fix getExtendedConfig in commandLineParser

* remove invalid assertion
* fix invalid array spread on possibly undefined value
* only add unique files to extendedSourceFiles, preventing the array from growing infinitely

Author: ajafff

src/compiler/commandLineParser.ts

@@ -1993,7 +1993,7 @@ namespace ts {
         if (ownConfig.extendedConfigPath) {
             // copy the resolution stack so it is never reused between branches in potential diamond-problem scenarios.
             resolutionStack = resolutionStack.concat([resolvedPath]);
-            const extendedConfig = getExtendedConfig(sourceFile!, ownConfig.extendedConfigPath, host, basePath, resolutionStack, errors);
+            const extendedConfig = getExtendedConfig(sourceFile, ownConfig.extendedConfigPath, host, basePath, resolutionStack, errors);
             if (extendedConfig && isSuccessfulParsedTsconfig(extendedConfig)) {
                 const baseRaw = extendedConfig.raw;
                 const raw = ownConfig.raw;
@@ -2134,7 +2134,7 @@ namespace ts {
     }
 
     function getExtendedConfig(
-        sourceFile: TsConfigSourceFile,
+        sourceFile: TsConfigSourceFile | undefined,
         extendedConfigPath: string,
         host: ParseConfigHost,
         basePath: string,
@@ -2143,7 +2143,12 @@ namespace ts {
     ): ParsedTsconfig | undefined {
         const extendedResult = readJsonConfigFile(extendedConfigPath, path => host.readFile(path));
         if (sourceFile) {
-            (sourceFile.extendedSourceFiles || (sourceFile.extendedSourceFiles = [])).push(extendedResult.fileName);
+            if (sourceFile.extendedSourceFiles) {
+                pushIfUnique(sourceFile.extendedSourceFiles, extendedResult.fileName);
+            }
+            else {
+                sourceFile.extendedSourceFiles = [extendedResult.fileName];
+            }
         }
         if (extendedResult.parseDiagnostics.length) {
             errors.push(...extendedResult.parseDiagnostics);
@@ -2153,8 +2158,10 @@ namespace ts {
         const extendedDirname = getDirectoryPath(extendedConfigPath);
         const extendedConfig = parseConfig(/*json*/ undefined, extendedResult, host, extendedDirname,
             getBaseFileName(extendedConfigPath), resolutionStack, errors);
-        if (sourceFile) {
-            sourceFile.extendedSourceFiles!.push(...extendedResult.extendedSourceFiles!);
+        if (sourceFile && extendedResult.extendedSourceFiles) {
+            for (const extended of extendedResult.extendedSourceFiles) {
+                pushIfUnique(sourceFile.extendedSourceFiles!, extended);
+            }
         }
 
         if (isSuccessfulParsedTsconfig(extendedConfig)) {

src/testRunner/unittests/configurationExtension.ts

@@ -244,6 +244,20 @@ namespace ts {
                 }, [
                     combinePaths(basePath, "main.ts")
                 ]);
+
+                it("adds extendedSourceFiles only once", () => {
+                    const sourceFile = readJsonConfigFile("configs/fourth.json", (path) => host.readFile(path));
+                    const dir = combinePaths(basePath, "configs");
+                    const expected = [
+                        combinePaths(dir, "third.json"),
+                        combinePaths(dir, "second.json"),
+                        combinePaths(dir, "base.json"),
+                    ];
+                    parseJsonSourceFileConfigFileContent(sourceFile, host, dir, {}, "fourth.json");
+                    assert.deepEqual(sourceFile.extendedSourceFiles, expected);
+                    parseJsonSourceFileConfigFileContent(sourceFile, host, dir, {}, "fourth.json");
+                    assert.deepEqual(sourceFile.extendedSourceFiles, expected);
+                });
             });
         });
     });