JavaScript

JavaScript runs one call stack per thread, then coordinates asynchronous work through queues.

• Synchronous code runs first on the call stack.
• Promise callbacks run in the microtask queue before timer callbacks in the macrotask queue.
• Long CPU work blocks rendering and I/O callbacks, so split work or move it to a worker.

A closure lets a function remember variables from the scope where it was created.

• Closures power function factories, memoization, modules, callbacks, and private state.
• They can also accidentally retain memory if long-lived callbacks capture large objects.
• In loops, let creates a new binding per iteration while var shares a function-scoped binding.

JavaScript inheritance is prototype-based; class syntax is a cleaner layer on top of prototypes.

• Property lookup checks the object first, then climbs the prototype chain.
• Arrow functions do not bind their own this, arguments, or super.
• Use composition when inheritance makes behavior harder to reason about.