Merge pull request #879 from LinusU/async-bug

Backport #874 to v1.x

lib/canvas.js

@@ -247,11 +247,6 @@ Canvas.prototype.toDataURL = function(a1, a2, a3){
   // ['image/jpeg', qual, fn] -> ['image/jpeg', {quality: qual}, fn]
   // ['image/jpeg', undefined, fn] -> ['image/jpeg', null, fn]
 
-  if (this.width === 0 || this.height === 0) {
-    // Per spec, if the bitmap has no pixels, return this string:
-    return "data:,";
-  }
-
   var type = 'image/png';
   var opts = {};
   var fn;
@@ -280,6 +275,17 @@ Canvas.prototype.toDataURL = function(a1, a2, a3){
     }
   }
 
+  if (this.width === 0 || this.height === 0) {
+    // Per spec, if the bitmap has no pixels, return this string:
+    var str = "data:,";
+    if (fn) {
+      setTimeout(function() {
+        fn(null, str);
+      });
+    }
+    return str;
+  }
+
   if ('image/png' === type) {
     if (fn) {
       this.toBuffer(function(err, buf){

test/canvas.test.js

@@ -526,6 +526,14 @@ describe('Canvas', function () {
       });
     });
 
+    it('toDataURL(function (err, str) {...}) is async even with no canvas data', function (done) {
+      new Canvas().toDataURL(function(err, str){
+        assert.ifError(err);
+        assert.ok('data:,' === str);
+        done();
+      });
+    });
+
     it('toDataURL(0.5, function (err, str) {...}) works and defaults to PNG', function (done) {
       new Canvas(200,200).toDataURL(0.5, function(err, str){
         assert.ifError(err);