Cache the theme file to avoid redundant reads

Thanks to @alessioalex for the suggestion in #3!

index.js

@@ -5,21 +5,8 @@ var juice = require('juice');
 // Package constructor
 function Mailgen(options) {
     // Set options as instance members
-    this.theme = options.theme;
     this.product = options.product;
-
-    // No theme provided - use default
-    if (!this.theme) {
-        this.theme = 'default';
-    }
-
-    // Build path to theme template
-    this.templatePath = __dirname + '/themes/' + this.theme + '.html';
-
-    // Bad theme?
-    if (!fs.existsSync(this.templatePath)) {
-        throw new Error('You have specified an invalid theme.');
-    }
+    this.theme = options.theme || 'default';
 
     // No product?
     if (!this.product || typeof this.product !== 'object') {
@@ -30,6 +17,17 @@ function Mailgen(options) {
     if (!this.product.name || !this.product.link) {
         throw new Error('Please provide the product name and link.');
     }
+
+    // Build path to theme's template file
+    var templatePath = __dirname + '/themes/' + this.theme + '.html';
+
+    // Bad path?
+    if (!fs.existsSync(templatePath)) {
+        throw new Error('You have specified an invalid theme.');
+    }
+
+    // Load template (sync) and cache it for later
+    this.template = fs.readFileSync(templatePath, 'utf8');
 }
 
 // HTML e-mail generator
@@ -47,9 +45,6 @@ Mailgen.prototype.generate = function (params) {
         throw new Error('Please provide the `body` parameter as an object.');
     }
 
-    // Load it (sync)
-    var output = fs.readFileSync(this.templatePath, 'utf8');
-
     // Prepare data to be passed to ejs engine
     var templateData = {
         product: this.product
@@ -59,7 +54,10 @@ Mailgen.prototype.generate = function (params) {
     for (var k in body) {
         templateData[k] = body[k];
     }
-
+
+    // Fetch cached theme HTML
+    var output = this.template;
+
     // Render the template with ejs
     output = ejs.render(output, templateData);