Add a new compiler option moduleSuffixes to expand the node module resolver's search algorithm (#48189)

* Add moduleSuffixes compiler option and related tests. Update baselines for compiler options tests.

* Add a flag to the command-line parser which allows "list" params to preserve "falsy" values such as empty strings. Falsy values are normally stripped out.

* Add tests. Rework resolver logic to only run module-suffix code when needed.

* PR feedback

* Add test

* Remove unnecessary conditional.

Author: afoxman

src/compiler/commandLineParser.ts

@@ -884,6 +884,18 @@ namespace ts {
             description: Diagnostics.Allow_accessing_UMD_globals_from_modules,
             defaultValueDescription: false,
         },
+        {
+            name: "moduleSuffixes",
+            type: "list",
+            element: {
+                name: "suffix",
+                type: "string",
+            },
+            listPreserveFalsyValues: true,
+            affectsModuleResolution: true,
+            category: Diagnostics.Modules,
+            description: Diagnostics.List_of_file_name_suffixes_to_search_when_resolving_a_module,
+        },
 
         // Source Maps
         {
@@ -3192,7 +3204,7 @@ namespace ts {
         if (option.type === "list") {
             const listOption = option;
             if (listOption.element.isFilePath || !isString(listOption.element.type)) {
-                return filter(map(value, v => normalizeOptionValue(listOption.element, basePath, v)), v => !!v) as CompilerOptionsValue;
+                return filter(map(value, v => normalizeOptionValue(listOption.element, basePath, v)), v => listOption.listPreserveFalsyValues ? true : !!v) as CompilerOptionsValue;
             }
             return value;
         }
@@ -3233,7 +3245,7 @@ namespace ts {
     }
 
     function convertJsonOptionOfListType(option: CommandLineOptionOfListType, values: readonly any[], basePath: string, errors: Push<Diagnostic>): any[] {
-        return filter(map(values, v => convertJsonOption(option.element, v, basePath, errors)), v => !!v);
+        return filter(map(values, v => convertJsonOption(option.element, v, basePath, errors)), v => option.listPreserveFalsyValues ? true : !!v);
     }
 
     /**

src/compiler/diagnosticMessages.json

@@ -5878,6 +5878,10 @@
         "category": "Message",
         "code": 6930
     },
+    "List of file name suffixes to search when resolving a module." : {
+      "category": "Error",
+      "code": 6931
+    },
 
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",

src/compiler/moduleNameResolver.ts

@@ -1576,6 +1576,16 @@ namespace ts {
 
     /** Return the file if it exists. */
     function tryFile(fileName: string, onlyRecordFailures: boolean, state: ModuleResolutionState): string | undefined {
+        if (!state.compilerOptions.moduleSuffixes?.length) {
+            return tryFileLookup(fileName, onlyRecordFailures, state);
+        }
+
+        const ext = tryGetExtensionFromPath(fileName) ?? "";
+        const fileNameNoExtension = ext ? removeExtension(fileName, ext) : fileName;
+        return forEach(state.compilerOptions.moduleSuffixes, suffix => tryFileLookup(fileNameNoExtension + suffix + ext, onlyRecordFailures, state));
+    }
+
+    function tryFileLookup(fileName: string, onlyRecordFailures: boolean, state: ModuleResolutionState): string | undefined {
         if (!onlyRecordFailures) {
             if (state.host.fileExists(fileName)) {
                 if (state.traceEnabled) {

src/compiler/types.ts

@@ -6149,6 +6149,7 @@ namespace ts {
         maxNodeModuleJsDepth?: number;
         module?: ModuleKind;
         moduleResolution?: ModuleResolutionKind;
+        moduleSuffixes?: string[];
         moduleDetection?: ModuleDetectionKind;
         newLine?: NewLineKind;
         noEmit?: boolean;
@@ -6453,6 +6454,7 @@ namespace ts {
     export interface CommandLineOptionOfListType extends CommandLineOptionBase {
         type: "list";
         element: CommandLineOptionOfCustomType | CommandLineOptionOfStringType | CommandLineOptionOfNumberType | CommandLineOptionOfBooleanType | TsConfigOnlyOption;
+        listPreserveFalsyValues?: boolean;
     }
 
     /* @internal */

src/testRunner/unittests/config/convertCompilerOptionsFromJson.ts

@@ -420,6 +420,70 @@ namespace ts {
             );
         });
 
+        it("Convert empty string option of moduleSuffixes to compiler-options ", () => {
+            assertCompilerOptions(
+                {
+                    compilerOptions: {
+                        moduleSuffixes: [".ios", ""]
+                    }
+                }, "tsconfig.json",
+                {
+                    compilerOptions: {
+                        moduleSuffixes: [".ios", ""]
+                    },
+                    errors: []
+                }
+            );
+        });
+
+        it("Convert empty string option of moduleSuffixes to compiler-options ", () => {
+            assertCompilerOptions(
+                {
+                    compilerOptions: {
+                        moduleSuffixes: [""]
+                    }
+                }, "tsconfig.json",
+                {
+                    compilerOptions: {
+                        moduleSuffixes: [""]
+                    },
+                    errors: []
+                }
+            );
+        });
+
+        it("Convert trailing-whitespace string option of moduleSuffixes to compiler-options ", () => {
+            assertCompilerOptions(
+                {
+                    compilerOptions: {
+                        moduleSuffixes: ["   "]
+                    }
+                }, "tsconfig.json",
+                {
+                    compilerOptions: {
+                        moduleSuffixes: ["   "]
+                    },
+                    errors: []
+                }
+            );
+        });
+
+        it("Convert empty option of moduleSuffixes to compiler-options ", () => {
+            assertCompilerOptions(
+                {
+                    compilerOptions: {
+                        moduleSuffixes: []
+                    }
+                }, "tsconfig.json",
+                {
+                    compilerOptions: {
+                        moduleSuffixes: []
+                    },
+                    errors: []
+                }
+            );
+        });
+
         it("Convert incorrectly format tsconfig.json to compiler-options ", () => {
             assertCompilerOptions(
                 {

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -2977,6 +2977,7 @@ declare namespace ts {
         maxNodeModuleJsDepth?: number;
         module?: ModuleKind;
         moduleResolution?: ModuleResolutionKind;
+        moduleSuffixes?: string[];
         moduleDetection?: ModuleDetectionKind;
         newLine?: NewLineKind;
         noEmit?: boolean;

tests/baselines/reference/api/typescript.d.ts

@@ -2977,6 +2977,7 @@ declare namespace ts {
         maxNodeModuleJsDepth?: number;
         module?: ModuleKind;
         moduleResolution?: ModuleResolutionKind;
+        moduleSuffixes?: string[];
         moduleDetection?: ModuleDetectionKind;
         newLine?: NewLineKind;
         noEmit?: boolean;

tests/baselines/reference/moduleResolutionWithSuffixes_empty.js

@@ -0,0 +1,17 @@
+//// [tests/cases/compiler/moduleResolutionWithSuffixes_empty.ts] ////
+
+//// [index.ts]
+import { base } from "./foo";
+//// [foo.ts]
+export function base() {}
+
+
+//// [foo.js]
+"use strict";
+exports.__esModule = true;
+exports.base = void 0;
+function base() { }
+exports.base = base;
+//// [index.js]
+"use strict";
+exports.__esModule = true;

tests/baselines/reference/moduleResolutionWithSuffixes_empty.symbols

@@ -0,0 +1,8 @@
+=== /index.ts ===
+import { base } from "./foo";
+>base : Symbol(base, Decl(index.ts, 0, 8))
+
+=== /foo.ts ===
+export function base() {}
+>base : Symbol(base, Decl(foo.ts, 0, 0))
+

tests/baselines/reference/moduleResolutionWithSuffixes_empty.trace.json

@@ -0,0 +1,7 @@
+[
+    "======== Resolving module './foo' from '/index.ts'. ========",
+    "Explicitly specified module resolution kind: 'NodeJs'.",
+    "Loading module as file / folder, candidate module location '/foo', target file type 'TypeScript'.",
+    "File '/foo.ts' exist - use it as a name resolution result.",
+    "======== Module name './foo' was successfully resolved to '/foo.ts'. ========"
+]