Test Coverage

Coverage tools report which statements, functions, and branches ran during a test suite. High coverage can expose untested code, but it does not prove tests assert the right behavior. Aim for meaningful boundary and failure-case tests rather than maximizing one percentage in isolation.

Both branches are exercised by two calls, making branch coverage for this tiny function complete. A test runner must still assert that the outputs are correct, not just execute them.

100% line coverage is not proof of correctness, security, or adequate edge-case testing.

Save as test-coverage.ts. The following program passed strict TypeScript compilation, and the displayed output was captured by executing its emitted JavaScript with Node.js 22.16.0.

TypeScript source

function describeScore(score: number): string {
  if (score >= 50) return "Pass";
  return "Try again";
}
console.log(describeScore(80));
console.log(describeScore(25));

Verified output

Pass
Try again

Compile: npx tsc --strict --target es2022 --module commonjs test-coverage.ts; run: node test-coverage.js. For Node.js API examples, install @types/node first. For browser projects, use the browser-specific instructions below.

The example demonstrates the code-level behavior; the related configuration or test commands are shown separately when needed.

Both branches are exercised by two calls, making branch coverage for this tiny function complete. A test runner must still assert that the outputs are correct, not just execute them.

Predict what changes if you alter one input. Then run the code to compare your reasoning with the real output.

Practical project lab: These are real configuration and source-file examples, not a TypeScript object that merely describes settings. Place the files at the named paths relative to one project directory.

score.ts

export const pass = (n:number):boolean => n>=50;

score.test.ts

import {test} from "node:test";
import {strict as assert} from "node:assert";
import {pass} from "./score";
test("both branches",()=>{ assert.equal(pass(60),true); assert.equal(pass(40),false); });

Commands

npx tsc --strict --module commonjs --target es2022 score.ts score.test.ts
node --experimental-test-coverage --test score.test.js

Coverage describes executed code paths, not correctness of expectations.

This local compiler or test-runner mini-project was validated in addition to the primary program.

100% line coverage is not proof of correctness, security, or adequate edge-case testing.

Identify whether the failure is a type-checking issue, a configuration issue, or a runtime behavior issue before choosing a fix.

Use a runner's --coverage option. Remove the failing-score case and observe the branch coverage report.

Write down your expected result first. For configuration examples, compare compiler output and diagnostics; for test examples, make the assertion fail deliberately to verify that the tool reports it.

Coverage tools report which statements, functions, and branches ran during a test suite.

Run the primary example, then complete the practice task or environment-specific lab.