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testland/jest-coverage-analysis

Configures Jest's built-in coverage (Istanbul-instrumented `babel` provider or V8-native `v8` provider), wires the right `coverageReporters` for downstream consumption (`lcov` for SaaS / cross-tool, `cobertura` for Jenkins, `text-summary` for terminal, `html` for human review), authors per-file `coverageThreshold` rules that focus the gate on critical paths (vs the global-only foot-gun), and parses the per-file JSON output for PR-time deltas. Use when the project tests with Jest (or Vitest, which uses the same Istanbul/V8 provider) and the team needs PR-time coverage signal that's both local-runnable and CI-gateable.

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Files

parsing-and-anti-patterns.mdreferences/

Parsing coverage-final.json and coverage-gate anti-patterns

Companion detail for jest-coverage-analysis. The Jest / Vitest config in SKILL.md is the runnable core; this file holds the full JSON parser and the anti-pattern catalog.

The coverage-final.json shape

The json reporter writes coverage/coverage-final.json with a per-file structure keyed by absolute path:

{
  "/abs/path/src/checkout/cart.ts": {
    "path": "/abs/path/src/checkout/cart.ts",
    "statementMap": { "0": { "start": {}, "end": {} } },
    "fnMap": { },
    "branchMap": { },
    "s": { "0": 42, "1": 42, "2": 0 },
    "f": { "0": 42, "1": 0 },
    "b": { "0": [42, 0] }
  }
}

s = per-statement hit counts; f = per-function; b = per-branch arm.

Full per-file parser

// scripts/parse_jest_coverage.js
import { readFileSync } from 'node:fs';

const data = JSON.parse(readFileSync('coverage/coverage-final.json', 'utf8'));

for (const [absPath, file] of Object.entries(data)) {
  const stmts = Object.values(file.s);
  const stmtPct = (stmts.filter(c => c > 0).length / stmts.length) * 100;

  const fns = Object.values(file.f);
  const fnPct = (fns.filter(c => c > 0).length / fns.length) * 100;

  // Branch coverage: each entry is an array of arm hit counts.
  const branchEntries = Object.values(file.b);
  const branchTotal = branchEntries.flat().length;
  const branchHit = branchEntries.flat().filter(c => c > 0).length;
  const brPct = branchTotal === 0 ? 100 : (branchHit / branchTotal) * 100;

  console.log({ path: absPath, stmtPct, fnPct, brPct });
}

The coverage-summary.json file (from the json-summary reporter) is the pre-aggregated version when per-statement detail isn't needed.

Anti-patterns

Anti-patternWhy it failsFix
collectCoverage: false in CINo coverage data emitted; downstream gate is empty.--coverage flag in the test command (Step 7).
Skipping collectCoverageFromFiles with no test silently absent from denominator; coverage inflated.Always set explicitly (Step 4).
coverageThreshold.global only, no per-path rulesA new module under src/api/ joins at 0% coverage; global drops by 0.3pp; gate passes.Per-path rules for critical modules (Step 3).
Mixing babel and v8 ignore commentsOne provider misses the ignore; coverage drops mysteriously.Pick one; grep-replace if switching.
Using coverage-final.json as the gate inputPer-statement detail is huge; gate scripts slow.coverage-summary.json for whole-repo + lcov.info for per-line drilldown.
coverageDirectory: '/tmp/...' outside the repoCI artifact upload step misses it.Keep in coverage/ (default).
Threshold set at 100% on a global ruleFirst refactor fails the build; team disables coverage entirely.Set globals at the maintainable floor, not aspirational ceiling.

references

parsing-and-anti-patterns.md

SKILL.md

tile.json