Difference between `var`, `let`, and `const`

`var` is function-scoped and hoisted to `undefined`. `let`/`const` are block-scoped with a temporal dead zone. `const` forbids reassignment but the value can still be mutated.

5 min read·~8 min to think through

Three axes to compare:

	`var`	`let`	`const`
Scope	function	block	block
Hoisting	hoisted, initialized to `undefined`	hoisted, but in TDZ until declaration	same as let
Reassign	yes	yes	no
Redeclare in same scope	yes (silently)	no (SyntaxError)	no

The Temporal Dead Zone (TDZ) is the window between entering a block and the actual let/const line — accessing the binding throws ReferenceError. This catches typos that var would silently let through.

const blocks reassignment of the binding, not mutation of the value. const arr = []; arr.push(1) is fine; const arr = []; arr = [1] is not.

Default to const. Use let only when you actually reassign. Reach for var essentially never in modern code — the only legitimate case is hot-patching legacy scripts that rely on its hoisting.

Code

Follow-up questions

•What is the temporal dead zone, and why does it exist?
•How does hoisting differ between function declarations and var?
•Why does using let in a for loop fix the classic closure-in-loop bug?

Common mistakes

•Thinking `const` makes the value immutable — it doesn't.
•Using `var` in a `for` loop and being surprised that callbacks all see the final value.
•Believing TDZ means the variable isn't hoisted — it is, but accessing it throws.

Performance considerations

•TDZ checks have negligible runtime cost; modern engines elide them after the first read.
•Block scoping enables tighter escape analysis and smaller closure environments — generally a win.

Edge cases

•`typeof` on a TDZ binding still throws (unlike on a truly undeclared identifier, where it returns 'undefined').
•`var` declarations at the top of a module bind to the module scope, not the global object — unlike scripts.

Real-world examples

•ESLint's `prefer-const` codifies the 'default to const' rule across most modern codebases.
•TypeScript narrowing works better with `const` because the binding can't change between checks.

Senior engineer discussion

At senior level you should be able to talk about how V8 represents these in its environment records, why `let` in a for-loop creates a per-iteration binding (a spec choice for closure correctness), and how this interacts with `for..in` / `for..of` iteration order.