执行javascript脚本

UIWebView的实例方法：

- (NSString * _Nullable)stringByEvaluatingJavaScriptFromString:(NSString * _Nonnull)script  返回执行javascript脚本的结果

需要注意的是：JavaScript allocations are limited to 10 MB. The web view raises an exception if you exceed this limit on the total memory allocation for JavaScript.

实例：　　

　　获取当前的url

NSString *currentURL = [webView stringByEvaluatingJavaScriptFromString:@"document.location.href"];

JavaScriptCore