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diff --git a/lib/fs/index.js b/lib/fs/index.js
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+var Q = require('q');
+var _ = require('lodash');
+var path = require('path');
+var Buffer = require('buffer').Buffer;
+var destroy = require('destroy');
+
+/*
+A filesystem is an interface to read/write files
+GitBook can works with a virtual filesystem, for example in the browser.
+*/
+
+// .readdir return files/folder as a list of string, folder ending with '/'
+function pathIsFolder(filename) {
+    return _.last(filename) == '/' || _.last(filename) == '\\';
+}
+
+
+function FS() {
+
+}
+
+// Check if a file exists, run a Promise(true) if that's the case, Promise(false) otherwise
+FS.prototype.exists = function(filename) {
+    // To implement for each fs
+};
+
+// Read a file and returns a promise with the content as a buffer
+FS.prototype.read = function(filename) {
+    // To implement for each fs
+};
+
+// Write a file and returns a promise
+FS.prototype.write = function(filename, buffer) {
+    // To implement for each fs
+};
+
+// List files/directories in a directory
+FS.prototype.readdir = function(folder) {
+    // To implement for each fs
+};
+
+
+// These methods don't require to be redefined, by default it uses .exists, .read, .write, .list
+// For optmization, it can be redefined:
+
+// List files in a directory
+FS.prototype.listFiles = function(folder) {
+    return this.readdir(folder)
+    .then(function(files) {
+        return _.reject(files, pathIsFolder);
+    });
+};
+
+// List all files in the fs
+FS.prototype.listAllFiles = function(folder) {
+    var that = this;
+
+    return this.readdir(folder)
+    .then(function(files) {
+        return _.reduce(files, function(prev, file) {
+            return prev.then(function(output) {
+                var isDirectory = pathIsFolder(file);
+
+                if (!isDirectory) {
+                    output.push(file);
+                    return output;
+                } else {
+                    return that.listAllFiles(path.join(folder, file))
+                    .then(function(files) {
+                        return output.concat(_.map(files, function(_file) {
+                            return path.join(file, _file);
+                        }));
+                    });
+                }
+            });
+        }, Q([]));
+    });
+};
+
+// Read a file as a string (utf-8)
+FS.prototype.readAsString = function(filename) {
+    return this.read(filename)
+    .then(function(buf) {
+        return buf.toString('utf-8');
+    });
+};
+
+// Write a stream to a file and returns a promise
+FS.prototype.writeStream = function(filename, stream) {
+    var bufs = [];
+    var d = Q.defer();
+
+    var cleanup = function() {
+        destroy(stream);
+        stream.removeAllListeners();
+    };
+
+    stream.on('data', function(d) {
+        bufs.push(d);
+    });
+
+    stream.on('error', function(err) {
+        cleanup();
+
+        d.reject(err);
+    });
+
+    stream.on('end', function(){
+        cleanup();
+
+        var buf = Buffer.concat(bufs);
+        d.resolve(buf);
+    });
+
+    return d.promise;
+};
+
+// Copy a file
+FS.prototype.copy = function(from, to) {
+    var that = this;
+
+    return this.read(from)
+    .then(function(buf) {
+        return that.write(to, buf);
+    });
+};
+
+// Find a file in a folder (case incensitive)
+// Return the real filename
+FS.prototype.findFile = function findFile(root, filename) {
+    return this.listFiles(root)
+    .then(function(files) {
+        return _.find(files, function(file) {
+            return (file.toLowerCase() == filename.toLowerCase());
+        });
+    });
+};
+
+// Load a JSON file
+// By default, fs only supports JSON
+FS.prototype.loadAsObject = function(filename) {
+    return this.readAsString(filename)
+    .then(function(str) {
+        return JSON.parse(str);
+    });
+};
+
+module.exports = FS;