Refactor how we (internally) expose JS module resolution

Also, provide a useful error if resolution fails.

Author: Andy Hanson

src/compiler/moduleNameResolver.ts

@@ -2,7 +2,6 @@
 /// <reference path="diagnosticInformationMap.generated.ts" />
 
 namespace ts {
-
     /* @internal */
     export function trace(host: ModuleResolutionHost, message: DiagnosticMessage, ...args: any[]): void;
     export function trace(host: ModuleResolutionHost): void {
@@ -15,6 +14,7 @@ namespace ts {
     }
 
     /** Array that is only intended to be pushed to, never read. */
+    /* @internal */
     export interface Push<T> {
         push(value: T): void;
     }
@@ -675,12 +675,25 @@ namespace ts {
     }
 
     export function nodeModuleNameResolver(moduleName: string, containingFile: string, compilerOptions: CompilerOptions, host: ModuleResolutionHost, cache?: ModuleResolutionCache): ResolvedModuleWithFailedLookupLocations {
-        return nodeModuleNameResolverWorker(moduleName, containingFile, compilerOptions, host, cache, /*jsOnly*/ false);
+        return nodeModuleNameResolverWorker(moduleName, getDirectoryPath(containingFile), compilerOptions, host, cache, /*jsOnly*/ false);
     }
 
+    /**
+     * Expose resolution logic to allow us to use Node module resolution logic from arbitrary locations.
+     * No way to do this with `require()`: https://github.com/nodejs/node/issues/5963
+     * Throws an error if the module can't be resolved.
+     */
     /* @internal */
-    export function nodeModuleNameResolverWorker(moduleName: string, containingFile: string, compilerOptions: CompilerOptions, host: ModuleResolutionHost, cache?: ModuleResolutionCache, jsOnly = false): ResolvedModuleWithFailedLookupLocations {
-        const containingDirectory = getDirectoryPath(containingFile);
+    export function resolveJavaScriptModule(moduleName: string, initialDir: string, host: ModuleResolutionHost): string {
+        const { resolvedModule, failedLookupLocations } =
+            nodeModuleNameResolverWorker(moduleName, initialDir, { moduleResolution: ts.ModuleResolutionKind.NodeJs, allowJs: true }, host, /*cache*/ undefined, /*jsOnly*/ true);
+        if (!resolvedModule) {
+            throw new Error(`Could not resolve JS module ${moduleName} starting at ${initialDir}. Looked in: ${failedLookupLocations.join(", ")}`);
+        }
+        return resolvedModule.resolvedFileName;
+    }
+
+    function nodeModuleNameResolverWorker(moduleName: string, containingDirectory: string, compilerOptions: CompilerOptions, host: ModuleResolutionHost, cache: ModuleResolutionCache | undefined, jsOnly: boolean): ResolvedModuleWithFailedLookupLocations {
         const traceEnabled = isTraceEnabled(compilerOptions, host);
 
         const failedLookupLocations: string[] = [];

src/server/server.ts

@@ -702,12 +702,11 @@ namespace ts.server {
     }
 
     sys.require = (initialDir: string, moduleName: string): RequireResult => {
-        const result = nodeModuleNameResolverWorker(moduleName, initialDir + "/program.ts", { moduleResolution: ts.ModuleResolutionKind.NodeJs, allowJs: true }, sys, undefined, /*jsOnly*/ true);
         try {
-            return { module: require(result.resolvedModule.resolvedFileName), error: undefined };
+            return { module: require(resolveJavaScriptModule(moduleName, initialDir, sys)), error: undefined };
         }
-        catch (e) {
-            return { module: undefined, error: e };
+        catch (error) {
+            return { module: undefined, error };
         }
     };