var is function-scoped and can be redeclared; let and const are block-scoped. const can't be reassigned after declaration, while let can.

Every element is a box made of content, padding, border, and margin, from inside out.

A closure is a function that retains access to variables from its outer scope even after that outer function has returned.

INNER JOIN returns only matching rows in both tables; LEFT JOIN returns all rows from the left table plus matches from the right.

The virtual DOM is a lightweight in-memory representation of the UI. React diffs it against the previous version and only updates the real DOM where necessary, minimizing expensive DOM operations.

Use a join table with foreign keys referencing both related tables' primary keys.

Authentication verifies who a user is (login); authorization determines what that authenticated user is allowed to do.

Use EXPLAIN ANALYZE to see the query plan, add appropriate indexes, consider caching, and check for N+1 query patterns.

DNS lookup resolves the domain to an IP, browser opens a TCP/TLS connection, sends an HTTP request, server responds, browser parses HTML/CSS/JS and renders the page.

This is a behavioral question — answer with a real example from your own projects, following a structure like: the bug, your debugging process, the root cause, and the fix.