Study notes · 15% of the exam

Objects and Arrays

Objects and arrays are references. Know which methods mutate, which return new values, and how to copy safely.

Key points

1
Assigning an object or array copies the reference, not the value. Two variables can point at the same array.
2
const stops you reassigning a variable; it does not make the object immutable.
3
Non-mutating methods (map, filter, slice, concat, toSorted) return new arrays; mutating ones (push, splice, sort, reverse) change the original.
4
Spread and Object.assign make shallow copies. Use structuredClone for deep copies that keep Dates, Maps and Sets.
5
Object.freeze is shallow; nested objects stay mutable.

JSON.parse(JSON.stringify(x)) turns Dates into strings and drops undefined and functions.
sort() sorts as strings by default: [10, 9, 1].sort() gives [1, 10, 9].

Ten questions, with the answer and explanation after each one.