diff --git a/packages/test-runner-core/src/coverage/getTestCoverage.ts b/packages/test-runner-core/src/coverage/getTestCoverage.ts
index ffeca3dea..d8a0110d4 100644
--- a/packages/test-runner-core/src/coverage/getTestCoverage.ts
+++ b/packages/test-runner-core/src/coverage/getTestCoverage.ts
@@ -4,6 +4,7 @@ import {
   CoverageMap,
   CoverageMapData,
   BranchMapping,
+  FunctionMapping,
   Location,
   Range,
 } from 'istanbul-lib-coverage';
@@ -26,61 +27,105 @@ export interface TestCoverage {
 const locEquals = (a: Location, b: Location) => a.column === b.column && a.line === b.line;
 const rangeEquals = (a: Range, b: Range) => locEquals(a.start, b.start) && locEquals(a.end, b.end);
 
-function findBranchKey(branches: Record<string, BranchMapping>, branch: BranchMapping) {
-  for (const [key, m] of Object.entries(branches)) {
-    if (rangeEquals(m.loc, branch.loc)) {
+function findKey<T extends BranchMapping | FunctionMapping>(items: Record<string, T>, item: T) {
+  for (const [key, m] of Object.entries(items)) {
+    if (rangeEquals(m.loc, item.loc)) {
       return key;
     }
   }
 }
 
+function collectCoverageItems<T extends BranchMapping | FunctionMapping>(
+  filePath: string,
+  itemsPerFile: Map<string, Record<string, T>>,
+  itemMap: Record<string, T>,
+) {
+  let items = itemsPerFile.get(filePath);
+  if (!items) {
+    items = {};
+    itemsPerFile.set(filePath, items);
+  }
+
+  for (const item of Object.values(itemMap)) {
+    if (findKey(items, item) == null) {
+      const key = Object.keys(items).length;
+      items[key] = item;
+    }
+  }
+}
+
+function patchCoverageItems<T extends BranchMapping | FunctionMapping, U extends number | number[]>(
+  filePath: string,
+  itemsPerFile: Map<string, Record<string, T>>,
+  itemMap: Record<string, T>,
+  itemIndex: Record<string, U>,
+  defaultIndex: () => U,
+) {
+  const items = itemsPerFile.get(filePath)!;
+  const originalItems = itemMap;
+  const originalIndex = itemIndex;
+  itemMap = items;
+  itemIndex = {};
+
+  for (const [key, mapping] of Object.entries(items)) {
+    const originalKey = findKey(originalItems, mapping);
+    if (originalKey != null) {
+      itemIndex[key] = originalIndex[originalKey];
+    } else {
+      itemIndex[key] = defaultIndex();
+    }
+  }
+
+  return { itemMap, itemIndex };
+}
+
 /**
- * Cross references coverage mapping data, looking for missing code branches
- * and adding empty entries for them if found. This is necessary because istanbul
- * expects code branch data to be equal for all coverage entries, while v8 only
- * outputs actual covered code branches.
+ * Cross references coverage mapping data, looking for missing code branches and
+ * functions and adding empty entries for them if found. This is necessary
+ * because istanbul expects code branch and function data to be equal for all
+ * coverage entries. V8 only outputs actual covered code branchesm and functions
+ * that are defined at runtime (for example methods defined in a constructor
+ * that isn't run will not be included).
  *
- * See https://github.com/istanbuljs/istanbuljs/issues/531 for more.
+ * See https://github.com/istanbuljs/istanbuljs/issues/531,
+ * https://github.com/istanbuljs/v8-to-istanbul/issues/121 and
+ * https://github.com/modernweb-dev/web/issues/689 for more.
  * @param coverages
  */
-function addingMissingCoverageBranches(coverages: CoverageMapData[]) {
+function addingMissingCoverageItems(coverages: CoverageMapData[]) {
   const branchesPerFile = new Map<string, Record<string, BranchMapping>>();
+  const functionsPerFile = new Map<string, Record<string, FunctionMapping>>();
 
-  // collect code branches from all code coverage entries
+  // collect functions and code branches from all code coverage entries
   for (const coverage of coverages) {
     for (const [filePath, fileCoverage] of Object.entries(coverage)) {
-      let branches = branchesPerFile.get(filePath);
-      if (!branches) {
-        branches = {};
-        branchesPerFile.set(filePath, branches);
-      }
-
-      for (const branch of Object.values(fileCoverage.branchMap)) {
-        if (findBranchKey(branches, branch) == null) {
-          const key = Object.keys(branches).length;
-          branches[key] = branch;
-        }
-      }
+      collectCoverageItems(filePath, branchesPerFile, fileCoverage.branchMap);
+      collectCoverageItems(filePath, functionsPerFile, fileCoverage.fnMap);
     }
   }
 
   // patch coverage entries to add missing code branches
   for (const coverage of coverages) {
     for (const [filePath, fileCoverage] of Object.entries(coverage)) {
-      const branches = branchesPerFile.get(filePath)!;
-      const originalBranches = fileCoverage.branchMap;
-      const originalB = fileCoverage.b;
-      fileCoverage.branchMap = branches;
-      fileCoverage.b = {};
-
-      for (const [key, mapping] of Object.entries(branches)) {
-        const originalKey = findBranchKey(originalBranches, mapping);
-        if (originalKey != null) {
-          fileCoverage.b[key] = originalB[originalKey];
-        } else {
-          fileCoverage.b[key] = [0];
-        }
-      }
+      const patchedBranches = patchCoverageItems(
+        filePath,
+        branchesPerFile,
+        fileCoverage.branchMap,
+        fileCoverage.b,
+        () => [0],
+      );
+      fileCoverage.branchMap = patchedBranches.itemMap;
+      fileCoverage.b = patchedBranches.itemIndex;
+
+      const patchedFunctions = patchCoverageItems(
+        filePath,
+        functionsPerFile,
+        fileCoverage.fnMap,
+        fileCoverage.f,
+        () => 0,
+      );
+      fileCoverage.fnMap = patchedFunctions.itemMap;
+      fileCoverage.f = patchedFunctions.itemIndex;
     }
   }
 }
@@ -98,7 +143,7 @@ export function getTestCoverage(
   // because we're only working with objects and arrays
   coverages = JSON.parse(JSON.stringify(coverages));
 
-  addingMissingCoverageBranches(coverages);
+  addingMissingCoverageItems(coverages);
 
   for (const coverage of coverages) {
     coverageMap.merge(coverage);