fix(coverage): istanbul provider to preserve implicit else

packages/coverage-istanbul/src/provider.ts

@@ -108,6 +108,8 @@ export class IstanbulCoverageProvider implements CoverageProvider {
     if (this.options.all)
       await this.includeUntestedFiles(mergedCoverage)
 
+    includeImplicitElseBranches(mergedCoverage)
+
     const sourceMapStore = libSourceMaps.createSourceMapStore()
     const coverageMap: CoverageMap = await sourceMapStore.transformCoverage(mergedCoverage)
 
@@ -238,3 +240,44 @@ function resolveIstanbulOptions(options: CoverageIstanbulOptions, root: string)
 function removeQueryParameters(filename: string) {
   return filename.split('?')[0]
 }
+
+/**
+ * Work-around for #1887 and #2239 while waiting for https://github.com/istanbuljs/istanbuljs/pull/706
+ *
+ * Goes through all files in the coverage map and checks if branchMap's have
+ * if-statements with implicit else. When finds one, copies source location of
+ * the if-statement into the else statement.
+ */
+function includeImplicitElseBranches(coverageMap: CoverageMap) {
+  for (const file of coverageMap.files()) {
+    const fileCoverage = coverageMap.fileCoverageFor(file)
+
+    for (const branchMap of Object.values(fileCoverage.branchMap)) {
+      if (branchMap.type === 'if') {
+        const lastIndex = branchMap.locations.length - 1
+
+        if (lastIndex > 0) {
+          const elseLocation = branchMap.locations[lastIndex]
+
+          if (elseLocation && isEmptyCoverageRange(elseLocation)) {
+            const ifLocation = branchMap.locations[0]
+
+            elseLocation.start = { ...ifLocation.start }
+            elseLocation.end = { ...ifLocation.end }
+          }
+        }
+      }
+    }
+  }
+}
+
+function isEmptyCoverageRange(range: libCoverage.Range) {
+  return (
+    range.start === undefined
+    || range.start.line === undefined
+    || range.start.column === undefined
+    || range.end === undefined
+    || range.end.line === undefined
+    || range.end.column === undefined
+  )
+}

test/coverage-test/coverage-test/coverage.istanbul.test.ts

@@ -1,5 +1,5 @@
 import fs from 'fs'
-import { resolve } from 'pathe'
+import { normalize, resolve } from 'pathe'
 import { expect, test } from 'vitest'
 
 test('istanbul html report', async () => {
@@ -39,3 +39,14 @@ test('files should not contain query parameters', () => {
   expect(files).toContain('Counter.component.ts.html')
   expect(files).not.toContain('Counter.component.ts?vue&type=script&src=true&lang.ts.html')
 })
+
+test('implicit else is included in branch count', async () => {
+  // @ts-expect-error -- generated file
+  const { default: coverageMap } = await import('./coverage/coverage-final.json')
+
+  const filename = normalize(resolve('./src/implicitElse.ts'))
+  const fileCoverage = coverageMap[filename]
+
+  expect(fileCoverage.b).toHaveProperty('0')
+  expect(fileCoverage.b['0']).toHaveLength(2)
+})

test/coverage-test/src/implicitElse.ts

@@ -0,0 +1,8 @@
+export function implicitElse(condition: boolean) {
+  let a = 1
+
+  if (condition)
+    a = 2
+
+  return a
+}

test/coverage-test/test/coverage.test.ts

@@ -1,6 +1,11 @@
 import { expect, test } from 'vitest'
 import { pythagoras } from '../src'
+import { implicitElse } from '../src/implicitElse'
 
 test('Math.sqrt()', async () => {
   expect(pythagoras(3, 4)).toBe(5)
 })
+
+test('implicit else', () => {
+  expect(implicitElse(true)).toBe(2)
+})

test/coverage-test/vitest.config.ts

@@ -20,7 +20,7 @@ export default defineConfig({
       enabled: true,
       clean: true,
       all: true,
-      reporter: ['html', 'text', 'lcov'],
+      reporter: ['html', 'text', 'lcov', 'json'],
     },
   },
 })