Solved: run test%3Acoverage command in jest

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Testing is an integral part of any software development process. It is how you ensure that the code you write does what it was intended to do and how you catch any bugs that may have slipped in. One common way of reporting on how much of your code is reached by your tests is through test coverage reports. In JavaScript, one popular testing framework that provides functionality to generate coverage reports is Jest. Running ‘test:coverage’ command in Jest will provide a detailed test coverage.

npm test -- --coverage

Understanding Jest Coverage Report

Upon running the ‘test:coverage’ command, Jest produces a comprehensive coverage report. This report includes several categories – Statements, Branches, Functions, and Lines.

A brief description of these categories is as follows –

  • Statements: This defines the total number of statements in your script, with the percentage of statements that are covered by tests.
  • Branches: This specifies the number of decision points (e.g., if statements) in your script, alongside the proportion that are covered by tests.
  • Functions: This stands for the total number of functions, showing the percentage of functions that are covered by tests.
  • Lines: This demonstrates the number of executable lines of code in your script, also showing the proportion that are covered by tests.

Steps To Generate a Jest Coverage Report

Here is a step-by-step guide to generate a test coverage report using Jest –

1. Install Jest if not already done so using npm or yarn:

npm install --save-dev jest

2. Add a test script in your package.json file. Example as follows:

 "scripts": {
   "test": "jest"

3. Now, you can easily run your tests with coverage as follows:

npm test -- --coverage

This command will generate a coverage report in your terminal and also a more detailed HTML report in a coverage directory at the root of your project.

Benefit of Using Jest for Test Coverage

Jest is a powerful tool for running tests and has a great feature for code coverage. Using Jest to report test coverage ensures that you are writing tests that are effective at testing the functionality of your code, it can prove useful when attempting to find untested parts of your software. Also, Jest offers a “watch” mode, automatically re-running your tests when it detects a change in your files.

Remember, while high test coverage can be a great goal, it’s important to write meaningful tests and not to get hung up on achieving 100% coverage. It’s much more valuable to have a smaller percentage of well-thought-out, effective tests than a larger percentage of ineffective tests. Testing is about quality, not quantity.


In conclusion, Jest provides a full-featured, easy-to-use test coverage tool that is built into the testing framework itself. With this tool, developers can confidently analyze the effectiveness of their test suite, identify areas that lack coverage and ensure that all edge cases are accounted for. Maintenance, refactoring, or adding new features to the codebase can be performed with peace of mind, knowing that any breaking changes can be caught promptly by the testing suite.

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