Const in ReactJS Explained: What It Is and Why It Matters

Published November 5, 2025 | By admin

When writing modern React applications, you’ll frequently see developers using the const keyword in their code. At first glance, it might look like just another way to declare variables, but in practice, const plays an important role in maintaining clean, predictable, and bug-free React projects. In this article, we’ll break down what const really means, why it matters in React, and how you can use it effectively to write better code.

What Does const Really Mean in JavaScript?

Before we talk about React, it’s important to revisit what const means in JavaScript itself. Many developers mistakenly believe that const makes values completely unchangeable. The reality is a bit more subtle and worth understanding fully before applying it in React.

The Basics of const

Constant bindings, not values: const prevents reassignment of a variable’s identifier, but it does not make the actual value immutable.
Block-scoped: Variables declared with const are confined to the block in which they are declared, unlike var which is function-scoped.
Initialization required: You cannot declare a const variable without assigning a value.

Behavior with Different Data Types

Primitives (string, number, boolean, null, undefined, bigint, symbol): Once assigned, these cannot be changed.

const language = “JavaScript”;

// language = “TypeScript”; ❌ Error

Reference Types (objects, arrays, functions): The reference itself cannot be reassigned, but internal properties or items can be modified.

const user = { name: “Ana” };

user.name = “Maria”; // ✅ allowed

// user = { name: “Lia” }; ❌ not allowed

This distinction is especially important in React, where state and props often involve objects and arrays. Knowing that const doesn’t freeze the content helps avoid confusion when updating state immutably.

How const Differs from let and var

Here’s a comparison table to clarify:

Feature	var	let	const
Scope	Function	Block	Block
Hoisted	Yes (initialized as undefined)	No	No
Reassignment Allowed	Yes	Yes	No
Redeclaration Allowed	Yes	No	No
Initialization Required	No	No	Yes

Understanding these mechanics ensures React developers avoid subtle bugs that arise when using the wrong declaration keyword.

Why This Matters for React Developers

React relies heavily on immutability for state management. If you misunderstand const and mutate objects or arrays incorrectly, React may not recognize changes and fail to re-render. For example, mutating a state object declared with const directly will not trigger React’s reconciliation properly.

Key takeaway: const does not make values truly immutable—it locks the binding, not the content. In React, this distinction is crucial for working correctly with state and props.

Why const Is the Default Choice in Modern React Code

Now that we’ve covered the mechanics of const, let’s look at why modern React developers default to using it in most scenarios.

Emphasis on Predictability

React thrives on predictable state and component behavior. Declaring variables with const signals that the identifier will not change, which reduces confusion and unexpected reassignments in complex apps.

Readability and Clarity

When reading code, const immediately communicates intent. Another developer reviewing your code knows at a glance that the variable should remain fixed. This small signal has a large impact on teamwork, especially in large codebases where many contributors are working together.

ESLint and Style Guides

Popular linting tools and JavaScript style guides (such as Airbnb’s) recommend using const by default. ESLint even has a rule—prefer-const—that automatically flags variables that can be const but are declared with let. This enforces consistency and reduces bugs caused by unintended reassignment.

Comparison in React Context

Here’s how each keyword typically applies in React projects:

var: Almost never used. It introduces hoisting and function-scoping, which can cause unexpected bugs.
let: Occasionally used in loops or when the variable value must be reassigned (like iterating through arrays or toggling conditions inside a function).
const: The standard for declaring React components, hooks, utility functions, and most values that don’t require reassignment.

Example of Best Practice

// Recommended

const Header = () => <h1>Welcome</h1>;

// With hooks

const [theme, setTheme] = useState(“light”);

Using const for both the component and the state declaration makes the intent crystal clear.

Key takeaway: const is favored in React because it communicates intent, enforces predictability, and aligns with modern style guides that help maintain consistency in large projects.

Using const with React Components and Hooks

In React, const is deeply tied to how we write both components and hooks. Understanding this relationship ensures code that is both idiomatic and maintainable.

Functional Components

Most modern React apps are built with functional components instead of class components. By declaring these components with const, you ensure that the function reference itself cannot be reassigned:

const Navbar = () => {

return <nav>Home | About | Contact</nav>;

};

This not only makes the component definition consistent but also prevents accidental reassignments later in the file.

Hooks and State Management

Hooks like useState, useReducer, and useEffect are always declared with const:

const [count, setCount] = useState(0);

Even though count changes as the user interacts, the variable binding stays constant. React manages updates internally while const ensures the developer cannot reassign count directly, forcing updates through React’s state mechanism.

Utility Functions and Helpers

Often, developers define helper functions or constants alongside their components:

const formatCurrency = (value) => `$${value.toFixed(2)}`;

This makes the code easier to test, reuse, and understand.

Example: Combining Concepts

const Counter = () => {

const [count, setCount] = useState(0);

const increment = () => setCount(count + 1);

return (

<div>

<p>{count}</p>

</div>

);

};

Here, const is used consistently for the component, state variables, and helper functions. This pattern is standard across professional React projects.

Key takeaway: const is essential in React’s functional paradigm. It guarantees stability for components, hooks, and helpers while aligning with React’s declarative design.

Common Mistakes Developers Make with const in React

While const is powerful, misusing it can lead to confusion and bugs. Recognizing these pitfalls early makes code more reliable.

Misunderstanding Immutability

One of the biggest misconceptions is believing const makes objects or arrays immutable. In reality, you can still mutate them:

const arr = [1, 2];

arr.push(3); // ✅ Works

To achieve immutability, you must use spread syntax, Object.freeze(), or libraries like Immer.

Overusing const

Not every variable should be const. For example, loop counters work best with let:

for (let i = 0; i < 5; i++) {

console.log(i);

}

Using const here would be invalid since reassignment is required.

Incorrect State Updates

React relies on immutability for detecting changes. Mutating a state object—even if declared with const—does not notify React:

// ❌ Wrong

user.name = “Updated”;

// ✅ Correct

setUser({ …user, name: “Updated” });

Declaring Variables Too Early

Another mistake is declaring a const variable before knowing if it will be used, which clutters code and reduces readability.

Key takeaway: Misusing const often stems from misunderstanding its limits. Developers should remember that it prevents reassignment but not mutation, and apply it thoughtfully.

Best Practices: Writing Clean and Maintainable React Code with const

To maximize the benefits of const, developers should adopt practices that improve readability, maintainability, and alignment with React’s design.

Use const by Default

Start with const for all variables, only switching to let if reassignment is necessary. This reduces mental overhead and enforces stable code patterns.

Combine with Immutability Principles

Since const alone doesn’t guarantee immutability, pair it with immutable updates when managing state. For example, always use spread operators or immutability helpers when modifying arrays and objects.

const updatedTodos = […todos, newTodo];

Maintain Consistency in Components

Declare all components and hooks with const. This improves readability and ensures predictability across the codebase.

Document Intent with const

Using const communicates to your team that the variable should remain unchanged. This implicit documentation reduces errors when multiple developers work on the same project.

Best Practices Checklist

Default to const
Use let only when reassignment is required
Never use var in React code
Apply immutability patterns for objects and arrays
Keep const declarations close to where they’re used

Key takeaway: Adopting consistent const usage across a React project leads to cleaner, more maintainable codebases that scale well with teams and complexity.

Conclusion

The const keyword may seem small, but it has a big impact on how you write and maintain React applications. By understanding its true behavior, using it consistently, and avoiding common mistakes, you’ll write code that’s easier to debug, collaborate on, and scale. In short, const isn’t just a keyword—it’s a mindset that supports the declarative, predictable nature of React.

FAQs

Does const make objects immutable in React?

No. const prevents reassignment but doesn’t stop mutation of objects or arrays.

Can I use let instead of const in React?

Yes, but only when reassignment is required. By default, const is recommended.

Why shouldn’t I use var in React code?

Because var introduces function scope and hoisting, leading to unpredictable behavior.

Is const faster than let or var?

Performance differences are negligible. The choice is more about readability and predictability.

Should I declare React components with const?

Yes. Defining components as const ensures they can’t be accidentally reassigned, improving code clarity.