fix: exclude coverage of the CJS-ESM bridge from results (bcoe#83)

lib/is-cjs-esm-bridge.js

@@ -0,0 +1,10 @@
+module.exports = ({ functions }) => {
+  // https://github.com/nodejs/node/blob/v12.1.0/lib/internal/modules/esm/create_dynamic_module.js#L11-L19
+  return functions.length === 3 &&
+  functions[0].functionName === '' &&
+  functions[0].isBlockCoverage === true &&
+  functions[1].functionName === 'get' &&
+  functions[1].isBlockCoverage === false &&
+  functions[2].functionName === 'set' &&
+  functions[2].isBlockCoverage === true
+}

lib/report.js

@@ -8,6 +8,7 @@ const { isAbsolute, resolve } = require('path')
 // TODO: switch back to @c88/v8-coverage once patch is landed.
 const { mergeProcessCovs } = require('@bcoe/v8-coverage')
 const v8toIstanbul = require('v8-to-istanbul')
+const isCjsEsmBridgeCov = require('./is-cjs-esm-bridge')
 
 class Report {
   constructor ({
@@ -55,21 +56,43 @@ class Report {
     if (this._allCoverageFiles) return this._allCoverageFiles
 
     const v8ProcessCov = this._getMergedProcessCov()
-
     const map = libCoverage.createCoverageMap({})
+    const resultCountPerPath = new Map()
+    const possibleCjsEsmBridges = new Map()
 
     for (const v8ScriptCov of v8ProcessCov.result) {
       try {
         const path = resolve(this.resolve, v8ScriptCov.url)
         const converter = v8toIstanbul(path, this.wrapperLength)
         await converter.load()
-        converter.applyCoverage(v8ScriptCov.functions)
-        map.merge(converter.toIstanbul())
+
+        if (resultCountPerPath.has(path)) {
+          resultCountPerPath.set(path, resultCountPerPath.get(path) + 1)
+        } else {
+          resultCountPerPath.set(path, 0)
+        }
+
+        if (isCjsEsmBridgeCov(v8ScriptCov)) {
+          possibleCjsEsmBridges.set(converter, {
+            path,
+            functions: v8ScriptCov.functions
+          })
+        } else {
+          converter.applyCoverage(v8ScriptCov.functions)
+          map.merge(converter.toIstanbul())
+        }
       } catch (err) {
         console.warn(`file: ${v8ScriptCov.url} error: ${err.stack}`)
       }
     }
 
+    for (const [converter, { path, functions }] of possibleCjsEsmBridges) {
+      if (resultCountPerPath.get(path) <= 1) {
+        converter.applyCoverage(functions)
+        map.merge(converter.toIstanbul())
+      }
+    }
+
     this._allCoverageFiles = map
     return this._allCoverageFiles
   }

test/fixtures/export.cjs

@@ -0,0 +1 @@
+module.exports = () => 'foo';

test/fixtures/import.mjs

@@ -1,3 +1,4 @@
-import foo from './export.mjs'
+import esm from './export.mjs'
+import cjs from './export.cjs'
 
-console.info(foo())
+console.log(esm(), cjs())

test/integration.js.snap

@@ -1,11 +1,12 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`c8 ESM Modules collects coverage for ESM modules 1`] = `
-",bar
+",bar foo
 ------------|----------|----------|----------|----------|-------------------|
 File        |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
 ------------|----------|----------|----------|----------|-------------------|
-All files   |       80 |      100 |       50 |       80 |                   |
+All files   |    83.33 |      100 |    66.67 |    83.33 |                   |
+ export.cjs |      100 |      100 |      100 |      100 |                   |
  export.mjs |    71.43 |      100 |       50 |    71.43 |               2,3 |
  import.mjs |      100 |      100 |      100 |      100 |                   |
 ------------|----------|----------|----------|----------|-------------------|