Kakšna je razlika med plitvo in globoko kopijo?

Question

Accepted Answer

Objekti so dodeljeni **po referenci**, zato `b = a` ne kopira — oba kažeta na isti objekt. **Plitva** kopija podvoji zgornjo raven, vendar deli ugniježđene reference; **globoka** kopija podvoji vse, popolnoma neodvisno.

```js
const original = { name: "Ann", address: { city: "NY" } };

// shallow copy: spread / Object.assign
const shallow = { ...original };
shallow.name = "Bob";            // ✅ independent (top-level primitive)
shallow.address.city = "LA";     // ⚠️ ALSO changes original.address.city — shared!

original.address.city; // "LA"  ← mutated through the shallow copy
```

Ugniježđeni objekt `address` je **ista referenca** v obeh, zato его mutacija vpliva na oba.

## Ustvarjanje globoke kopije

```js
const deep = structuredClone(original); // modern, built-in, handles most types
deep.address.city = "Chicago";          // ✅ original is untouched
```

Druge možnosti in njihove predpostavke:

```js
JSON.parse(JSON.stringify(obj)); // simple, but LOSES functions, Dates → strings, undefined
// lodash: _.cloneDeep(obj)       // robust, for complex cases
```

## Zakaj je to pomembno

To je poglavitni vir hroščev "zakaj se je moje stanje nepričakovano spremenilo?", posebej v React/Redux, kjer ne smete mutirati stanja.

Raztros je primeren, kadar spremenite le polja zgornje ravni, vendar za ugniježđene posodobitve morate kopirati tudi ugniježđeno raven (`{ ...obj, address: { ...obj.address, city } }`) ali narediti globoko kloniranje.