chore: transformers must return objects, not strings

CHANGELOG.md

@@ -26,6 +26,7 @@
 - `[jest-transform]` Show enhanced `SyntaxError` message for all `SyntaxError`s ([#10749](https://github.com/facebook/jest/pull/10749))
 - `[jest-transform]` [**BREAKING**] Refactor API to pass an options bag around rather than multiple boolean options ([#10753](https://github.com/facebook/jest/pull/10753))
 - `[jest-transform]` [**BREAKING**] Refactor API of transformers to pass an options bag rather than separate `config` and other options
+- `[jest-transform]` [**BREAKING**] All transformers must now return `{code: string, map?: SourceMap | string | null}`. Returning a string is no longer supported ([#10773](https://github.com/facebook/jest/pull/10773))
 - `[pretty-format]` [**BREAKING**] Convert to ES Modules ([#10515](https://github.com/facebook/jest/pull/10515))
 
 ### Chore & Maintenance

packages/babel-jest/src/index.ts

@@ -155,7 +155,7 @@ const createTransformer: CreateTransformer = userOptions => {
         }
       }
 
-      return sourceText;
+      return {code: sourceText};
     },
   };
 };

packages/jest-repl/src/cli/repl.ts

@@ -38,10 +38,7 @@ const evalCommand: repl.REPLEval = (
           supportsTopLevelAwait: false,
         },
       );
-      cmd =
-        typeof transformResult === 'string'
-          ? transformResult
-          : transformResult.code;
+      cmd = transformResult.code;
     }
     result = runInThisContext(cmd);
   } catch (e) {

packages/jest-transform/src/ScriptTransformer.ts

@@ -204,10 +204,7 @@ export default class ScriptTransformer {
     canMapToInput: boolean,
     options: ReducedTransformOptions,
   ): TransformedSource {
-    const inputCode = typeof input === 'string' ? input : input.code;
-    const inputMap = typeof input === 'string' ? null : input.map;
-
-    const result = babelTransform(inputCode, {
+    const result = babelTransform(input.code, {
       auxiliaryCommentBefore: ' istanbul ignore next ',
       babelrc: false,
       caller: {
@@ -228,15 +225,15 @@ export default class ScriptTransformer {
             cwd: this._config.rootDir,
             exclude: [],
             extension: false,
-            inputSourceMap: inputMap,
+            inputSourceMap: input.map,
             useInlineSourceMaps: false,
           },
         ],
       ],
       sourceMaps: canMapToInput ? 'both' : false,
     });
 
-    if (result && result.code) {
+    if (result?.code) {
       return result as TransformResult;
     }
 
@@ -292,14 +289,13 @@ export default class ScriptTransformer {
         configString: this._cache.configString,
       });
 
-      if (typeof processed === 'string') {
-        transformed.code = processed;
-      } else if (processed != null && typeof processed.code === 'string') {
+      if (processed != null && typeof processed.code === 'string') {
         transformed = processed;
       } else {
         throw new TypeError(
-          "Jest: a transform's `process` function must return a string, " +
-            'or an object with `code` key containing this string.',
+          "Jest: a transform's `process` function must return an object with " +
+            '`code` key containing a string and an optional `map` key with ' +
+            'the source map.',
         );
       }
     }
@@ -344,9 +340,8 @@ export default class ScriptTransformer {
         options,
       );
 
-      code =
-        typeof instrumented === 'string' ? instrumented : instrumented.code;
-      map = typeof instrumented === 'string' ? null : instrumented.map;
+      code = instrumented.code;
+      map = instrumented.map;
     } else {
       code = transformed.code;
     }

packages/jest-transform/src/types.ts

@@ -31,10 +31,10 @@ interface FixedRawSourceMap extends Omit<RawSourceMap, 'version'> {
   version: number;
 }
 
-// TODO: For Jest 26 normalize this (always structured data, never a string)
-export type TransformedSource =
-  | {code: string; map?: FixedRawSourceMap | string | null}
-  | string;
+export type TransformedSource = {
+  code: string;
+  map?: FixedRawSourceMap | string | null;
+};
 
 export type TransformResult = TransformTypes.TransformResult;