WKWebView -> evaluateJavaScript

- (void)webView:(WKWebView *)webView didFinishNavigation:(null_unspecified WKNavigation *)navigation{
    [webView evaluateJavaScript:@"document.title" completionHandler:^(NSString * _Nullable title, NSError * _Nullable error) {
        self.navigationItem.title = title;
    }];
}

UIWebView -> stringByEvaluatingJavaScriptFromString

- (void)webViewDidFinishLoad:(UIWebView *)webView{   
    self.navigationItem.title = [webView stringByEvaluatingJavaScriptFromString:@"document.title"];
}

WKWebView可以通过category方法实现stringByEvaluatingJavaScriptFromString，代码如下

@interface WKWebView(SynchronousEvaluateJavaScript)
- (NSString *)stringByEvaluatingJavaScriptFromString:(NSString *)script;
@end

@implementation WKWebView(SynchronousEvaluateJavaScript)

- (NSString *)stringByEvaluatingJavaScriptFromString:(NSString *)script
{
    __block NSString *resultString = nil;
    __block BOOL finished = NO;

    [self evaluateJavaScript:script completionHandler:^(id result, NSError *error) {
        if (error == nil) {
            if (result != nil) {
                resultString = [NSString stringWithFormat:@"%@", result];
            }
        } else {
            NSLog(@"evaluateJavaScript error : %@", error.localizedDescription);
        }
        finished = YES;
    }];

    while (!finished)
    {
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    }

    return resultString;
}
@end

//具体使用
NSString *userAgent = [_webView stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"];

NSLog(@"userAgent: %@", userAgent);

JSContext -> evaluateScript

只有UIWebView可以拿到JSContext，WKWebView不能

- (void)webViewDidFinishLoad:(UIWebView *)webView{
    //获取到JS的context
    JSContext *jsContext = [self.webView_1 valueForKeyPath:@"documentView.webView.mainFrame.javaScriptContext"];
    //设置JS执行报错捕获
    [jsContext setExceptionHandler:^(JSContext *context, JSValue *exception){
        NSLog(@"%@", exception);
    }];
    
    JSValue *value = [jsContext evaluateScript:@"document.title"];
    self.navigationItem.title = value.toString;
}