Allow a path mapping to provide a file extension

src/compiler/core.ts

@@ -2210,6 +2210,17 @@ namespace ts {
      * Path must have a valid extension.
      */
     export function extensionFromPath(path: string): Extension {
+        const ts = tryGetTypeScriptExtensionFromPath(path);
+        if (ts !== undefined) {
+            return ts;
+        }
+        const js = tryGetJavaScriptExtensionFromPath(path);
+        if (js !== undefined) {
+            return js;
+        }
+        Debug.fail(`File ${path} has unknown extension.`);
+    }
+    export function tryGetTypeScriptExtensionFromPath(path: string): Extension | undefined {
         if (fileExtensionIs(path, ".d.ts")) {
             return Extension.Dts;
         }
@@ -2219,13 +2230,13 @@ namespace ts {
         if (fileExtensionIs(path, ".tsx")) {
             return Extension.Tsx;
         }
+    }
+    function tryGetJavaScriptExtensionFromPath(path: string): Extension | undefined {
         if (fileExtensionIs(path, ".js")) {
             return Extension.Js;
         }
         if (fileExtensionIs(path, ".jsx")) {
             return Extension.Jsx;
         }
-        Debug.fail(`File ${path} has unknown extension.`);
-        return Extension.Js;
     }
 }

src/compiler/moduleNameResolver.ts

@@ -514,18 +514,21 @@ namespace ts {
             if (state.traceEnabled) {
                 trace(state.host, Diagnostics.Module_name_0_matched_pattern_1, moduleName, matchedPatternText);
             }
-            for (const subst of state.compilerOptions.paths[matchedPatternText]) {
+            return forEach(state.compilerOptions.paths[matchedPatternText], subst => {
                 const path = matchedStar ? subst.replace("*", matchedStar) : subst;
                 const candidate = normalizePath(combinePaths(state.compilerOptions.baseUrl, path));
                 if (state.traceEnabled) {
                     trace(state.host, Diagnostics.Trying_substitution_0_candidate_module_location_Colon_1, subst, path);
                 }
-                const resolved = loader(extensions, candidate, failedLookupLocations, !directoryProbablyExists(getDirectoryPath(candidate), state.host), state);
-                if (resolved) {
-                    return resolved;
+                // A path mapping may have a ".ts" extension; in contrast to an import, which should omit it.
+                const tsExtension = tryGetTypeScriptExtensionFromPath(candidate);
+                if (tsExtension !== undefined) {
+                    const path = tryFile(candidate, failedLookupLocations, /*onlyRecordFailures*/false, state);
+                    return path && { path, extension: tsExtension };
                 }
-            }
-            return undefined;
+
+                return loader(extensions, candidate, failedLookupLocations, !directoryProbablyExists(getDirectoryPath(candidate), state.host), state);
+            });
         }
         else {
             const candidate = normalizePath(combinePaths(state.compilerOptions.baseUrl, moduleName));

tests/baselines/reference/pathMappingBasedModuleResolution_withExtension.js

@@ -0,0 +1,16 @@
+//// [tests/cases/compiler/pathMappingBasedModuleResolution_withExtension.ts] ////
+
+//// [foo.ts]
+
+export function foo() {}
+
+//// [a.ts]
+import { foo } from "foo";
+
+
+//// [foo.js]
+"use strict";
+function foo() { }
+exports.foo = foo;
+//// [a.js]
+"use strict";

tests/baselines/reference/pathMappingBasedModuleResolution_withExtension.symbols

@@ -0,0 +1,9 @@
+=== /a.ts ===
+import { foo } from "foo";
+>foo : Symbol(foo, Decl(a.ts, 0, 8))
+
+=== /foo/foo.ts ===
+
+export function foo() {}
+>foo : Symbol(foo, Decl(foo.ts, 0, 0))
+

tests/baselines/reference/pathMappingBasedModuleResolution_withExtension.trace.json

@@ -0,0 +1,11 @@
+[
+    "======== Resolving module 'foo' from '/a.ts'. ========",
+    "Module resolution kind is not specified, using 'NodeJs'.",
+    "'baseUrl' option is set to '/', using this value to resolve non-relative module name 'foo'",
+    "'paths' option is specified, looking for a pattern to match module name 'foo'.",
+    "Module name 'foo', matched pattern 'foo'.",
+    "Trying substitution 'foo/foo.ts', candidate module location: 'foo/foo.ts'.",
+    "File '/foo/foo.ts' exist - use it as a name resolution result.",
+    "Resolving real path for '/foo/foo.ts', result '/foo/foo.ts'",
+    "======== Module name 'foo' was successfully resolved to '/foo/foo.ts'. ========"
+]

tests/baselines/reference/pathMappingBasedModuleResolution_withExtension.types

@@ -0,0 +1,9 @@
+=== /a.ts ===
+import { foo } from "foo";
+>foo : () => void
+
+=== /foo/foo.ts ===
+
+export function foo() {}
+>foo : () => void
+

tests/baselines/reference/pathMappingBasedModuleResolution_withExtension_failedLookup.errors.txt

@@ -0,0 +1,9 @@
+/a.ts(2,21): error TS2307: Cannot find module 'foo'.
+
+
+==== /a.ts (1 errors) ====
+
+    import { foo } from "foo";
+                        ~~~~~
+!!! error TS2307: Cannot find module 'foo'.
+

tests/baselines/reference/pathMappingBasedModuleResolution_withExtension_failedLookup.js

@@ -0,0 +1,7 @@
+//// [a.ts]
+
+import { foo } from "foo";
+
+
+//// [a.js]
+"use strict";

tests/baselines/reference/pathMappingBasedModuleResolution_withExtension_failedLookup.trace.json

@@ -0,0 +1,41 @@
+[
+    "======== Resolving module 'foo' from '/a.ts'. ========",
+    "Module resolution kind is not specified, using 'NodeJs'.",
+    "'baseUrl' option is set to '/', using this value to resolve non-relative module name 'foo'",
+    "'paths' option is specified, looking for a pattern to match module name 'foo'.",
+    "Module name 'foo', matched pattern 'foo'.",
+    "Trying substitution 'foo/foo.ts', candidate module location: 'foo/foo.ts'.",
+    "File '/foo/foo.ts' does not exist.",
+    "Loading module 'foo' from 'node_modules' folder.",
+    "File '/node_modules/foo.ts' does not exist.",
+    "File '/node_modules/foo.tsx' does not exist.",
+    "File '/node_modules/foo.d.ts' does not exist.",
+    "File '/node_modules/foo/package.json' does not exist.",
+    "File '/node_modules/foo/index.ts' does not exist.",
+    "File '/node_modules/foo/index.tsx' does not exist.",
+    "File '/node_modules/foo/index.d.ts' does not exist.",
+    "File '/node_modules/@types/foo.ts' does not exist.",
+    "File '/node_modules/@types/foo.tsx' does not exist.",
+    "File '/node_modules/@types/foo.d.ts' does not exist.",
+    "File '/node_modules/@types/foo/package.json' does not exist.",
+    "File '/node_modules/@types/foo/index.ts' does not exist.",
+    "File '/node_modules/@types/foo/index.tsx' does not exist.",
+    "File '/node_modules/@types/foo/index.d.ts' does not exist.",
+    "'baseUrl' option is set to '/', using this value to resolve non-relative module name 'foo'",
+    "'paths' option is specified, looking for a pattern to match module name 'foo'.",
+    "Module name 'foo', matched pattern 'foo'.",
+    "Trying substitution 'foo/foo.ts', candidate module location: 'foo/foo.ts'.",
+    "File '/foo/foo.ts' does not exist.",
+    "Loading module 'foo' from 'node_modules' folder.",
+    "File '/node_modules/foo.js' does not exist.",
+    "File '/node_modules/foo.jsx' does not exist.",
+    "File '/node_modules/foo/package.json' does not exist.",
+    "File '/node_modules/foo/index.js' does not exist.",
+    "File '/node_modules/foo/index.jsx' does not exist.",
+    "File '/node_modules/@types/foo.js' does not exist.",
+    "File '/node_modules/@types/foo.jsx' does not exist.",
+    "File '/node_modules/@types/foo/package.json' does not exist.",
+    "File '/node_modules/@types/foo/index.js' does not exist.",
+    "File '/node_modules/@types/foo/index.jsx' does not exist.",
+    "======== Module name 'foo' was not resolved. ========"
+]

tests/cases/compiler/pathMappingBasedModuleResolution_withExtension.ts

@@ -0,0 +1,18 @@
+// @noImplicitReferences: true
+// @traceResolution: true
+
+// @Filename: /foo/foo.ts
+export function foo() {}
+
+// @Filename: /a.ts
+import { foo } from "foo";
+
+// @Filename: /tsconfig.json
+{
+    "compilerOptions": {
+        "baseUrl": ".",
+        "paths": {
+            "foo": ["foo/foo.ts"]
+        }
+    }
+}

tests/cases/compiler/pathMappingBasedModuleResolution_withExtension_failedLookup.ts

@@ -0,0 +1,15 @@
+// @noImplicitReferences: true
+// @traceResolution: true
+
+// @Filename: /a.ts
+import { foo } from "foo";
+
+// @Filename: /tsconfig.json
+{
+    "compilerOptions": {
+        "baseUrl": ".",
+        "paths": {
+            "foo": ["foo/foo.ts"]
+        }
+    }
+}