JS Const

JavaScript const: Constant Bindings and Object Mutation

The const keyword declares a block-scoped variable whose binding cannot be reassigned after initialization. You must assign a value to a const variable on the exact line you declare it.

Binding Immutability vs. Value Mutation

Many learners think const makes an object or array completely read-only. In reality, const locks the pointer connecting the variable name to its value in memory.

• Primitives (number, string, boolean): Cannot be reassigned or mutated, because primitive values are immutable.
• Objects and Arrays: You cannot point the const variable to a brand-new object, yet you can still update properties or push items inside it.

Practical const Example with Object.freeze()

If you want to prevent accidental property changes on a configuration object, combine const with Object.freeze().

const apiConfig = {
  baseUrl: "https://api.example.com",
  timeoutMs: 3000,
};

// Allowed: mutating a property inside a const object
apiConfig.timeoutMs = 5000;
console.log("Updated timeout:", apiConfig.timeoutMs);

// True shallow immutability with Object.freeze
const roles = Object.freeze({ ADMIN: "admin", EDITOR: "editor" });
console.log("Role:", roles.ADMIN);

When to Use const

Use const for configuration constants, imported modules, function expressions, objects, arrays, and any computed value you do not replace. Defaulting to const makes your codebase easier to reason about because bindings stay stable.

Common Mistakes to Avoid

• Declaring const without a value: Writing const taxRate; throws a SyntaxError immediately. Always initialize on the same line.
• Reassigning a const variable: Trying maxRetries = 5 on const maxRetries = 3 throws a TypeError: Assignment to constant variable.
• Assuming Object.freeze() is deep: Object.freeze(user) locks top-level keys only; nested objects inside user.address can still change unless frozen recursively.

Frequently Asked Questions

Can I push items into an array declared with const?

Yes, const tags = ["js"]; tags.push("es6"); works because tags still points to the exact same array in memory. Only tags = ["node"] is blocked.

Does const improve JavaScript performance?

Modern engines optimize both let and const well, so speed is nearly identical. The main benefit of const is preventing accidental reassignment bugs for humans.

How do I make a const object deeply immutable?

You can clone with structuredClone() when updating state, or recursively call Object.freeze() on nested objects in strict mode.