diff --git a/src/compiler/builder.ts b/src/compiler/builder.ts
index 4c2f6258ddf..b5803932c4e 100644
--- a/src/compiler/builder.ts
+++ b/src/compiler/builder.ts
@@ -900,7 +900,7 @@ namespace ts {
         /**
          * Computing hash to for signature verification
          */
-        const computeHash = host.createHash || generateDjb2Hash;
+        const computeHash = maybeBind(host, host.createHash);
         let state = createBuilderProgramState(newProgram, getCanonicalFileName, oldState);
         let backupState: BuilderProgramState | undefined;
         newProgram.getProgramBuildInfo = () => getProgramBuildInfo(state, getCanonicalFileName);
diff --git a/src/compiler/builderState.ts b/src/compiler/builderState.ts
index 69b3f2558e5..72c5c37b233 100644
--- a/src/compiler/builderState.ts
+++ b/src/compiler/builderState.ts
@@ -77,7 +77,7 @@ namespace ts {
         /**
          * Compute the hash to store the shape of the file
          */
-        export type ComputeHash = (data: string) => string;
+        export type ComputeHash = ((data: string) => string) | undefined;
 
         /**
          * Exported modules to from declaration emit being computed.
@@ -337,7 +337,7 @@ namespace ts {
                     emitOutput.outputFiles.length > 0 ? emitOutput.outputFiles[0] : undefined;
                 if (firstDts) {
                     Debug.assert(fileExtensionIs(firstDts.name, Extension.Dts), "File extension for signature expected to be dts", () => `Found: ${getAnyExtensionFromPath(firstDts.name)} for ${firstDts.name}:: All output files: ${JSON.stringify(emitOutput.outputFiles.map(f => f.name))}`);
-                    latestSignature = computeHash(firstDts.text);
+                    latestSignature = (computeHash || generateDjb2Hash)(firstDts.text);
                     if (exportedModulesMapCache && latestSignature !== prevSignature) {
                         updateExportedModules(sourceFile, emitOutput.exportedModulesFromDeclarationEmit, exportedModulesMapCache);
                     }
@@ -521,7 +521,7 @@ namespace ts {
         /**
          * When program emits modular code, gets the files affected by the sourceFile whose shape has changed
          */
-        function getFilesAffectedByUpdatedShapeWhenModuleEmit(state: BuilderState, programOfThisState: Program, sourceFileWithUpdatedShape: SourceFile, cacheToUpdateSignature: ESMap<Path, string>, cancellationToken: CancellationToken | undefined, computeHash: ComputeHash | undefined, exportedModulesMapCache: ComputingExportedModulesMap | undefined) {
+        function getFilesAffectedByUpdatedShapeWhenModuleEmit(state: BuilderState, programOfThisState: Program, sourceFileWithUpdatedShape: SourceFile, cacheToUpdateSignature: ESMap<Path, string>, cancellationToken: CancellationToken | undefined, computeHash: ComputeHash, exportedModulesMapCache: ComputingExportedModulesMap | undefined) {
             if (isFileAffectingGlobalScope(sourceFileWithUpdatedShape)) {
                 return getAllFilesExcludingDefaultLibraryFile(state, programOfThisState, sourceFileWithUpdatedShape);
             }
@@ -544,7 +544,7 @@ namespace ts {
                 if (!seenFileNamesMap.has(currentPath)) {
                     const currentSourceFile = programOfThisState.getSourceFileByPath(currentPath)!;
                     seenFileNamesMap.set(currentPath, currentSourceFile);
-                    if (currentSourceFile && updateShapeSignature(state, programOfThisState, currentSourceFile, cacheToUpdateSignature, cancellationToken, computeHash!, exportedModulesMapCache)) { // TODO: GH#18217
+                    if (currentSourceFile && updateShapeSignature(state, programOfThisState, currentSourceFile, cacheToUpdateSignature, cancellationToken, computeHash, exportedModulesMapCache)) {
                         queue.push(...getReferencedByPaths(state, currentSourceFile.resolvedPath));
                     }
                 }
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 76fa4b62aab..70ea81c35d5 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -73,6 +73,7 @@ namespace ts {
     export function createCompilerHostWorker(options: CompilerOptions, setParentNodes?: boolean, system = sys): CompilerHost {
         const existingDirectories = new Map<string, boolean>();
         const getCanonicalFileName = createGetCanonicalFileName(system.useCaseSensitiveFileNames);
+        const computeHash = maybeBind(system, system.createHash) || generateDjb2Hash;
         function getSourceFile(fileName: string, languageVersion: ScriptTarget, onError?: (message: string) => void): SourceFile | undefined {
             let text: string | undefined;
             try {
@@ -128,7 +129,7 @@ namespace ts {
 
         let outputFingerprints: ESMap<string, OutputFingerprint>;
         function writeFileWorker(fileName: string, data: string, writeByteOrderMark: boolean) {
-            if (!isWatchSet(options) || !system.createHash || !system.getModifiedTime) {
+            if (!isWatchSet(options) || !system.getModifiedTime) {
                 system.writeFile(fileName, data, writeByteOrderMark);
                 return;
             }
@@ -137,7 +138,7 @@ namespace ts {
                 outputFingerprints = new Map<string, OutputFingerprint>();
             }
 
-            const hash = system.createHash(data);
+            const hash = computeHash(data);
             const mtimeBefore = system.getModifiedTime(fileName);
 
             if (mtimeBefore) {
diff --git a/src/compiler/watch.ts b/src/compiler/watch.ts
index 724d57f48b2..422dc13ab22 100644
--- a/src/compiler/watch.ts
+++ b/src/compiler/watch.ts
@@ -345,11 +345,11 @@ namespace ts {
 
     export function setGetSourceFileAsHashVersioned(compilerHost: CompilerHost, host: { createHash?(data: string): string; }) {
         const originalGetSourceFile = compilerHost.getSourceFile;
-        const computeHash = host.createHash || generateDjb2Hash;
+        const computeHash = maybeBind(host, host.createHash) || generateDjb2Hash;
         compilerHost.getSourceFile = (...args) => {
             const result = originalGetSourceFile.call(compilerHost, ...args);
             if (result) {
-                result.version = computeHash.call(host, result.text);
+                result.version = computeHash(result.text);
             }
             return result;
         };
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index d0b4891c963..9c8c198e1b2 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -730,7 +730,6 @@ namespace ts.server {
                 this.syntaxOnly = false;
             }
 
-            Debug.assert(!!this.host.createHash, "'ServerHost.createHash' is required for ProjectService");
             if (this.host.realpath) {
                 this.realpathToScriptInfos = createMultiMap();
             }
diff --git a/src/server/project.ts b/src/server/project.ts
index e3720383543..e7bb3c857d3 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -631,8 +631,16 @@ namespace ts.server {
             }
             updateProjectIfDirty(this);
             this.builderState = BuilderState.create(this.program!, this.projectService.toCanonicalFileName, this.builderState);
-            return mapDefined(BuilderState.getFilesAffectedBy(this.builderState, this.program!, scriptInfo.path, this.cancellationToken, data => this.projectService.host.createHash!(data)), // TODO: GH#18217
-                sourceFile => this.shouldEmitFile(this.projectService.getScriptInfoForPath(sourceFile.path)) ? sourceFile.fileName : undefined);
+            return mapDefined(
+                BuilderState.getFilesAffectedBy(
+                    this.builderState,
+                    this.program!,
+                    scriptInfo.path,
+                    this.cancellationToken,
+                    maybeBind(this.projectService.host, this.projectService.host.createHash)
+                ),
+                sourceFile => this.shouldEmitFile(this.projectService.getScriptInfoForPath(sourceFile.path)) ? sourceFile.fileName : undefined
+            );
         }
 
         /**
@@ -654,7 +662,10 @@ namespace ts.server {
                     const dtsFiles = outputFiles.filter(f => fileExtensionIs(f.name, Extension.Dts));
                     if (dtsFiles.length === 1) {
                         const sourceFile = this.program!.getSourceFile(scriptInfo.fileName)!;
-                        BuilderState.updateSignatureOfFile(this.builderState, this.projectService.host.createHash!(dtsFiles[0].text), sourceFile.resolvedPath);
+                        const signature = this.projectService.host.createHash ?
+                            this.projectService.host.createHash(dtsFiles[0].text) :
+                            generateDjb2Hash(dtsFiles[0].text);
+                        BuilderState.updateSignatureOfFile(this.builderState, signature, sourceFile.resolvedPath);
                     }
                 }
             }