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Grokking Node.js Design Patterns for Interviews

Problem: Prevent Accidental Mutation of Read-Only Settings

Easy
15 min
Protect configuration data by blocking all write attempts through a Proxy.

Problem statement

Your shared configuration object, config, is being mutated by multiple modules, which sometimes overwrites critical values, such as API URLs or version tags. This leads to unpredictable bugs in production.

You need a safeguard that prevents any property from being modified once loaded. The original configuration should remain immutable throughout the application’s lifetime.

Goal

Create a Proxy that blocks any write attempts and throws an error message such as "Cannot modify read-only settings" whenever code tries to change a property.

Constraints

  • You must use the set trap.

  • Do not freeze or clone the object. The restriction should come purely from Proxy behavior.

  • The Proxy must preserve normal read access to all properties.

  • Writing to a key should not change the original object.

Sample output

The examples below illustrate what the output should look like:

console.log(readOnlyConfig.appName);
/* Expected output:
NodeMaster
*/
try {
  readOnlyConfig.apiUrl = 'https://evil.com';
} catch (err) {
  console.error(err.message);
}
/* Expected console error:
Cannot modify read-only settings
*/
console.log(readOnlyConfig.apiUrl);
/* Expected output:
https://api.nodeapp.dev
*/

Good luck trying the problem! If you’re unsure how to proceed, check the “Solution” tab above.

Similar Problems
Track API Usage Metrics Transparently
Cache Expensive Function Calls via Proxy
Proxy a Remote Object over Network Simulation
Log Every Property Read on a Config Object
Auto-Default Missing Values
Lazy-Load a Heavy Service Only When First Used
Enforce Role-Based Access on Service Methods
Shape Legacy API Responses Without Touching the Source
Home
Courses
Grokking Node.js Design Patterns for Interviews

Problem: Prevent Accidental Mutation of Read-Only Settings

Easy
15 min
Protect configuration data by blocking all write attempts through a Proxy.

Problem statement

Your shared configuration object, config, is being mutated by multiple modules, which sometimes overwrites critical values, such as API URLs or version tags. This leads to unpredictable bugs in production.

You need a safeguard that prevents any property from being modified once loaded. The original configuration should remain immutable throughout the application’s lifetime.

Goal

Create a Proxy that blocks any write attempts and throws an error message such as "Cannot modify read-only settings" whenever code tries to change a property.

Constraints

  • You must use the set trap.

  • Do not freeze or clone the object. The restriction should come purely from Proxy behavior.

  • The Proxy must preserve normal read access to all properties.

  • Writing to a key should not change the original object.

Sample output

The examples below illustrate what the output should look like:

console.log(readOnlyConfig.appName);
/* Expected output:
NodeMaster
*/
try {
  readOnlyConfig.apiUrl = 'https://evil.com';
} catch (err) {
  console.error(err.message);
}
/* Expected console error:
Cannot modify read-only settings
*/
console.log(readOnlyConfig.apiUrl);
/* Expected output:
https://api.nodeapp.dev
*/

Good luck trying the problem! If you’re unsure how to proceed, check the “Solution” tab above.

Similar Problems
Track API Usage Metrics Transparently
Cache Expensive Function Calls via Proxy
Proxy a Remote Object over Network Simulation
Log Every Property Read on a Config Object
Auto-Default Missing Values
Lazy-Load a Heavy Service Only When First Used
Enforce Role-Based Access on Service Methods
Shape Legacy API Responses Without Touching the Source
Node.js
// Application configuration
const config = {
  appName: 'NodeMaster',
  version: '1.2.0',
  apiUrl: 'https://api.nodeapp.dev'
};
// Your code here
// Example usage
console.log(readOnlyConfig.appName);
/* Expected output:
NodeMaster
*/
try {
  readOnlyConfig.apiUrl = 'https://evil.com';
} catch (err) {
  console.error(err.message);
}
/* Expected console error:
Cannot modify read-only settings
*/
console.log(readOnlyConfig.apiUrl);
/* Expected output:
https://api.nodeapp.dev
*/