06 — Objects

One line: An object is a set of properties keyed only by strings and symbols, and the everyday tools that copy or lock one — spread, Object.assign, Object.freeze — stop at the first level, leaving nested objects shared and writable.

The chapter starts with the literal, the way most objects are made, and the keys it accepts, then the order those keys come back in. Next come the flags every property carries and the three locks built from them, then getters and setters, properties that run code. Copying follows, because every copy has to decide what to do with nested objects, flags and getters. ?. and ?? read through missing values, and JSON closes the chapter as the format that keeps only part of an object.

The pages

Page	Level	In one line	Status
Object literals	101	An object literal is more than pairs: { name } copies a variable in, [expr]: v computes a key at run time, and __proto__: p sets the prototype instead of making a key — unless written ["__proto__"]: p, which makes an ordinary property.	stub
Property keys	201	A property key is always a string or a symbol — o[1] and o["1"] are the same property, and o[{}] is o["[object Object]"] — and own keys come back integer-like first in ascending order, then other strings in insertion order, then symbols.	stub
Property descriptors	301	Every data property carries three flags; assignment sets all three to true, but Object.defineProperty defaults each to false, so the property it creates is hidden from Object.keys and JSON.stringify, and writing to it throws in a module and silently does nothing in sloppy code.	stub
freeze, seal and preventExtensions	201	preventExtensions blocks new properties, seal also blocks deletes, freeze also blocks changes — and all three stop at the first level, so a frozen object's nested array still takes push; a blocked write throws in a module and is silently ignored in sloppy code.	stub
Getters and setters	201	A getter runs on every read and a setter on every write, but spread, Object.assign and JSON.stringify call the getter once and keep only the value, so a copied object holds plain data that no longer computes anything.	stub
Copying objects	201	Spread and Object.assign copy one level, so nested objects stay shared with the original; structuredClone copies all the way down and keeps Date, Map and even cycles, but throws on a function and hands back a class instance as a plain object.	stub
?. and ??	101	?? falls back only on null and undefined, so 0 ?? 50 is 0 where 0 || 50 is 50; ?. stops the whole chain at the first null or undefined and skips every call and index expression after it.	stub
JSON	201	JSON.stringify drops undefined, functions and symbols from objects and writes them as null in arrays, turns NaN and Infinity into null, a Date into a string and a Map into {}, and throws a TypeError on a BigInt or a cycle.	stub

Boundaries

Prototypes, inheritance and classes are in Prototypes and classes; Map and Set, for keys of any type, are in Arrays and collections; types for object shapes are in Everyday types.