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Test Coverage with Jest's Built in Coverage

Introduction​

Code coverage shows how much of your code is tested. Jest's built in coverage can easily be used to show code coverage reports.

Concepts​

Coverage reports show:

  • Statements covered.
  • Branches (if/else) covered.
  • Functions covered.
  • Lines covered.

Helps identify untested parts of your code.

Examples​

Running Coverage with Jest​

Jest includes built-in coverage reporting, so no additional packages are required.

Run coverage directly from the command line:

npx jest --coverage

Or add a script to your package.json:

"scripts": {
  "test": "jest",
  "coverage": "jest --coverage"
},

Running coverage will output a report showing:

  • Statements : 80% ( 8/10 )
  • Branches : 75% ( 3/4 )
  • Functions : 100% ( 2/2 )
  • Lines : 80% ( 8/10 )

Challenges​

1. Run jest coverage​

  • Run jest coverage on your project and check coverage.
💡 Show Answer
npm run coverage

2. Improve coverage by adding tests​

  • Add tests until coverage improves, then rerun.
💡 Show Answer
// add missing branch test to increase coverage

3. grade function test​

  • Write a function with an if/else branch.
  • See how coverage reports branch testing.
💡 Show Answer
// testCode.js
function checkNumber(num) {
  if (num > 0) {
    return "Positive";
  } else if (num < 0) {
    return "Negative";
  } else {
    return "Zero";
  }
}

module.exports = { checkNumber };

// testCode.test.js
const { checkNumber } = require("./testCode");

test("checkNumber returns 'Positive' for positive numbers", () => {
  expect(checkNumber(5)).toBe("Positive");
});

test("checkNumber returns 'Negative' for negative numbers", () => {
  expect(checkNumber(-3)).toBe("Negative");
});

test("checkNumber returns 'Zero' for zero", () => {
  expect(checkNumber(0)).toBe("Zero");
});

4. Unused code uncovered​

  • Write a function with unused code and observe how coverage reports it.
💡 Show Answer
function unused(){
  return "never tested";
}
// shows up as uncovered in report

5. HTML report​

  • Configure Jest to output an HTML report and open it in a browser.
💡 Show Answer
// Add a jest config file jest.config.js
module.exports = {
  collectCoverage: false, // We'll enable this via command line
  coverageDirectory: 'coverage',
  coverageReporters: [
    'text',
    'html',
    'lcov'
  ],
  collectCoverageFrom: [
    '**/*.js',
    '!**/*.test.js',
    '!**/*.spec.js',
    '!**/node_modules/**',
    '!**/coverage/**',
    '!jest.config.js'
  ],
  testEnvironment: 'node'
};

6. Coverage thresholds​

  • Add coverage thresholds in jest.config.js to fail below 80%.
💡 Show Answer
// Update jest.config.js
module.exports = {
  collectCoverage: false, // We'll enable this via command line
  coverageDirectory: "coverage",
  coverageReporters: ["text", "html", "lcov"],
  collectCoverageFrom: [
    "**/*.js",
    "!**/*.test.js",
    "!**/*.spec.js",
    "!**/node_modules/**",
    "!**/coverage/**",
    "!jest.config.js",
  ],
  coverageThreshold: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80,
    },
  },
  testEnvironment: "node",
};

7. Exclude files​

  • Exclude the test file from coverage with and see the effect.
💡 Show Answer
// Update jest.config.js to include
collectCoverageFrom: [
  "**/*.js",
  "!**/*.test.js",
  "!**/*.spec.js",
  "!**/node_modules/**",
  "!**/coverage/**",
  "!jest.config.js",
  // Additional exclusion examples:
  // "!**/dist/**",           // Exclude build/dist directories
  // "!**/build/**",          // Exclude build directories
  // "!**/*.config.js",       // Exclude all config files
  // "!**/scripts/**",        // Exclude scripts directory
  // "!**/docs/**",           // Exclude documentation
  // "!**/examples/**",       // Exclude example files
  // "!**/migrations/**",     // Exclude database migrations
  // "!**/seeds/**",          // Exclude database seeds
  // "!index.js",             // Exclude specific files
],

8. Untested error handling​

  • Show how untested error handling paths reduce coverage.
💡 Show Answer
function safeDivide(a, b) {
  if (b === 0) throw new Error();
  return a / b;
}

module.exports = { safeDivide };

9. Incremental coverage​

  • Add a test at a time to see how coverage changes (any project)
💡 Show Answer
// add one test at a time and rerun coverage to see % increase

10. Async coverage​

💡 Show Answer
// testCode.js
async function fetch(flag) {
  if (flag) return "ok";
  else throw "fail";
}

module.exports = { fetch };

// testCode.test.js
const { fetch } = require("./testCode");

test("ok", async () => {
  expect(await fetch(true)).toBe("ok");
});
test("fail", async () => {
  await expect(fetch(false)).rejects.toBe("fail");
});