanced Logic: Short Circuiting & Conditional Rendering
React code regularly uses “short circuit” logic; for example, isActive && <Component /> will only render <Component /> if isActive is truthy. Understanding this pattern—and the broader mechanics of boolean logic (ANDs, ORs, and evaluation orders)—will help React code look logical instead of alien.
6. Mastering Advanced Array Methods
React structures often process and display data using methods like map, filter, and sort. For example, listing todos involves mapping over an array of todo objects and rendering a component for each. Know exactly which array methods mutate the data and which return new arrays; React relies heavily on this to manage state without unwanted side-effects.
7. Immutability: Crafting Unchangeable State
Immutability is at the heart of React. You’ll see frequent warnings against mutating state directly—because React’s update mechanisms depend on detecting changes in immutable structures. If you’re shaky on the idea of mutability versus immutability, React’s update model will mystify you.
8. Asynchronous Code: Callbacks, Promises & Async/Await
Modern web apps, React included, constantly fetch or compute things in the background. Understand asynchronous code flow—especially how callbacks, Promises, and async/await work. You’ll encounter these all the time in React, from fetching API data with useEffect to handling user input.
9. JavaScript Modules: Import, Export, and Organization
Success with React often comes down to the ability to break your code into manageable components and modules. Know how import and export work, both for installing Node modules and for organizing your own code. Don’t fall for the myth that module systems are “React-only”—they’re standard JavaScript!
10. ES6 Features in the Wild
React makes heavy use of ES6 enhancements—think template literals (backticks), destructuring, the spread operator (...), and arrow functions. Mastering these means less confusion (“is this React magic?”) and more confidence (“oh, that’s just modern JS”).
The Big Idea: Almost everything in React is just regular JavaScript in a new context. The more fluent you are in JavaScript, the faster you’ll pick up React. Instead of being bogged down by both JS and React quirks, you’ll only need to absorb a handful of React-specific patterns.
A Resource Checklist: What to Study, and Where
For a practical, in-depth checklist of the exact topics to cover, check the Table of Contents for my JavaScript Simplified course. You don’t need to buy anything—just reviewing the list ensures you won’t overlook a critical concept. But if you want a comprehensive, guided learning experience, the course itself is designed to take you from beginner to ready-for-React, step by step.
Final Thoughts: Make Your Transition to React Effortless
React is an incredible tool, but its real power depends on a rock-solid JavaScript foundation. Invest time understanding these essential concepts, and you won’t just breeze through React—you’ll build better apps, faster, and with fewer headaches.
Thanks for reading, and best of luck on your JavaScript and React journey!
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