Adding support for promises. Closes #379, #382 (#380)

* adding promise-based filedescriptor

* fixing promisify dep

* promisifying shell

* deprecating 'exists' method on fs

* adding docs

* fixing docs

* removing redundant code.

.gitignore

@@ -3,6 +3,7 @@ bower_components
 .env
 *~
 dist/filer-issue225.js
+.vscode
 
 # Parcel build dirs
 .cache

README.md

@@ -80,6 +80,7 @@ instance.  By default, a new [IndexedDB](https://developer.mozilla.org/en/docs/I
 database is created for each file system. The file system can also use other
 backend storage providers, for example [WebSQL](http://en.wikipedia.org/wiki/Web_SQL_Database)
 or even RAM (i.e., for temporary storage). See the section on [Storage Providers](#providers).
+<a name="overviewExample"></a>
 
 ```javascript
 var fs = new Filer.FileSystem();
@@ -110,6 +111,21 @@ Like node.js, callbacks for methods that accept them are optional but suggested
 you omit the callback, errors will be thrown as exceptions). The first callback parameter is
 reserved for passing errors. It will be `null` if no errors occurred and should always be checked.
 
+#### Support for Promises
+The Promise based API mimics the way Node [implements](https://nodejs.org/api/fs.html#fs_fs_promises_api) them. Both `Shell` and `FileSystem` now have a `promises` object attached alongside the regular callback style methods. Method names are identical to their callback counterparts with the difference that instead of receiving a final argument as a callback, they return a Promise that is resolved or rejected based on the success of method execution.
+> Please note that `exists` method will always throw, since it was [deprecated](https://nodejs.org/api/fs.html#fs_fs_exists_path_callback) by Node.
+
+See example below:
+
+```javascript
+const fs = new Filer.FileSystem().promises;
+fs.open('/myfile', 'w+')
+  .then(fd => fs.close(fd))
+  .then(() => fs.stat('/myfile'))
+  .then(stats => { console.log(`stats: ${JSON.stringify(stats)}`); })
+  .catch(err => { console.error(err); })
+```
+> The callback style usage could be found [here](#overviewExample).
 #### Filer.FileSystem(options, callback) constructor
 
 File system constructor, invoked to open an existing file system or create a new one.

package.json

@@ -35,6 +35,7 @@
   },
   "dependencies": {
     "base64-arraybuffer": "^0.1.5",
+    "es6-promisify": "^6.0.0",
     "minimatch": "^3.0.4"
   },
   "devDependencies": {

src/filesystem/implementation.js

@@ -1901,6 +1901,7 @@ function exists(fs, context, path, callback) {
   function cb(err) {
     callback(err ? false : true);
   }
+  console.warn('This method is deprecated. For more details see https://nodejs.org/api/fs.html#fs_fs_exists_path_callback');// eslint-disable-line no-console
   stat(fs, context, path, cb);
 }
 

src/filesystem/interface.js

@@ -1,4 +1,5 @@
 var _ = require('../../lib/nodash.js');
+var { promisify } = require('es6-promisify');
 
 var isNullPath = require('../path.js').isNull;
 var nop = require('../shared.js').nop;
@@ -262,11 +263,7 @@ function FileSystem(options, callback) {
       impl.ensureRootDirectory(context, complete);
     }
   });
-}
-
-// Expose storage providers on FileSystem constructor
-FileSystem.providers = providers;
-
+  FileSystem.prototype.promises = {};
   /**
    * Public API for FileSystem
   */
@@ -350,6 +347,13 @@ FileSystem.providers = providers;
         callback(error);
       }
     };
+    
+    FileSystem.prototype.promises[methodName] = promisify(FileSystem.prototype[methodName].bind(fs));
   });
 
+}
+
+// Expose storage providers on FileSystem constructor
+FileSystem.providers = providers;
+
 module.exports = FileSystem;

src/shell/shell.js

@@ -1,3 +1,4 @@
+var {promisify} = require('es6-promisify');
 var Path = require('../path.js');
 var Errors = require('../errors.js');
 var Environment = require('./environment.js');
@@ -56,6 +57,24 @@ function Shell(fs, options) {
   this.pwd = function() {
     return cwd;
   };
+
+  this.promises = {};
+  /**
+  * Public API for Shell converted to Promise based
+  */
+  [
+    'cd',
+    'exec',
+    'touch',
+    'cat',
+    'ls',
+    'rm',
+    'tempDir',
+    'mkdirp',
+    'find'
+  ].forEach((methodName)=>{
+    this.promises[methodName] = promisify(this[methodName].bind(this));
+  });
 }
 
 /**