How can I ignore a function for coverage?

handler.ts:

Coverage report:

-----------------|---------|----------|---------|---------|-------------------
File             | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
-----------------|---------|----------|---------|---------|-------------------
All files        |   97.05 |      100 |      75 |   96.96 |
 src             |   96.87 |      100 |   66.66 |   96.77 |
  handler.ts     |   96.87 |      100 |   66.66 |   96.77 | 90

jest.config.js:

module.exports = {
  clearMocks: true,
  transform: {
    '.(js|ts)$': ['esbuild-jest', { sourcemap: true }],
  },
  setupFilesAfterEnv: ['./jest.setup.ts'],
  collectCoverage: true,
};

I bet the transform is the issue, just don't know how to fix it.

Tried:

adding /* istanbul ignore next */ on line 88
adding /* istanbul ignore next */ after the word export on line 89
adding /* istanbul ignore next */ after the word return on line 90

When I do I still get an uncovered line. All the middleware etc makes actually testing that function not worth the effort.

Solution

Adding a parsing hint comment is the correct way to ignore a function, but esbuild only includes "legal comments" in the build output (emphasis mine):

A "legal comment" is considered to be any statement-level comment in JS or rule-level comment in CSS that contains @license or @preserve or that starts with //! or /*!. These comments are preserved in output files by default since that follows the intent of the original authors of the code.

So given an actual MRE:

// index.js

export default function foo() {
    console.log("Can't touch this");
}

// index.test.js

import func from "./index";

it("doesn't test anything", () => {
    expect(true).toBe(true);
});

// jest.config.js

module.exports = {
    transform: {
        "\\.js$": ["esbuild-jest", { sourcemap: true }],
    },
};

// package.json

{
  "devDependencies": {
    "esbuild": "^0.17.14",
    "esbuild-jest": "^0.5.0",
    "jest": "^29.5.0"
  },
  "scripts": {
    "test": "jest"
  }
}

with the output:

$ npm t -- --coverage

> test
> jest --coverage

 PASS  ./index.test.js
  ✓ doesn't test anything (1 ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files |      50 |      100 |       0 |      50 |
 index.js |      50 |      100 |       0 |      50 | 2
----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.16 s, estimated 1 s
Ran all test suites.

you can get to 100% coverage by adding a parsing hint containing @preserve so that it still appears in the output from esbuild:

// index.js

/* istanbul ignore next @preserve */
export default function foo() {
    console.log("Can't touch this");
}

New output:

$ npm t -- --coverage

> test
> jest --coverage

 PASS  ./index.test.js
  ✓ doesn't test anything (1 ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |       0 |     100 |
 index.js |     100 |      100 |       0 |     100 |
----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.363 s, estimated 1 s
Ran all test suites.