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A variable is a labeled box that holds a value. JavaScript has three keywords for declaring variables: var, let, and const. The decision tree for modern JavaScript is simple:

• Default to const. Most variables don't need to change after their first assignment.
• Use let only when you'll reassign. A loop counter, an accumulating value, a flag.
• Avoid var. Its scoping rules cause real bugs. We're not skipping var because it's hard — we're skipping it because modern code doesn't use it.

Declaring with const and let

The = is assignment, not equality. Read it left-to-right: "the name on the left now refers to the value on the right."

const protects the BINDING, not the value

A common confusion: const doesn't make the value immutable, just the variable name. You still can't reassign the variable, but if it points at an object or array, you can mutate that object's contents:

The constant part is the reference, not the contents. To get true immutability, use Object.freeze() or libraries.

Block scope

Both const and let are block-scoped — they exist only within the { ... } block where they were declared:

This is what you'd expect from most languages. It prevents leaks and accidental clashes.

Why we avoid var

var is FUNCTION-scoped, not block-scoped. It also gets "hoisted" — accessible anywhere in the function before it's even declared:

That's a footgun. Modern code avoids it.

Naming rules

• Letters, digits, _, $. Can't start with a digit.
• Case-sensitive: name and Name are different.
• Reserved words can't be names: if, return, class, etc.

Convention: camelCase for variables and functions, PascalCase for classes, UPPER_SNAKE_CASE for true constants:

Common mistakes

• Using == (loose equality) when checking values — use ===.
• Mistaking const for deep immutability. It only locks the binding.
• Reaching for let when const would do — if you don't reassign, prefer const.
• Using var because old tutorials still mention it.