Declarations and Types

In JavaScript, variable declaration has evolved significantly. Understanding the differences between var, let, and const—and how the engine handles memory for different types—is fundamental to technical excellence.

1. Declarations: var, let, and const

The Temporal Dead Zone (TDZ)

Unlike var, which is initialized as undefined when hoisted, let and const variables are not initialized. Any attempt to access them before their declaration results in a ReferenceError. This period is known as the Temporal Dead Zone.

2. Primitive vs. Reference Types

JavaScript divides its types into two categories based on how they are stored in memory.

I. Primitives (Stored by Value)

Stored directly on the Stack. They are immutable.

• Number, String, Boolean, Null, Undefined, Symbol, BigInt.

II. Reference Types (Stored by Reference)

The variable stores a pointer (on the stack) to the actual data stored on the Heap.

• Object, Array, Function, Date, RegExp.

3. The "Gotcha" of const with Objects

A common interview question: Can you change the properties of a const object?

• The Answer: Yes. const prevents Reassignment (changing the pointer), but it does not prevent Mutation of the underlying object in the heap.

const user = { name: "Alice" };
user.name = "Bob"; // Works
// user = { name: "Charlie" }; // Error: Assignment to constant variable

4. Dynamic and Weak Typing

JavaScript is Weakly Typed, meaning it performs implicit type conversion (coercion) when operators are used between different types.

console.log(1 + "2"); // "12" (Number is coerced to string)
console.log(5 - "2"); // 3 (String is coerced to number)

Interview Pro-Tips: typeof Quirks

• typeof null: Returns "object". This is a legacy bug in JavaScript that was never fixed to avoid breaking the web.
• typeof NaN: Returns "number". NaN (Not a Number!) is technically a numeric type used to represent undefined mathematical results.

Technical Summary

1. Memory: Primitives are copied by value; objects are copied by reference.
2. Immutability: Use Object.freeze() if you want to make an object truly immutable (shallowly).
3. Best Practice: Default to const. Use let only if you explicitly need to reassign a variable. Never use var in modern development.