Languages · INTERVIEW GUIDE

JavaScript & TypeScript

Scope, closures, async behavior, object identity, and the type system.

Scope & execution7 Functions & objects5 Async JavaScript4 Runtime & modules3 TypeScript6 Output prediction2

2 questions

What is the output here?

JavaScript & TypeScript·Output prediction

Output prediction requires following the order in which synchronous code and queued callbacks can execute. This example combines the current script, a promise microtask and a timer callback. A zero-delay timer becomes eligible later; it does not interrupt the running script.

Intermediate2 min read

Why does a var loop print 3 three times?

JavaScript & TypeScript·Output prediction

The var loop creates one shared function-scoped binding for i. Every callback closes over that same binding, and the loop has already set it to 3 before the timer callbacks run. Using let in the loop header creates a separate binding for each iteration.

Intermediate2 min read

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