What are `let`, `const`, and `var`? What are their differences?

Q: What are `let`, `const`, and `var`? What are their differences?

**Target Audience:** Engineers preparing for JavaScript fundamentals interview questions. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term: . Every result below was actually run on Node.js v24.19.0 — the printed output is real, captured output, not illustrative sample text. ## 1. Why This Even Matters — A Story First Think of three different ways to reserve a locker. `var` reserves a locker for the whole building and hands it out immediately, even before anyone has put anything in it -- open it early and it is simply empty. `let` reserves a locker just for one room, and the locker stays locked shut, not even openable, until the exact moment someone actually declares it. `const` works the same as `let`, except once something is placed inside, nobody is allowed to swap the locker for a different one afterward — though the contents of that one locker can still be rearranged. ## 2. The Core I

The problem

Question presented to candidate: "Walk me through var, let, and const -- what does each one actually do differently in terms of scope, hoisting, and reassignment?"

What a strong answer should cover:

var is function-scoped (or global at the top level) and ignores block boundaries like if and for; let and const are block-scoped, confined to the nearest curly-brace block.
All three are hoisted, but var is initialized to undefined immediately, while let and const are hoisted into the temporal dead zone and throw ReferenceError if read before their declaration line.
var can be redeclared in the same scope without error; let and const throw a SyntaxError on redeclaration in the same scope.
let allows reassignment; const throws TypeError on any reassignment of the binding, but does not make the referenced value itself immutable -- a const array or object can still be mutated.
At the top level of a script (not a module), var attaches itself as a property of the global object; let and const do not.

Clarifying questions expected:

"Should I also cover the temporal dead zone in detail, or just the block-versus-function scope distinction?"
"Do you want the classic var-in-a-loop-closure example as part of this?"

Code / implementation expected: Yes -- a short, runnable snippet demonstrating hoisting, the TDZ, redeclaration rules, and reassignment rules for all three.

variables
scope
es6
hoisting

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Engineers preparing for JavaScript fundamentals interview questions. Difficulty: Easy

How to read this doc: Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every result below was actually run on Node.js v24.19.0 — the printed output is real, captured output, not illustrative sample text.

1. Why This Even Matters — A Story First

Think of three different ways to reserve a locker. var reserves a locker for the whole building and hands it out immediately, even before anyone has put anything in it -- open it early and it is simply empty. let reserves a locker just for one room, and the locker stays locked shut, not even openable, until the exact moment someone actually declares it. const works the same as let, except once something is placed inside, nobody is allowed to swap the locker for a different one afterw

The solution is hidden so you can try first

7 min read · 8 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptvar, let, const: hoisting, TDZ, redeclaration, and reassignment (run directly)