feat: support promises in patchFs (#217)

Author: aleung

docs/api/patchFs.md

@@ -27,3 +27,14 @@ vol.fromJSON({'/dir/foo': 'bar'});
 patchFs(vol);
 console.log(require('fs').readdirSync('/')); // [ 'dir' ]
 ```
+
+Promises API is supported as well:
+
+```js
+import {vol} from 'memfs';
+import {patchFs} from 'fs-monkey';
+
+vol.fromJSON({'/dir/foo': 'bar'});
+patchFs(vol);
+require('fs').promises.readFile('/dir/foo', 'UTF-8').then(console.log); // bar
+```

src/__tests__/patchFs.test.js

@@ -20,6 +20,29 @@ describe('patchFs', () => {
         expect(fs.F_OK).toBe(vol.F_OK);
     });
 
+    it('should patch promises', () => {
+        const vol = {
+            get promises() {
+                return {
+                    readFile: () => 'foo'
+                }
+            }
+        };
+        const fs = {
+            get promises() {
+                return {
+                    readFile: () => 'bar'
+                }
+            }
+        };
+
+        const unpatch = patchFs(vol, fs);
+        expect(fs.promises.readFile()).toBe('foo');
+        
+        unpatch();
+        expect(fs.promises.readFile()).toBe('bar');
+    })
+
     describe('unpatch()', () => {
         it('should return "unpatch" method', () => {
             const vol = {

src/patchFs.js

@@ -48,8 +48,24 @@ export default function patchFs(vol, fs = require('fs')) {
         if(typeof vol[method] === 'function')
             patchMethod(method);
 
+    // Promises API
+    let promisesBackup;
+    try {
+        promisesBackup = fs.promises;
+        Object.defineProperty(fs, 'promises', {
+            get: () => vol.promises
+        });
+    } catch {
+        undefined;
+    }
+
     // Give user back a method to revert the changes.
     return function unpatch () {
         for (const key in bkp) fs[key] = bkp[key];
+        if (promisesBackup) {
+            Object.defineProperty(fs, 'promises', {
+                get: () => promisesBackup
+            });
+        }
     };
 };