javascript

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.

Destructuring extracts values from arrays or properties from objects into distinct variables. The spread operator (...) expands iterables into individual elements, while rest (...) collects remaining elements into an array or object.

Key string methods include: length, indexOf/includes for searching, slice/substring for extracting, toUpperCase/toLowerCase for case, trim for whitespace, split for converting to arrays, replace/replaceAll for substitution, and template literals for interpolation.

Use querySelector/querySelectorAll (CSS selectors) or getElementById to select elements. Manipulate them via textContent, innerHTML, classList, style, setAttribute, and createElement/appendChild for creating new elements.

try/catch wraps code that might throw errors: the try block runs the code, catch handles any error that occurs, and the optional finally block always executes regardless of the outcome. Use it for operations that can fail at runtime like API calls, JSON parsing, and user input processing.

Template literals use backticks (`) instead of quotes, supporting string interpolation with ${expression}, multi-line strings, and embedded expressions. Tagged templates allow you to parse template literals with a custom function for advanced string processing.

An IIFE is a function expression that is defined and executed immediately, creating a private scope to avoid polluting the global namespace.

A polyfill is code that provides modern functionality in older environments that don't natively support it. Implementing Array.prototype.map requires iterating over the array, calling the callback with each element, index, and the original array, and collecting results into a new array.