Fixing "TypeError: Cannot redefine property x" Error in Jest Tests

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Fixing "TypeError: Cannot redefine property x" Error in Jest Tests

Quick Solution Overview

Encountering a TypeError: Cannot redefine property x error in Jest tests can be resolved using two main approaches:

Approach 1: Using jest.mock

Mock the module directly:

import yourModule from "@/path/to/yourModule";

// ๐Ÿ‘‡ Mock the module
jest.mock("@/path/to/yourModule");

Approach 2: Using Barrel Imports

Re-export functions from a new file:

// chakra.ts
import { useColorMode } from "@chakra-ui/react";

const chakra = {
  // ๐Ÿ‘‡ Re-export the function
  useColorMode,
};

export default chakra;

Then, mock the function in your test file:

// your.spec.ts
import chakra from "./chakra";

const useColorModeMock = jest.spyOn(chakra, "useColorMode");
useColorModeMock.mockReturnValue({ colorMode: "light" });

Understanding the Problem

The error typically arises when attempting to mock a function that's directly exported from a package. This is common with certain npm packages, like @chakra-ui/react, where redefining properties directly can trigger a TypeError.

Example Error Scenario

Here's an example error encountered while testing Risedle Interface:

TypeError: Cannot redefine property: useColorMode

The error occurred when trying to mock useColorMode from @chakra-ui/react directly:

import chakra from "@chakra-ui/react";

const useColorModeMock = jest.spyOn(chakra, "useColorMode");
useColorModeMock.mockReturnValue({ colorMode: "light" });

Detailed Solutions

Solution 1: Using jest.mock

Mocking the entire module is straightforward and usually the quickest fix:

import yourModule from "@/path/to/yourModule";

jest.mock("@/path/to/yourModule");

Solution 2: Using Barrel Imports

  1. Create a New File for Re-exports:

    • Example chakra.ts:

        import { useColorMode } from "@chakra-ui/react";
      
        const chakra = {
          useColorMode,
        };
      
        export default chakra;
      
  2. Mock in Test Files:

    • In your test file (your.spec.ts):

        import chakra from "./chakra";
      
        const useColorModeMock = jest.spyOn(chakra, "useColorMode");
        useColorModeMock.mockReturnValue({ colorMode: "light" });
      

Conclusion

Both methods effectively solve the "TypeError: Cannot redefine property: x" issue in Jest. The choice depends on your specific testing scenario and preferences. Happy testing!

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