Spread operator

What is the spread operator?

The spread operator ... expands an array or object into its individual elements. It’s how you copy, merge, and build new data structures without mutating the originals.

const numbers = [1, 2, 3];
const moreNumbers = [...numbers, 4, 5, 6];

console.log(moreNumbers); // [1, 2, 3, 4, 5, 6]
console.log(numbers);     // [1, 2, 3] — original unchanged

You’ll use the spread operator constantly in React, where you need to create new arrays and objects instead of mutating state.

Array spreading

Copying an array

const original = [1, 2, 3];
const copy = [...original];

copy.push(4);
console.log(original); // [1, 2, 3] — unaffected
console.log(copy);     // [1, 2, 3, 4]

Python mental model: copy = [...original] is like copy = [*original] or copy = original.copy(). In all cases, this is a shallow copy.

Merging arrays

const frontend = ["React", "Vue", "Angular"];
const backend = ["Node", "Django", "FastAPI"];

const allFrameworks = [...frontend, ...backend];
console.log(allFrameworks);
// ["React", "Vue", "Angular", "Node", "Django", "FastAPI"]

Adding items without mutation

const colors = ["red", "green", "blue"];

// Add to the end
const withYellow = [...colors, "yellow"];
// ["red", "green", "blue", "yellow"]

// Add to the beginning
const withWhite = ["white", ...colors];
// ["white", "red", "green", "blue"]

// Insert in the middle
const withOrange = [...colors.slice(0, 2), "orange", ...colors.slice(2)];
// ["red", "green", "orange", "blue"]

In React, never use .push() or .pop() on state arrays. Instead, use spread to create new arrays: setItems([...items, newItem]). This tells React that the data changed and it needs to re-render.

Object spreading

Copying an object

const user = { name: "Sarah", age: 28, role: "admin" };
const copy = { ...user };

copy.age = 29;
console.log(user.age); // 28 — original unchanged
console.log(copy.age); // 29

Merging objects

const defaults = { theme: "light", language: "en", fontSize: 14 };
const userPrefs = { theme: "dark", fontSize: 16 };

const settings = { ...defaults, ...userPrefs };
console.log(settings);
// { theme: "dark", language: "en", fontSize: 16 }

Properties from later objects override earlier ones. userPrefs.theme (“dark”) overrides defaults.theme (“light”).

Updating a property without mutation

const user = { name: "Sarah", age: 28, role: "admin" };

// Create a new object with one property changed
const updatedUser = { ...user, age: 29 };

console.log(user);        // { name: "Sarah", age: 28, role: "admin" }
console.log(updatedUser); // { name: "Sarah", age: 29, role: "admin" }

This is the pattern you’ll use in React state updates:

const [user, setUser] = useState({ name: "Sarah", age: 28 });

// Update age without mutating
setUser({ ...user, age: 29 });

Spread creates a shallow copy. If your object contains nested objects, the inner objects are still shared references. For deep nested updates, spread each level: { ...user, address: { ...user.address, city: "Seattle" } }.

Python mental model: this is the same behavior as dict.copy() or {**user} — nested dicts/lists are still shared unless you copy those nested levels too.

Practical patterns

Building API request bodies

function createUserPayload(formData) {
  return {
    ...formData,
    createdAt: new Date().toISOString(),
    status: "active",
  };
}

const payload = createUserPayload({ name: "Sarah", email: "sarah@example.com" });
// { name: "Sarah", email: "sarah@example.com", createdAt: "2024-01-15T...", status: "active" }

Removing a property (rest + spread)

const user = { name: "Sarah", password: "secret123", role: "admin" };

// Remove password, keep everything else
const { password, ...safeUser } = user;
console.log(safeUser); // { name: "Sarah", role: "admin" }

This combines destructuring (to grab password) with rest syntax (...safeUser to collect everything else). It’s a clean way to strip sensitive fields.

Conditional properties

const isAdmin = true;

const user = {
  name: "Sarah",
  email: "sarah@example.com",
  ...(isAdmin && { role: "admin", permissions: ["read", "write", "delete"] }),
};

console.log(user);
// { name: "Sarah", email: "sarah@example.com", role: "admin", permissions: [...] }

The ...(condition && { props }) pattern conditionally includes properties. If the condition is false, false is spread (which adds nothing). You’ll see this when building request bodies with optional fields.

Python mental model: this is similar to {**base, **({"role": "admin"} if is_admin else {})}.

Comparing to Python

JavaScript
Python

// Spread arrays
const combined = [...arr1, ...arr2];

// Spread objects
const merged = { ...obj1, ...obj2 };

// Copy + update
const updated = { ...user, age: 29 };

# Unpack lists
combined = [*list1, *list2]

# Merge dicts (3.9+)
merged = {**dict1, **dict2}
# or: merged = dict1 | dict2

# Copy + update
updated = {**user, "age": 29}

JavaScript’s ... is like Python’s * for lists and ** for dicts. Same concept, same patterns.

What’s next?

You can work with arrays and objects. Now let’s learn about JSON — the format your frontend and backend use to exchange data.

JSON basics

Convert between JSON strings and JavaScript objects

Data structures

Working with JSON

Control flow

What is the spread operator?

Array spreading

Copying an array

Merging arrays

Adding items without mutation

Object spreading

Copying an object

Merging objects

Updating a property without mutation

Practical patterns

Building API request bodies

Removing a property (rest + spread)

Conditional properties

Comparing to Python

What’s next?

JSON basics

​What is the spread operator?

​Array spreading

​Copying an array

​Merging arrays

​Adding items without mutation

​Object spreading

​Copying an object

​Merging objects

​Updating a property without mutation

​Practical patterns

​Building API request bodies

​Removing a property (rest + spread)

​Conditional properties

​Comparing to Python

​What’s next?

JSON basics

What is the spread operator?

Array spreading

Copying an array

Merging arrays

Adding items without mutation

Object spreading

Copying an object

Merging objects

Updating a property without mutation

Practical patterns

Building API request bodies

Removing a property (rest + spread)

Conditional properties

Comparing to Python

What’s next?