accept custom id generators

Author: JonghunYu

README.md

@@ -107,6 +107,18 @@ or a custom ID
 var db = new Store("data",{saveId:'myKey'});
 ```
 
+### custom ID generator
+We use uuid v4 for ID generation if you don't pass an id when save a data. If you want, you can pass custom generator.
+```javascript
+var i = 0;
+var db = new Store("data",{
+  idGenerator: function() {
+    i = i + 1;
+    return i;
+  }
+});
+```
+
 
 ## Tests
 

Store.es6.js

@@ -102,7 +102,7 @@ const save = function(id, o, cb) {
     id = null;
   }
   if (typeof id !== "string") {
-    id = uuid.v4();
+    id = this._idGenerator();
   }
   const file = this._getFileName(id);
   o = clone(o);
@@ -243,6 +243,7 @@ class Store {
     this._pretty = opt.pretty === true;
     this._memory = opt.memory === true || opt.type === 'memory';
     this._saveId = opt.saveId;
+    this._idGenerator = typeof opt.idGenerator === "function" ? opt.idGenerator : uuid.v4;
 
     if (isJSONFile(this.name)) {
       this.name = this.name.split(".json")[0];

Store.spec.es6.js

@@ -14,7 +14,7 @@ describe("The jfs module", () => {
     fs.unlink(NAME + '.json', (err) => {
       exec("rm -rf ./" + NAME, (err, out) => {
         console.log(out);
-        if (err != null) {
+        if (err !== null) {
           console.error(err);
         }
         done();
@@ -84,6 +84,19 @@ describe("The jfs module", () => {
       });
     });
 
+    it("can autosave the id with a custom generator", (done) => {
+      const store = new Store(NAME, {
+        idGenerator: () => "customId",
+        saveId: true
+      });
+      store.save({}, (err, id) => {
+        store.get(id, (err, o) => {
+          o.id.should.equal("customId");
+          done();
+        });
+      });
+    });
+
     it("can save an object synchronously", () => {
       const store = new Store(NAME);
       const data = {