Fix #12273. Don't call easing functions for duration 0 animations. Close gh-895.

src/effects.js

@@ -376,7 +376,13 @@ Tween.prototype = {
 		var eased,
 			hooks = Tween.propHooks[ this.prop ];
 
-		this.pos = eased = jQuery.easing[ this.easing ]( percent, this.options.duration * percent, 0, 1, this.options.duration );
+		if ( this.options.duration ) {
+			this.pos = eased = jQuery.easing[ this.easing ](
+				percent, this.options.duration * percent, 0, 1, this.options.duration
+			);
+		} else {
+			this.pos = eased = percent;
+		}
 		this.now = ( this.end - this.start ) * eased + this.start;
 
 		if ( this.options.step ) {

test/unit/effects.js

@@ -1811,4 +1811,22 @@ test( "Animate properly sets overflow hidden when animating width/height (#12117
 	});
 });
 
+test( "Animations with 0 duration don't ease (#12273)", 1, function() {
+	jQuery.easing.test = function() {
+		ok( false, "Called easing" );
+	};
+
+	jQuery( "#foo" ).animate({
+		height: 100
+	}, {
+		duration: 0,
+		easing: "test",
+		complete: function() {
+			equal( jQuery( this ).height(), 100, "Height is 100" );
+		}
+	});
+
+	delete jQuery.easing.test;
+});
+
 } // if ( jQuery.fx )