Snippet

Dynamic Object Mocking with Property Descriptors for Next.js Headers Unit Testing

This technique leverages low-level JavaScript property descriptors (Object.getOwnPropertyDescriptor and Object.defineProperty) to safely isolate and mock the global Headers constructor used in Next.js server runtime environments during unit tests. It returns a cleanup teardown function that restores the original global state.

snippet.js

javascript

export function mockNextHeaders(headerMap) {
  const originalDescriptor = Object.getOwnPropertyDescriptor(globalThis, 'Headers');
  
  const MockedHeaders = class extends Headers {
    constructor() {
      super();
      Object.entries(headerMap).forEach(([key, val]) => this.set(key, val));
    }
  };
 
  Object.defineProperty(globalThis, 'Headers', {
    value: MockedHeaders,
    configurable: true,
    writable: true
  });
 
  return () => {
    if (originalDescriptor) {
      Object.defineProperty(globalThis, 'Headers', originalDescriptor);
    } else {
      delete globalThis.Headers;
    }
  };
}

nextjs

Breakdown

const originalDescriptor = Object.getOwnPropertyDescriptor(globalThis, 'Headers');

Captures the exact property descriptor metadata of the global Headers object prior to modification.

Object.defineProperty(globalThis, 'Headers', { value: MockedHeaders, configurable: true, writable: true });

Overrides the global scope with the custom MockedHeaders implementation using precise configuration flags.

return () => { ... };

Returns a teardown function that restores the original global property descriptor to prevent test pollution.

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Dynamic Object Mocking with Property Descriptors for Next.js Headers Unit Testing

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