Two names can point to one object

First, build a mental model

A variable is a named binding to a value. When that value is an object, copying it copies a reference to the same object, not the whole object graph.

1. Primitive values compare by value; objects compare by identity.
2. Assignment of an object reference creates another path to the same object.
3. Object spread copies immediate properties, not nested objects.
4. Immutable updates must replace each modified level that needs a new identity.

const original = { name: 'Mira', stats: { score: 10 } };
const alias = original;
const shallow = { ...original };
shallow.stats.score = 99;
console.log(original.stats.score);
console.log(alias === original, shallow === original);
const next = { ...original, stats: { ...original.stats, score: 100 } };
console.log(next.stats === original.stats);

1. Alias

   alias and original identify one object.

2. Shallow copy

   shallow is new, but shallow.stats still identifies original.stats.

3. Nested update

   next creates a new outer object and a new stats object.