JavaScript fundamentals checklist for frontend interviews

Interview framing: This is usually a breadth question. Give a structured checklist and show that you can connect language rules to real bugs.

Core topics:

1. Scope and closures:

• let/const are block-scoped; var is function-scoped.
• A closure lets a function remember variables from its outer scope.
• Common use cases: event handlers, memoization, debounce, private state.

2. Hoisting and TDZ:

• Function declarations are callable before their declaration.
• var is hoisted and initialized to undefined.
• let/const are hoisted but unavailable until initialized (temporal dead zone).

3. Equality and coercion:

• Prefer === because it avoids implicit coercion.
• Know common surprises: [] == false, '0' == 0, null == undefined.

4. this binding:

• Determined by call site: default, implicit, explicit call/apply/bind, or new.
• Arrow functions do not have their own this; they capture lexical this.

5. Prototypes and classes:

• Classes are syntax over prototype-based inheritance.
• Property lookup walks the prototype chain.

6. Async model:

• Synchronous code runs first.
• Promise callbacks run as microtasks.
• Timers, events, and network callbacks are macrotasks.

7. Common frontend pitfalls:

• Mutating shared objects accidentally.
• Missing return in promise chains.
• Forgetting cleanup for timers/listeners.
• Relying on stale closure values in callbacks.

Strong closing sentence: "I try to explain JS fundamentals by tying them to production bugs: stale state, wrong this, race conditions, memory leaks, and unexpected async ordering."