What are the most important JavaScript concepts every developer should know?
A handful of core concepts underpin most JavaScript work:
Scope & closures — how variables live and are remembered.
this & prototypes — context and inheritance.
The event loop — how async code is scheduled.
Promises & async/await — managing asynchronous flow.
Types & coercion — value vs reference, truthy/falsy.
Master these and most language quirks and patterns start to make sense.
This is often asked as 'what makes a strong JS developer?' Name 5-7 concepts with one sentence explaining each. Closures, event loop, and prototypes are the 'big three' that separate beginners from intermediates.
Showing depth in any of these topics is more impressive than listing many.