What are the most important JavaScript features every developer should know?

A few features every developer should be comfortable with:

let/const & block scope — modern variable declarations.
Arrow functions & this — concise syntax and lexical context.
Destructuring, spread & rest — clean data handling.
Template literals — readable string building.
Promises & async/await — managing asynchronous code.
Modules — organising code with import/export.

Together these define modern (ES6+) JavaScript — knowing them well covers the vast majority of day-to-day work.

When asked 'what makes a strong JS developer,' list 5-7 features with one-sentence explanations. Closures, event loop, and prototypes/this are the 'big three' that separate levels.

Showing depth in any one topic is better than listing many shallowly.