How to Test Promise Catch With Mocha.js?

To test a promise catch block with mocha.js, you can use the done callback function to handle asynchronous code in your test case.

First, create a test case that returns a promise that should be rejected:

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it('should handle promise rejection', function(done) {
    return new Promise((resolve, reject) => {
        reject(new Error('Something went wrong'));
    }).catch((error) => {
        // Perform assertions on the error
        expect(error.message).to.equal('Something went wrong');
        done(); // Call done to indicate the test has completed
    });
});

In this test case, we create a promise that is immediately rejected with an error. We then use the .catch() method to handle the rejection and perform assertions on the error object. Finally, we call the done() function to indicate that the test has completed.

When the promise is rejected, the .catch() block will execute, and the error object will be passed to it. You can then perform any necessary assertions on the error object within the .catch() block to ensure that the promise rejection is properly handled.

What is the difference between handling errors with catch and then in Mocha.js promises?

In Mocha.js, when handling errors in promises, using the catch method is specifically for handling errors that occur in the promise chain, while using then with two callback functions is used for handling both the success and error cases.

When using catch, it is specifically used to catch and handle errors that occur in the promise chain. If an error occurs in any of the previous promise callbacks, it will be caught by the catch method and the error can be handled within that block.

On the other hand, when using then with two callback functions, the first callback function is for handling the success case (when the promise is resolved), and the second callback function is for handling the error case (when the promise is rejected). This allows for specific handling of both success and error cases in separate blocks of code.

Overall, using catch is more concise and cleaner way to handle errors in promise chains, while using then with two callback functions allows for more explicit handling of both success and error cases.

What is the purpose of using catch with promises in Mocha.js testing?

The purpose of using catch with promises in Mocha.js testing is to handle any errors that may occur during the execution of asynchronous code. When testing asynchronous code using promises, it is important to ensure that any errors are properly handled to prevent the test from failing unexpectedly. By using catch with promises in Mocha.js testing, developers can ensure that any errors are caught and logged, allowing the test to continue running and providing useful information about what went wrong.

How to handle errors in promise catch with Mocha.js?

To handle errors in promise catch with Mocha.js, you can use the assert.rejects method provided by Chai Assertion Library.

Here's an example of how you can use assert.rejects in a Mocha test to handle errors in a promise catch block:

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const { assert } = require('chai');

describe('Promise Catch Error Handling', function () {
  it('should handle errors in promise catch block', async function () {
    const promise = new Promise((resolve, reject) => {
      setTimeout(() => {
        reject(new Error('Something went wrong'));
      }, 1000);
    });

    await assert.rejects(promise, Error, 'Something went wrong');
  });
});

In this example, we create a new promise that will be rejected with an error after 1 second. We then use assert.rejects to check if the promise is rejected with the expected error message.

Make sure to install Chai Assertion Library before running the test:

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npm install chai

To run the Mocha test, you can use the following command:

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mocha test.js

This will run the test and output the result in the console. If the promise catch block does not handle the error correctly, the test will fail.