Data Structures

Arrays

Arrays are ordered collections of values, accessed by index starting at zero. They're the workhorse data structure for lists, queues, and stacks. length is a property, not a method — no parentheses needed.

Arrays can hold mixed types, though in practice you rarely want that. Use push and pop for stack-like behavior, shift and unshift for queue-like behavior.

Objects

Objects store named properties — key-value pairs where keys are strings (or Symbols). Access properties with dot notation (obj.key) or bracket notation (obj["key"]). Brackets let you use dynamic keys.

delete removes a property. in checks if a property exists. Object.keys() returns an array of property names.

Mutability

Numbers, strings, and booleans are immutable — you can't change them, only create new values. Objects and arrays are mutable — they hold references, not copies. Two variables can point to the same object, so changing one changes the other.

Comparing objects with == checks identity (same reference), not structural equality. This is a common source of bugs.

Array Methods

map transforms each element. filter selects elements matching a condition. reduce accumulates a single value from left to right. These three methods replace the vast majority of manual loops.

Destructuring extracts values into bindings: let [a, b] = [1, 2]. Rest parameters (...rest) collect remaining arguments. Spread (...arr) expands an array into individual elements. JSON serializes objects to strings and back with JSON.stringify and JSON.parse.