JS Array Const

JavaScript Array const: Reference Binding & Immutability

Declaring an array with const prevents reassignment of the array identifier variable, but it does NOT freeze the elements contained within the array instance.

Array const Binding Rules

flowchart TD
    ConstArr["const list = [1, 2, 3]"] --> ReassignAttempt{"Attempt list = [4, 5]?"}
    ReassignAttempt -- "Yes" --> Err["TypeError: Assignment to constant variable"]
    
    ConstArr --> MutateAttempt{"Attempt list.push(4) or list[0] = 99?"}
    MutateAttempt -- "Yes" --> Allowed["Operation Allowed! Array reference is fixed, content is mutable."]

Mutability Matrix

Practical Code Example

// Demonstrating const array reference behavior and Object.freeze

// 1. Const Array Mutation Allowed
const numbers = [10, 20, 30];
numbers.push(40);     // Allowed
numbers[0] = 99;      // Allowed
console.log("Mutated const numbers array:", numbers); // [99, 20, 30, 40]

// 2. Reassignment Throws Error
try {
  // numbers = [1, 2, 3]; // Un-commenting throws TypeError
} catch (err) {
  console.error(err.message);
}

// 3. True Immutability with Object.freeze()
const immutableList = Object.freeze([1, 2, 3]);

// In non-strict mode mutates silently fail; in strict mode throws TypeError
// immutableList.push(4); // Throws TypeError: Cannot add property 3, object is not extensible

console.log("Frozen List:", immutableList);

Best Practices & Gotchas

• Always Declare Arrays with const: Using const for arrays prevents accidental variable overwrites while allowing necessary element operations.
• Object.freeze() is Shallow: Object.freeze() prevents adding, deleting, or editing top-level array elements, but objects nested inside the frozen array remain mutable unless recursively frozen.
• Use Spread Operator for Copying: To make an edited copy without mutating the original const array, use spread syntax: const updated = [...original, newItem].

Self-Check Challenge

Explain the difference between const arr = [1, 2]; arr.push(3); and let arr = [1, 2]; arr = [...arr, 3];.