Make .val(undefined) == .val("") and chainable; fixes #4130.

Ensure .val(null) sets an empty string on IE6/7; fixes #5163.

src/attributes.js

@@ -136,7 +136,7 @@ jQuery.fn.extend({
 	},
 
 	val: function( value ) {
-		if ( value === undefined ) {
+		if ( !arguments.length ) {
 			var elem = this[0];
 
 			if ( elem ) {
@@ -209,9 +209,10 @@ jQuery.fn.extend({
 				val = value.call(this, i, self.val());
 			}
 
-			// Typecast each time if the value is a Function and the appended
-			// value is therefore different each time.
-			if ( typeof val === "number" ) {
+			// Treat null/undefined as ""; convert numbers to string
+			if ( val == null ) {
+				val = "";
+			} else if ( typeof val === "number" ) {
 				val += "";
 			}
 

test/unit/attributes.js

@@ -359,14 +359,20 @@ test("val()", function() {
 });
 
 var testVal = function(valueObj) {
-	expect(6);
+	expect(8);
 
 	jQuery("#text1").val(valueObj( 'test' ));
 	equals( document.getElementById('text1').value, "test", "Check for modified (via val(String)) value of input element" );
 
+	jQuery("#text1").val(valueObj( undefined ));
+	equals( document.getElementById('text1').value, "", "Check for modified (via val(undefined)) value of input element" );
+
 	jQuery("#text1").val(valueObj( 67 ));
 	equals( document.getElementById('text1').value, "67", "Check for modified (via val(Number)) value of input element" );
 
+	jQuery("#text1").val(valueObj( null ));
+	equals( document.getElementById('text1').value, "", "Check for modified (via val(null)) value of input element" );
+
 	jQuery("#select1").val(valueObj( "3" ));
 	equals( jQuery("#select1").val(), "3", "Check for modified (via val(String)) value of select element" );