Built-ins
Standard objects, methods, and global functions.
- Array Methods (map, filter, find, some, every)The everyday transformation and search methods on Array.prototype — which mutate, which don't, and when to reach for each.
- Array.prototype.reduce()Combines every array element into a single accumulated result, using a callback and a starting value.
- Date and Intl.DateTimeFormatCreating and reading a Date correctly, a well-known month-indexing and time-zone-parsing pitfall, and formatting dates for a human reader.
- JSON.parse() and JSON.stringify()Convert between a JavaScript value and its JSON text representation — the standard way data crosses storage and network boundaries.
- Map, Set, WeakMap, and WeakSetDedicated collection types — Map/Set for general keyed/unique-value storage, WeakMap/WeakSet for weakly-held object keys that don't prevent garbage collection.
- Math and Intl.NumberFormatThe Math object's standard numeric operations, and Intl.NumberFormat for locale-correct display formatting instead of hand-built strings.
- Number Edge Cases (NaN, Infinity, Floating Point)JavaScript's single numeric type is 64-bit floating point, which produces predictable rounding error and two special non-finite values.
- PromiseA value representing an operation that hasn't completed yet, eventually settling as fulfilled (success) or rejected (failure).
- String MethodsThe everyday methods for searching, slicing, and transforming text — every one returns a new string, since strings are immutable.
- structuredClone()Deep-copies a value using the structured clone algorithm — handles plain data, Map, Set, and Date, but not functions or DOM nodes.