Allow setValueForKeyPath to create missing intermediate paths.

Author: robertjustjones

lib/underscore-keypath.js

@@ -212,16 +212,32 @@
 
 		setValueForKeyPath : function (obj, keypath, newValue) {
 			"use strict";
+
+			var createTarget = function( obj, segments ) {
+				var i, target;
+				for( i = 0; i < segments.length; i += 1 ) {
+					if( underscore.hasKeyPath( obj, segments.slice( 0, i + 1 ) ) ) {
+						continue;
+					}
+					if( i === 0 ) {
+						setProperty(obj, segments[ 0 ], {} );
+						continue;
+					}
+					target = underscore.valueForKeyPath( obj, segments.slice( 0, i ) );
+					setProperty(target, segments[ i ], {} );
+				}
+			};
+
 			var segments = toSegments(keypath), lastPropertyName, target;
 
 			lastPropertyName = segments.pop();
-			target = underscore(obj).valueForKeyPath(segments.join("."));
-
-			if (target !== null && target !== undefined) {
-				return setProperty(target, lastPropertyName, newValue);
+			target = underscore(obj).valueForKeyPath(segments);
+			
+			if (target === null || target === undefined) {
+				createTarget( obj, segments );
+				target = underscore(obj).valueForKeyPath(segments);
 			}
-
-			return undefined;
+			return setProperty(target, lastPropertyName, newValue);
 		},
 
 		pluckByKeyPath : function (array, keypath) {

test/setValueForKeyPath.js

@@ -40,4 +40,39 @@ describe("setValueForKeyPath", function () {
 			fixture.bar._age.should.be.exactly(99);
 		});
 	});
+
+  describe("set property when target missing", function () {
+		it("new path two deep must work", function () {
+			_(fixture).setValueForKeyPath("a.b", 2)
+				.should.be.exactly(2);
+			fixture.a.b.should.be.exactly(2);
+		});
+
+		it("new path three deep must work", function () {
+			_(fixture).setValueForKeyPath("c.d.e", 3)
+				.should.be.exactly(3);
+			fixture.c.d.e.should.be.exactly(3);
+		});
+
+		it("new path four deep must work", function () {
+			_(fixture).setValueForKeyPath("f.g.h.i", 4)
+				.should.be.exactly(4);
+			fixture.f.g.h.i.should.be.exactly(4);
+		});
+
+		it("new path three deep, down one must work", function () {
+			_(fixture).setValueForKeyPath("a.m.n.o", 5)
+				.should.be.exactly(5);
+			fixture.a.m.n.o.should.be.exactly(5);
+		});
+
+		it("new path three deep, down two must work", function () {
+			_(fixture).setValueForKeyPath("a.b.p.q.r", 6)
+				.should.be.exactly(6);
+			fixture.a.b.p.q.r.should.be.exactly(6);
+		});
+
+	});
+
+  
 });