JS Const

JavaScript const: Constant Bindings & Object Mutation

The const keyword declares block-scoped constants. A const variable creates an immutable binding to a value, meaning the variable identifier cannot be reassigned. However, the value itself is NOT immutable if it holds an object or array.

Reassignment vs Mutation Matrix

flowchart TD
    ConstDecl["const declaration"] --> TypeCheck{"Primitive or Reference?"}
    TypeCheck -- "Primitive (Number, String, Boolean)" --> Prim["Value is completely immutable"]
    TypeCheck -- "Reference (Object, Array)" --> Ref["Binding is fixed, but properties/elements remain mutable"]
    Ref --> Freeze["Use Object.freeze() for deep immutability"]

const Behavioral Matrix

Practical Code Example

// Demonstrating const primitive binding vs object mutation

// 1. Primitive const binding
const APP_VERSION = "2.4.0";
// APP_VERSION = "2.5.0"; // Throws TypeError: Assignment to constant variable.

// 2. Mutable Array under const
const categories = ["Frontend", "Backend"];
categories.push("DevOps"); // Allowed: mutating contents of the reference
console.log("Mutated categories array:", categories);

// 3. Mutable Object under const
const dbConfig = {
  host: "localhost",
  port: 5432
};
dbConfig.port = 5433; // Allowed: mutating property value
console.log("Updated dbConfig:", dbConfig);

// 4. Truly Immutable Object using Object.freeze()
const immutableConfig = Object.freeze({
  apiKey: "SECRET_KEY_12345",
  environment: "production"
});

// In non-strict mode, this silently fails; in strict mode ('use strict'), throws TypeError
immutableConfig.environment = "staging"; 
console.log("Immutable Config (Environment unchanged):", immutableConfig.environment);

Best Practices & Gotchas

• Default Choice for All Declarations: Declare every variable with const by default unless you intend to reassign it later with let.
• Must Initialize on Declaration: const x; is invalid syntax and triggers a SyntaxError: Missing initializer in const declaration.
• Use Object.freeze() for Read-Only Constants: If an object constant must never be mutated by external code, wrap it in Object.freeze().

Self-Check Challenge

What happens when you execute const colors = ["red", "blue"]; colors[0] = "green"; vs colors = ["green", "blue"];?