Fix: getExampleFilename should use default example if file does not exist (styleguidist#1362)

Closes styleguidist#1338

Author: mendrew

package-lock.json

@@ -126,6 +126,7 @@
     "jest-serializer-html": "^6.0.0",
     "keymirror": "^0.1.1",
     "lint-staged": "^8.1.5",
+    "memfs": "^2.15.2",
     "prettier": "1.16.4",
     "raf": "^3.4.1",
     "react": "^16.8.5",

package.json

@@ -1,18 +1,34 @@
 import deabsDeep from 'deabsdeep';
+import { vol } from 'memfs';
 import getExamples from '../getExamples';
 
+jest.mock('fs', () => {
+	return require('memfs').fs;
+});
+
 const file = '../pizza.js';
 const displayName = 'Pizza';
 const examplesFile = './Pizza.md';
 const defaultExample = './Default.md';
 
+afterEach(vol.reset.bind(vol));
+
 test('require an example file if component has example file', () => {
+	vol.fromJSON({ [examplesFile]: 'pizza' });
+
 	const result = getExamples(file, displayName, examplesFile);
 	expect(deabsDeep(result).require).toMatchInlineSnapshot(
 		`"!!~/src/loaders/examples-loader.js?displayName=Pizza&file=.%2F..%2Fpizza.js&shouldShowDefaultExample=false!./Pizza.md"`
 	);
 });
 
+test('require default example file if component has no example in the file system', () => {
+	const result = getExamples(file, displayName, examplesFile, defaultExample);
+	expect(deabsDeep(result).require).toMatchInlineSnapshot(
+		`"!!~/src/loaders/examples-loader.js?displayName=Pizza&file=.%2F..%2Fpizza.js&shouldShowDefaultExample=false!./Default.md"`
+	);
+});
+
 test('require default example has no example file', () => {
 	const result = getExamples(file, displayName, false, defaultExample);
 	expect(deabsDeep(result).require).toMatchInlineSnapshot(

src/loaders/utils/__tests__/getExamples.spec.js

@@ -1,5 +1,6 @@
 // @flow
 import path from 'path';
+import fs from 'fs';
 import { encode } from 'qss';
 import requireIt from './requireIt';
 
@@ -14,7 +15,7 @@ module.exports = function getExamples(
 	examplesFile: string,
 	defaultExample: string
 ): {} | null {
-	const examplesFileToLoad = examplesFile || defaultExample;
+	const examplesFileToLoad = (fs.existsSync(examplesFile) && examplesFile) || defaultExample;
 	if (!examplesFileToLoad) {
 		return null;
 	}