Fix options.moduleNameMapper override order with preset (jestjs#3565) (…

…jestjs#3689)

* Fix options.moduleNameMapper override order with preset (jestjs#3565)

Adjusted the order so that the options mappings are checked first and then the preset mappings are checked.
The preset mappings can be overridden by the options mappings.

* Fix prettier linting issues

* Fix eslint issues

* Workaround in test for eslint sort-keys rule

packages/jest-config/src/__tests__/normalize.test.js

@@ -827,14 +827,40 @@ describe('preset', () => {
       {},
     );
 
-    expect(options.moduleNameMapper).toEqual([['b', 'b'], ['a', 'a']]);
+    expect(options.moduleNameMapper).toEqual([['a', 'a'], ['b', 'b']]);
     expect(options.modulePathIgnorePatterns).toEqual(['b', 'a']);
     expect(options.setupFiles.sort()).toEqual([
       '/node_modules/a',
       '/node_modules/b',
       '/node_modules/regenerator-runtime/runtime',
     ]);
   });
+
+  test('merges with options and moduleNameMapper preset is overridden by options', () => {
+    // Object initializer not used for properties as a workaround for 
+    //  sort-keys eslint rule while specifing properties in 
+    //  non-alphabetical order for a better test
+    const moduleNameMapper = {};
+    moduleNameMapper.e = 'ee';
+    moduleNameMapper.b = 'bb';
+    moduleNameMapper.c = 'cc';
+    moduleNameMapper.a = 'aa';
+    const {options} = normalize(
+      {
+        moduleNameMapper,
+        preset: 'react-native',
+        rootDir: '/root/path/foo',
+      },
+      {},
+    );
+
+    expect(options.moduleNameMapper).toEqual([
+      ['e', 'ee'],
+      ['b', 'bb'],
+      ['c', 'cc'],
+      ['a', 'aa'],
+    ]);
+  });
 });
 
 describe('preset without setupFiles', () => {

packages/jest-config/src/normalize.js

@@ -76,6 +76,7 @@ const setupPreset = (
   if (options.moduleNameMapper && preset.moduleNameMapper) {
     options.moduleNameMapper = Object.assign(
       {},
+      options.moduleNameMapper,
       preset.moduleNameMapper,
       options.moduleNameMapper,
     );