What is a closure and why does it matter?
A closure is a function that remembers the variables from where it was created. It matters for data privacy, callbacks, and functional patterns like once and memoize, since anything that 'remembers' state across calls uses closures.
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More FAQs in JavaScript Closures, Event Loop, and Promises Interview Questions
Data privacy (encapsulating variables inside a factory function), callbacks like setTimeout, and patterns like once, memoize, and partial application. Anywhere you need to 'remember' state across calls without global variables uses closures.
How JavaScript handles async on a single thread. It processes the call stack, then microtasks like promises, then macrotasks like setTimeout and events. It matters because it explains the order async code runs.
Microtasks (promise callbacks) run after the current task and before the next macrotask. Macrotasks (setTimeout, events) run one per loop. This ordering is why promise callbacks run before setTimeout callbacks set at the same time.
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