Optimize buffer generate first and all edge cases

lib/handlebars/compiler/javascript-compiler.js

@@ -140,42 +140,27 @@ JavaScriptCompiler.prototype = {
   },
 
   preamble: function() {
-    var out = [];
-
-    if (!this.environment.isSimple) {
-      out.push(", buffer = " + this.initializeBuffer());
-    } else {
-      out.push("");
-    }
-
     // track the last context pushed into place to allow skipping the
     // getContext opcode when it would be a noop
     this.lastContext = 0;
-    this.source = out;
+    this.source = [];
   },
 
   createFunctionContext: function(asObject) {
-    var locals = this.stackVars.concat(this.registers.list);
+    var varDeclarations = '';
 
+    var locals = this.stackVars.concat(this.registers.list);
     if(locals.length > 0) {
-      this.source[0] += ", " + locals.join(", ");
+      varDeclarations += ", " + locals.join(", ");
     }
 
     // Generate minimizer alias mappings
     for (var alias in this.aliases) {
       if (this.aliases.hasOwnProperty(alias)) {
-        this.source[0] += ', ' + alias + '=' + this.aliases[alias];
+        varDeclarations += ', ' + alias + '=' + this.aliases[alias];
       }
     }
 
-    if (this.source[0]) {
-      this.source[0] = "var " + this.source[0].substring(2) + ";";
-    }
-
-    if (!this.environment.isSimple) {
-      this.pushSource("return buffer;");
-    }
-
     var params = ["depth0", "helpers", "partials", "data"];
 
     for(var i=0, l=this.environment.depths.list.length; i<l; i++) {
@@ -183,7 +168,7 @@ JavaScriptCompiler.prototype = {
     }
 
     // Perform a second pass over the output to merge content when possible
-    var source = this.mergeSource();
+    var source = this.mergeSource(varDeclarations);
 
     if (asObject) {
       params.push(source);
@@ -193,11 +178,12 @@ JavaScriptCompiler.prototype = {
       return 'function(' + params.join(',') + ') {\n  ' + source + '}';
     }
   },
-  mergeSource: function() {
-    // WARN: We are not handling the case where buffer is still populated as the source should
-    // not have buffer append operations as their final action.
+  mergeSource: function(varDeclarations) {
     var source = '',
-        buffer;
+        buffer,
+        appendOnly = !this.forceBuffer,
+        appendFirst;
+
     for (var i = 0, len = this.source.length; i < len; i++) {
       var line = this.source[i];
       if (line.appendToBuffer) {
@@ -208,12 +194,39 @@ JavaScriptCompiler.prototype = {
         }
       } else {
         if (buffer) {
-          source += 'buffer += ' + buffer + ';\n  ';
+          if (!source) {
+            appendFirst = true;
+            source = buffer + ';\n  ';
+          } else {
+            source += 'buffer += ' + buffer + ';\n  ';
+          }
           buffer = undefined;
         }
         source += line + '\n  ';
+
+        if (!this.environment.isSimple) {
+          appendOnly = false;
+        }
       }
     }
+
+    if (appendOnly) {
+      if (buffer || !source) {
+        source += 'return ' + (buffer || '""') + ';\n';
+      }
+    } else {
+      varDeclarations += ", buffer = " + (appendFirst ? '' : this.initializeBuffer());
+      if (buffer) {
+        source += 'return buffer + ' + buffer + ';\n';
+      } else {
+        source += 'return buffer;\n';
+      }
+    }
+
+    if (varDeclarations) {
+      source = 'var ' + varDeclarations.substring(2) + (appendFirst ? '' : ';\n  ') + source;
+    }
+
     return source;
   },
 

spec/expected/empty.amd.js

@@ -1,7 +1,6 @@
 define(['handlebars.runtime'], function(Handlebars) {
   Handlebars = Handlebars["default"];  var template = Handlebars.template, templates = Handlebars.templates = Handlebars.templates || {};
 return templates['empty'] = template({"compiler":[4,">= 1.0.0"],"main":function(depth0,helpers,partials,data) {
-  var buffer = "";
-  return buffer;
-  },"useData":true});
+  return "";
+},"useData":true});
 });

spec/javascript-compiler.js

@@ -45,10 +45,12 @@ describe('javascript-compiler api', function() {
   describe('buffer', function() {
     var $superAppend, $superCreate;
     beforeEach(function() {
+      handlebarsEnv.JavaScriptCompiler.prototype.forceBuffer = true;
       $superAppend = handlebarsEnv.JavaScriptCompiler.prototype.appendToBuffer;
       $superCreate = handlebarsEnv.JavaScriptCompiler.prototype.initializeBuffer;
     });
     afterEach(function() {
+      handlebarsEnv.JavaScriptCompiler.prototype.forceBuffer = false;
       handlebarsEnv.JavaScriptCompiler.prototype.appendToBuffer = $superAppend;
       handlebarsEnv.JavaScriptCompiler.prototype.initializeBuffer = $superCreate;
     });