$.extend: added support for extending deeply

README.md

@@ -890,10 +890,11 @@ $.each ( object, callback ) // => object
 
 #### $.extend ()
 
-Extends target object with properties from the source object.
+Extends target object with properties from the source object, potentially deeply too.
 
 ```js
 $.extend ( target, source ) // => object
+$.extend ( true, target, source ) // => object
 ```
 
 #### $.isArray ()

src/core/extend.ts

@@ -1,29 +1,45 @@
 
 // @require ./cash.ts
+// @require ./type_checking.ts
 
 interface CashStatic {
   extend (): any;
+  extend ( deep: true, target: any, ...sources: any[] ): any;
   extend ( target: any ): typeof cash;
-  extend ( target: any, ...objs: any[] ): any;
+  extend ( target: any, ...sources: any[] ): any;
 }
 
 interface Cash {
   extend ( plugins: Record<any, any> ): this;
 }
 
-function extend ( target?: any, ...objs: any[] ) {
+function extend ( ...sources: any[] ) {
 
-  const length = arguments.length;
+  const deep = isBoolean ( sources[0] ) ? sources.shift () : false,
+        target = sources.shift (),
+        length = sources.length;
 
-  if ( !length ) return {};
+  if ( !target ) return {};
 
-  if ( length === 1 ) return extend ( cash, target );
+  if ( !length ) return extend ( deep, cash, target );
 
-  for ( let i = 1; i < length; i++ ) {
+  for ( let i = 0; i < length; i++ ) {
 
-    for ( const key in arguments[i] ) {
+    const source = sources[i];
 
-      target[key] = arguments[i][key];
+    for ( const key in source ) {
+
+      if ( deep && ( isArray ( source[key] ) || isPlainObject ( source[key] ) ) ) {
+
+        if ( !target[key] || target[key].constructor !== source[key].constructor ) target[key] = new source[key].constructor ();
+
+        extend ( deep, target[key], source[key] );
+
+      } else {
+
+        target[key] = source[key];
+
+      }
 
     }
 

src/core/type_checking.ts

@@ -34,6 +34,12 @@ function isElement ( x: any ): x is HTMLElement {
 
 }
 
+function isBoolean ( x: any ): x is boolean {
+
+  return typeof x === 'boolean';
+
+}
+
 function isFunction ( x: any ): x is Function {
 
   return typeof x === 'function';

test/modules/utilities.js

@@ -73,6 +73,25 @@ describe ( 'Utilities', function () {
 
     });
 
+    it ( 'can extend deeply', function ( t ) {
+
+      var result1 = $.extend ( true, { foo: 1 }, { bar: { inner: 2 } } );
+      t.deepEqual ( result1, { foo: 1, bar: { inner: 2 } } );
+
+      var result2 = $.extend ( true, { foo: 1 }, { bar: [1, 2] } );
+      t.deepEqual ( result2, { foo: 1, bar: [1, 2] } );
+
+      var result3 = $.extend ( true, {}, { foo: 1 }, { bar: { inner: 2, baz: { inner2: 3 } } } );
+      t.deepEqual ( result3, { foo: 1, bar: { inner: 2, baz: { inner2: 3 } } } );
+
+      var result4 = $.extend ( true, {}, { foo: 1, bar: { baz: 3 } }, { bar: { baz: 4 } } );
+      t.deepEqual ( result4, { foo: 1, bar: { baz: 4 } } );
+
+      var result5 = $.extend ( true, {}, { foo: 1, bar: { baz: 3 } }, { bar: { baz: 4 } }, { bar: { qux: 5 }} );
+      t.deepEqual ( result5, { foo: 1, bar: { baz: 4, qux: 5 } } );
+
+    });
+
     it ( 'works also with either zero or one object(s)', function ( t ) {
 
       t.deepEqual ( $.extend (), {} );