assign process.env.JEST_WORKER_ID=1 when in runInBand mode

CHANGELOG.md

@@ -37,6 +37,8 @@
 
 ### Fixes
 
+* `[jest-runner]` Assign `process.env.JEST_WORKER_ID="1"` when in runInBand mode
+  ([#5860](https://github.com/facebook/jest/pull/5860))
 * `[jest-cli]` Add descriptive error message when trying to use
   `globalSetup`/`globalTeardown` file that doesn't export a function.
   ([#5835](https://github.com/facebook/jest/pull/5835))

packages/jest-runner/src/__tests__/test_runner.test.js

@@ -86,3 +86,22 @@ test('does not inject the rawModuleMap in serial mode', () => {
       ]);
     });
 });
+
+test('assign process.env.JEST_WORKER_ID = 1 when in runInBand mode', () => {
+  const globalConfig = {maxWorkers: 1, watch: false};
+  const config = {rootDir: '/path/'};
+  const context = {config};
+
+  return new TestRunner(globalConfig)
+    .runTests(
+      [{context, path: './file.test.js'}],
+      new TestWatcher({isWatchMode: globalConfig.watch}),
+      () => {},
+      () => {},
+      () => {},
+      {serial: true},
+    )
+    .then(() => {
+      expect(process.env.JEST_WORKER_ID).toBe('1');
+    });
+});

packages/jest-runner/src/index.js

@@ -61,6 +61,7 @@ class TestRunner {
     onResult: OnTestSuccess,
     onFailure: OnTestFailure,
   ) {
+    process.env.JEST_WORKER_ID = '1';
     const mutex = throat(1);
     return tests.reduce(
       (promise, test) =>