made syntax changes, updated readme, and added more tests

README.md

@@ -388,6 +388,7 @@ const fs = new Filer.FileSystem(options, callback);
 * [fs.removexattr(path, name, callback)](#removexattr)
 * [fs.fremovexattr(fd, name, callback)](#fremovexattr)
 * [fs.watch(filename, [options], [listener])](#watch)
+* [fs.watchFile(filename, [options], [listener])](#watchFile)
 
 #### fs.rename(oldPath, newPath, callback)<a name="rename"></a>
 
@@ -1323,6 +1324,24 @@ var watcher = fs.watch('/data', { recursive: true }, function(event, filename) {
 fs.writeFile('/data/subdir/file', 'data');
 ```
 
+### fs.watchFile(filename, [options], [listener])<a name="watch"></a> 
+
+Watch for changes on a file at `filename`. The callback `listener` will be called each time the file is accessed. 
+
+The `options` argument only supports the change in interval between checks measured in milliseconds and does not support perstistence like node.
+
+The `listener` receives two arguments that are the current stat object and previous stat object that are instances of `fs.Stat`. Reference to `fs.Stat` can be found 
+here: [`fs.Stat`](https://nodejs.org/api/fs.html#fs_class_fs_stats)
+
+Example: 
+```javascript 
+fs.watchFile('/myfile.txt', (curr, prev) => {
+  console.log(`the current mtime is: ${curr.mtime}`);
+  console.log(`the previous mtime was: ${prev.mtime}`);
+});
+```
+
+
 ### FileSystemShell<a name="FileSystemShell"></a>
 
 Many common file system shell operations are available by using a `FileSystemShell` object.

src/filesystem/interface.js

@@ -226,15 +226,16 @@ function FileSystem(options, callback) {
 
     let intervalValue = statWatchers.get(filename);
 
-    // Checks to see if there's a pre-existing watcher on the file
+    if(intervalValue) {
-    if (intervalValue === undefined) {
+      return;
-      // Stores initial prev value to compare 
+    }
+    else { 
       fs.stat(filename, function (err, stats) {
         var value = setInterval(function () {
           prevStat = currStat;
 
           //Conditional check for first run to set initial state for prevStat
-          if(prevStat === undefined) { 
+          if(!prevStat) { 
             prevStat = stats;
           } 
 
@@ -242,21 +243,21 @@ function FileSystem(options, callback) {
 
           if (err) {
             clearInterval(value);
-            console.warn('[Filer Error] fs.watchFile encountered an error: ' + err);
+            console.warn('[Filer Error] fs.watchFile encountered an error' + err.message);
           }
           if (JSON.stringify(prevStat) !== JSON.stringify(currStat)) {
             listener(prevStat, currStat);
           }
           // Set a new prevStat based on previous
           prevStat = currStat;
-        },
+        }, 
         interval
         );
 
         // Stores interval return values
         statWatchers.set(filename, value);
       });
-    }
+    } 
   };
 
   // Deal with various approaches to node ID creation

tests/spec/fs.watchFile.spec.js

@@ -19,7 +19,25 @@ describe('fs.watchFile', function() {
     expect(fn).to.throw();
   });
 
-  it('prev and curr should be populated', function() {
+  it('should throw an error if a file is deleted', function() {
+    const fs = util.fs(); 
+    
+    fs.writeFile('/myfile', 'data', function(error) {
+      if(error) throw error;
+    });  
+
+    fs.watchFile('/myfile', function(prev, curr) {
+      expect(prev).to.exist;
+      expect(curr).to.exist;
+    }); 
+
+    fs.unlink('/myfile'); 
+
+    const fn = () => fs.watchFile('/myfile');
+    expect(fn).to.throw();
+  });
+
+  it('prev and curr should be equal if nothing has been changed in the file', function() {
     const fs = util.fs(); 
 
     fs.writeFile('/myfile', 'data', function(error) {
@@ -27,8 +45,25 @@ describe('fs.watchFile', function() {
     }); 
     
     fs.watchFile('/myfile', function(prev, curr) {
-      expect(prev).to.exist;
+      expect(prev).to.equal(curr);
-      expect(curr).to.exist;
+    });
+  });
+
+  it('prev and curr should not be equal if something has been changed in the file', function() {
+    const fs = util.fs(); 
+
+    fs.writeFile('/myfile', 'data', function(error) {
+      if(error) throw error;
+    }); 
+
+    fs.watchFile('/myfile', function(prev, curr) {
+      expect(prev).to.equal(curr);
+
+      fs.writeFile('/myfile', 'data2', function(error) {
+        if(error) throw error;
+      }); 
+      
+      expect(prev).to.not.equal(curr);
     });
   });
 });