--- title: JSExport framework: javascriptcore role: symbol role_heading: Protocol path: javascriptcore/jsexport --- # JSExport The protocol for exporting Objective-C objects to JavaScript. ## Declaration ```swift protocol JSExport ``` ## Overview Overview Implement this protocol to export your Objective-C classes and their instance methods, class methods, and properties to JavaScript code. Export Objective-C Objects to JavaScript When you create a JavaScript value from an instance of an Objective-C class, and the JSValue class doesn’t specify a copying convention, JavaScriptCore creates a JavaScript wrapper object. For certain classes, JavaScriptCore automatically copies values to the appropriate JavaScript type; for example, NSString instances become JavaScript strings. JavaScript supports inheritance using a chain of prototype objects. For each Objective-C class you export, JavaScriptCore creates a prototype within the enclosing JavaScript context (a JSContext object). For the NSObject class, the prototype object is the JavaScript context’s Object prototype. For all other Objective-C classes, JavaScriptCore creates a prototype object with an internal [Prototype] property that points to the prototype property it creates for the Objective-C class’s superclass. As such, the prototype chain for a JavaScript wrapper object reflects the wrapped Objective-C type’s inheritance hierarchy. In addition to the prototype object, JavaScriptCore produces a JavaScript constructor object for each Objective-C class. Expose Objective-C Methods and Properties to JavaScript By default, no methods or properties of the Objective-C class have exposure to JavaScript. Instead, you must choose methods and properties to export. For each protocol that a class conforms to, if the protocol incorporates the JSExport protocol, JavaScriptCore interprets that protocol as a list of methods and properties to export to JavaScript. For each instance method you export, JavaScriptCore creates a corresponding JavaScript function as a property of the prototype object. For each Objective-C property you export, JavaScriptCore creates a JavaScript accessor property on the prototype. For each class method you export, JavaScriptCore creates a JavaScript function on the constructor object. The following code example illustrates adoption of the JSExport protocol: @protocol MyPointExports @property double x; @property double y; - (NSString *)description; - (instancetype)initWithX:(double)x y:(double)y; + (MyPoint *)makePointWithX:(double)x y:(double)y; @end @interface MyPoint : NSObject - (void)myPrivateMethod; // This isn't in the MyPointExports protocol, so it isn't visible to JavaScript code. @end @implementation MyPoint // ... @end The following code example illustrates the API that an exported class presents in JavaScript: // Objective-C properties become fields. point.x; point.x = 10; // Objective-C instance methods become functions. point.description(); // Call Objective-C initializers with constructor syntax. var p = MyPoint(1, 2); // Objective-C class methods become functions on the constructor object. var q = MyPoint.makePointWithXY(0, 0); The attributes of an Objective-C @property declaration determine the attributes of the corresponding JavaScript property as follows: If the Objective-C property is readonly, the JavaScript property has the attributes writable: false, enumerable: false, configurable: true. If the Objective-C property is readwrite, the JavaScript property has the attributes writable: true, enumerable: true, configurable: true. Wrapped Objective-C properties, parameters, and return values convert according to the copying convention that the JSValue class specifies for their types. See JSValue for the complete list of copying conventions. note: If a class declares conformance to the JSExport protocol, JavaScriptCore ignores its normal copying conventions for built-in types. For example, if you define a custom NSString subclass that conforms to the JSExport protocol and pass an instance of that class to the valueWithObject: method, the result is a JavaScript wrapper object for the custom class, not a JavaScript string primitive. Customize Export of Objective-C Selectors When exporting a selector that takes one or more arguments, JavaScriptCore generates a corresponding function name using the following conversion: It removes all colons from the selector. It capitalizes any lowercase letter that follows a colon. For example, under the default conversion, the Objective-C selector doX:withY: exports as the JavaScript function doXWithY. note: You can only apply the JSExportAs macro to a selector that takes one or more arguments. To rename a selector that you export to JavaScript, use the JSExportAs macro. For example, to instead export the Objective-C selector doX:withY: as the JavaScript function doX, use the following declaration: @protocol MyClassJavaScriptMethods JSExportAs(doX, - (void)doX:(id)x withY:(id)y ); @end