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var is function-scoped and can be redeclared, let is block-scoped and can be reassigned, const is block-scoped and cannot be reassigned after initialization.

JavaScript has 7 primitive data types: string, number, bigint, boolean, undefined, null, and symbol.

map() transforms each element and returns a new array of the same length, filter() returns elements that pass a test, and reduce() accumulates array elements into a single value.

== performs type coercion before comparison (loose equality), while === compares both value and type without coercion (strict equality).

Arrow functions have a shorter syntax, don't have their own 'this' binding (they inherit from the enclosing scope), cannot be used as constructors, and don't have access to the 'arguments' object.

A closure is a function that has access to variables from its outer (enclosing) scope, even after the outer function has returned. They're useful for data privacy, state management, and creating function factories.

A Promise is an object representing the eventual completion or failure of an asynchronous operation. It has three states: pending, fulfilled, or rejected, and provides .then(), .catch(), and .finally() methods for handling results.

async/await is syntactic sugar over Promises. An async function always returns a Promise, and await pauses execution until a Promise resolves, making asynchronous code look and behave more like synchronous code.

The Event Loop is JavaScript's mechanism for handling asynchronous operations. It continuously checks the call stack and task queues, executing callbacks from the microtask queue (Promises) before the macrotask queue (setTimeout, events).

The value of 'this' depends on how a function is called: in a method, it refers to the object; in a regular function, it's the global object (or undefined in strict mode); in arrow functions, it's lexically inherited from the enclosing scope.