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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.

Hoisting moves variable and function declarations to the top of their scope during compilation. var declarations are initialized as undefined, while let/const enter a Temporal Dead Zone until their declaration is reached.

Event delegation is a pattern where you attach a single event listener to a parent element instead of individual listeners on each child. It works because events bubble up through the DOM, and you can check event.target to determine which child was clicked.

Type coercion is JavaScript's automatic conversion of values from one type to another. It happens implicitly with operators like +, ==, and if(), or explicitly using Number(), String(), and Boolean().

The ternary operator (condition ? valueIfTrue : valueIfFalse) is a concise one-line alternative to if-else that returns a value. Unlike if-else, it's an expression that can be used in assignments, return statements, and JSX.

A Single Page Application loads one HTML page and dynamically updates content using JavaScript without full page reloads. The browser handles routing client-side, fetching data via APIs and re-rendering only the parts that change.