Dimensions: allow modification of coordinates argument

Fixes gh-1848
Closes gh-1853

2 files changed, 15 insertions, 1 deletions

diff --git a/src/offset.js b/src/offset.js
index aee95efde..69e571f7f 100644
--- a/src/offset.js
+++ b/src/offset.js
@@ -51,7 +51,9 @@ jQuery.offset = {
 		}
 
 		if ( jQuery.isFunction( options ) ) {
-			options = options.call( elem, i, curOffset );
+
+			// Use jQuery.extend here to allow modification of coordinates argument (gh-1848)
+			options = options.call( elem, i, jQuery.extend( {}, curOffset ) );
 		}
 
 		if ( options.top != null ) {
diff --git a/test/unit/dimensions.js b/test/unit/dimensions.js
index 8d51c6cde..7d37e13bc 100644
--- a/test/unit/dimensions.js
+++ b/test/unit/dimensions.js
@@ -455,4 +455,16 @@ testIframe( "dimensions/documentLarge", "window vs. large document", function( j
 	ok( jQuery( document ).width() > jQuery( window ).width(), "document width is larger than window width" );
 });
 
+test( "allow modification of coordinates argument (gh-1848)", 1, function() {
+	var element = jQuery( "<div/>" ).appendTo( "#qunit-fixture" );
+
+	element.offset(function( index, coords ) {
+		coords.top = 100;
+
+		return coords;
+	});
+
+	equal( element.offset().top, 100, "coordinates are modified" );
+});
+
 })();