Be Careful with the Spread Operator in JavaScript

The spread operator (...) in JavaScript is a powerful syntax that allows for expanding elements of iterables like arrays or strings into individual items. Its use simplifies many operations in coding but also comes with nuances that could lead to inefficiencies or unintended behaviors.

That's why it's important to be aware of these potential issues and prevent unnecessary problems. Today, we'll dive into this topic.

Understanding the Spread Operator

Introduced in ES6, the spread operator allows for the expansion of elements where literals are expected, such as in array literals, function calls, and object literals.

Examples and Basic Uses

Array Literals:

let firstArray = [1, 2, 3];
// Expands to [1, 2, 3, 4, 5]
let secondArray = [...firstArray, 4, 5];

Function Calls:

function sum(x, y, z) {
  return x + y + z;
let numbers = [1, 2, 3];
console.log(sum(...numbers)); // Outputs 6

Object Literals:

let obj1 = { foo: 'bar', x: 42 };
// Creates { foo: 'bar', x: 42, y: 13 }
let obj2 = { ...obj1, y: 13 };

Potential Pitfalls

Performance and Efficiency

Copying large arrays or objects using the spread operator is inefficient compared to methods like Array.slice() or Object.assign().

Before using it in complex algorithms, verify the impact, and use a faster approach.

Shallow Copy Limitations

The spread operator only performs shallow copies. For example:

let original = { a: 1, b: { c: 2 } };
let copied = { ...original };
// Also changes 'original.b.c'
copied.b.c = 3;

Misuse with Non-iterable Values

let value = 123;
// Throws TypeError: value is not iterable
let array = [...value];

Security Considerations on the Backend

When using the spread operator on the backend, there's a risk of accidentally returning sensitive data to clients. This happens if more data than necessary is included in the response using the spread operator.

It usually occurs, when you add new properties to an entity, and you've already implemented code that uses spread operator.

Unsafe Practice Example:

let loadedFromDbUser = { 
   username: 'username_hash', 
   password: 'password_hash', 
   email: '' 
let response = { ...loadedFromDbUser, token: 'someAuthToken' };
res.send(response); // Sends password hash

Mitigation Strategies:

  • Minimize Data Exposure: Explicitly define and send only necessary attributes.
  • Use Utility Functions for Data Sanitization: Employ utilities like lodash's _.pick to safely construct response objects.
  • Implement Strong Access Controls: Ensure robust authentication and authorization before sending out data.

So, instead of doing it with a spread operator, we may assign values explicitly.

let loadedFromDbUser = { 
   username: 'username_hash', 
   password: 'password_hash', 
   email: '' 
let response = { email:, token: 'someAuthToken' };
res.send(response); // Now it's fine ✅

Interface Segregation and the Spread Operator

One of the principles of SOLID design, Interface Segregation, advises that no client should be forced to depend on methods or properties it does not use. Applying this principle in JavaScript, particularly with the spread operator, it's vital to ensure that objects are constructed with care to prevent bloating interfaces with unnecessary data.

Example of Interface Bloating:

let userDetails = { 
    name: 'Jane Doe', 
    email: '', 
    age: 30 
function greet({ name, }) {
  console.log(`Hello, ${name}!`);
  // 'rest' includes unneeded properties.

Mitigating Interface Bloating:

Explicitly specifying only necessary properties ensures efficient and secure code:

function greet({ name }) {
  console.log(`Hello, ${name}!`);


While the spread operator enhances JavaScript's expressiveness, it requires careful handling to prevent performance penalties, bugs, and security issues. Proper understanding and thoughtful application are crucial for secure and efficient programming.

So, the important question arises: Should you use it or not? It's perfectly fine to use it in simple functions or UI components that do not involve critical business logic. However, you should consider using it more carefully. Remember to check, measure, and conduct tests to avoid security or performance issues.