1. Copies & Object References

In JavaScript, primitives copy by value, but objects and arrays copy by reference.

2. Why It Matters

Assigning an object to a new variable (const b = a) creates a shared reference pointer. Modifying a property on b silently mutates a!

3. Shallow Copy vs. Deep Copy

• Shallow Copy ({ ...obj } or Object.assign()): Copies top-level properties. Nested objects still share references in memory!
• Deep Copy (structuredClone(obj)): Creates a completely independent clone of the entire object graph, including all nested objects and arrays.

const original = {
  name: "App",
  settings: { theme: "dark" },
};

// Shallow Copy Problem
const shallow = { ...original };
shallow.settings.theme = "light";
console.log(original.settings.theme); // "light" (Mutated original settings!)

// Native Deep Clone (Modern ES2022 Standard)
const deepCopy = structuredClone(original);
deepCopy.settings.theme = "blue";
console.log(original.settings.theme); // "light" (Original untouched!)

4. Visual Memory Reference Layout

[ Shallow Copy ] -> copy.settings ---+
                                     +--> Shares same nested memory cell!
[ Original ]     -> orig.settings ---+

[ Deep Clone ]   -> clone.settings ----> Independent new memory cell!

5. Common Mistakes

• Using JSON.parse(JSON.stringify(obj)): The old JSON deep copy trick loses Functions, undefined, Date objects, Set, Map, and crashes on circular references. Always use native structuredClone().
• Assuming Spread is Deep: Spread operator ONLY copies 1 level deep!

6. Try It Yourself

Write it yourself. The tests below run against your code.

function cloneAndModify(original, newTheme) {
  return original;
}

7. Next Steps

Use structuredClone() for safe deep cloning. Next, let's explore modern native array grouping with Object.groupBy()!