fix: Restored getSource file fallback implementation

src/get-source-file.ts

@@ -0,0 +1,14 @@
+import * as ts from 'typescript';
+
+export function getSourceFile(program: ts.Program, fileName: string, sourceText?: string): ts.SourceFile {
+    if (program !== undefined) {
+        const sourceFile = program.getSourceFile(fileName);
+        if (sourceFile) {
+            return sourceFile;
+        }
+    }
+    if (sourceText === undefined) {
+        throw new Error(`Invalid source file: ${fileName}`);
+    }
+    return ts.createSourceFile(fileName, sourceText, ts.ScriptTarget.ES5, true);
+}

src/index.ts

@@ -1,5 +1,6 @@
 import * as ts from 'typescript';
 import { createProgram } from './create-program';
+import { getSourceFile } from './get-source-file';
 
 type createServiceOptions = {
     configFile: string;
@@ -18,7 +19,7 @@ export function createService({ compilerOptions, configFile }: createServiceOpti
                 program = ts.createProgram(rootFileNames, program.getCompilerOptions(), host, program);
                 sourceFile = program.getSourceFile(fileName);
             }
-            return sourceFile;
+            return sourceFile || getSourceFile(program, fileName, sourceText);
         },
         getDiagnostics: (fileName: string, sourceText?: string) => {
             const sourceFile = api.getSourceFile(fileName, sourceText);

tsconfig.json

@@ -7,7 +7,13 @@
     "declaration": false,
     "outDir": "dist",
     "removeComments": false,
-    "strict": true,
+    "noImplicitReturns": true,
+    "noImplicitAny": true,
+    "strictNullChecks": true,
+    "noUnusedParameters": true,
+    "noUnusedLocals": true,
+    "skipLibCheck": true,
+    "skipDefaultLibCheck": true,
     "sourceMap": true,
     "lib": [
       "esnext"