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Frequently Asked Questions ​

Property Descriptor Errors ​

"Descriptor for property X is non-configurable and non-writable" ​

If you encounter an error like this:

TypeError: Descriptor for property toBeMocked is non-configurable and non-writable

This error occurs when Sinon tries to stub or spy on a property that has been defined as immutable by JavaScript's property descriptor system. This is not a bug in Sinon, but rather a limitation imposed by the JavaScript engine itself.

Common Causes ​

  1. ES Module transpilation: When ES modules are transpiled to CommonJS (e.g., by TypeScript, Babel, or SWC), the exported properties often become non-configurable and non-writable.

  2. Object.freeze() or Object.seal(): Objects that have been frozen or sealed have immutable properties.

  3. Native browser/Node.js APIs: Some built-in objects and their properties are inherently immutable.

  4. Third-party libraries: Some libraries define their exports with non-configurable descriptors.

Solutions ​

  1. Use dependency injection: Instead of stubbing the import directly, pass the dependency as a parameter:

    javascript
    // Instead of this:
import { toBeMocked } from "./module";
sinon.stub(module, "toBeMocked"); // This might fail

// Do this:
function myFunction(dependency = toBeMocked) {
  return dependency();
}

// In tests:
const stub = sinon.stub();
myFunction(stub);

  2. Stub at the module level: For ES modules, consider using a tool like proxyquire or testdouble.js for module-level mocking.

  3. Use dynamic imports: Dynamic imports can sometimes work around transpilation issues:

    javascript
    // In your test
const module = await import("./module");
sinon.stub(module, "toBeMocked");

  4. Restructure your code: Consider whether the code under test can be refactored to be more testable.

For TypeScript Users ​

When using TypeScript with SWC or similar transpilers, see our TypeScript with SWC guide for specific solutions.

Further Reading ​