Add option preventCast.

.gitignore

@@ -1,2 +1,3 @@
 .DS_Store
 node_modules
+npm-debug.log

.npmignore

@@ -1,2 +1,3 @@
 test
 node_modules
+npm-debug.log

lib/express-csv.js

@@ -29,6 +29,12 @@ exports.version = package.version;
 
 exports.separator = ',';
 
+/**
+ * Prevent Excel's casting.
+ */
+
+exports.preventCast = false;
+
 /**
  * Escape CSV field
  *
@@ -38,6 +44,9 @@ exports.separator = ',';
  */
 
 function escape(field) {
+  if (exports.preventCast) {
+    return '"="' + field.toString().replace(/\"/g, '""') + '""';
+  }
   return '"' + field.toString().replace(/\"/g, '""') + '"';
 }
 

test/index.js

@@ -20,12 +20,18 @@ describe('express-csv', function() {
   it('should expose .separator', function() {
     csv.separator.should.be.a('string');
   });
+
+  it('should expose .preventCast', function() {
+    csv.preventCast.should.be.a('boolean');
+  });
 
   it('should extend http.ServerResponse.prototype.csv', function() {
     require('http').ServerResponse.prototype.csv.should.be.a('function');
   });
+});
 
-  it('should return csv', function(done) {
+describe('res.csv()', function() {
+  it('should response csv', function(done) {
     request 
       .get('http://127.0.0.1:8383/test/1')
       .end(function(res) {
@@ -34,7 +40,7 @@ describe('express-csv', function() {
       });
   });
 
-  it('should return tsv', function(done) {
+  it('should response tsv', function(done) {
     var prevSeparator = csv.separator;
     csv.separator = '\t';
     request
@@ -45,4 +51,16 @@ describe('express-csv', function() {
         done();
       });
   });
+
+  it('should response casted csv', function(done) {
+    var prevSetting = csv.preventCast;
+    csv.preventCast = true;
+    request
+      .get('http://127.0.0.1:8383/test/1')
+      .end(function(res) {
+        csv.preventCast = prevSetting;
+        res.text.should.equal('"="a"","="b"","="c""\r\n"="d"","="e"","="f""\r\n');
+        done();
+      });
+  });
 });