Getting "A JavaScript exception occurred" When evaluateJavaScript on WKWebView in Objective-C

Question

Here I am using Reference swift project to load a PDF file in PDFViewer in WKWebview.

Reference Project - https://github.com/Minitex/PDFJSTest/blob/master/PDFJSTest/ViewController.swift

Here in the reference project have a code look like this for load PDF file

func openPDFInViewer(myURL: URL)
{
    let pdf = NSData(contentsOf: myURL)

    let length = pdf?.length
    var myArray = [UInt8](repeating: 0, count: length!)
    pdf?.getBytes(&myArray, length: length!)

    webView?.evaluateJavaScript("PDFViewerApplication.open(new Uint8Array(\(myArray)))", completionHandler: { result, error in
        print("Completed Javascript evaluation.")
        print("Result: \(String(describing: result))")
        print("Error: \(String(describing: error))")
   })
}

And I tried to convert this function into Objective-C language and it looks like this:

-(void)renderPDF:(NSTimer*)theTimer
{
    NSMutableDictionary *dictData = [[NSMutableDictionary alloc] initWithDictionary:theTimer.userInfo];
    NSURL *requestURL =  (NSURL *)[dictData valueForKey:@"request_url"];

    NSData *data = [NSData dataWithContentsOfURL:requestURL];
    NSUInteger len = [data length];
    uint8_t myArray[len];
    [data getBytes:&myArray length:len];

    NSString *strForEvaluate = [NSString stringWithFormat:@"PDFViewerApplication.open(new Uint8Array('%s'));",myArray];


    [wkWebView evaluateJavaScript:strForEvaluate completionHandler:^(id Result, NSError * _Nullable error) {
        if (error)
        {
            NSLog(@"This is error....%@",error.description);
        }
        else if(Result)
        {
            NSLog(@"%@",Result);
        }
     }];
}

When I tried to run the app and evaluateJavaScript function that time getting error response like this

Error Domain=WKErrorDomain Code=4 "A JavaScript exception occurred" UserInfo={WKJavaScriptExceptionLineNumber=1, WKJavaScriptExceptionMessage=SyntaxError: Unexpected EOF, WKJavaScriptExceptionColumnNumber=0, WKJavaScriptExceptionSourceURL=file:///Users/sapanaranipa/Library/Developer/CoreSimulator/Devices/207FA841-3256-45D2-8698-2B769A72A2F4/data/Containers/Bundle/Application/4BD7E620-DF16-429B-BBC2-2A36BA2A2DE8/SureFox.app//PDF_Web/pdf.js-dist/web/viewer.html, NSLocalizedDescription=A JavaScript exception occurred}

Please help me, Thanks in Advance.

Answer 1

You treat an arbitrary byte array as a C-string and that might not be null-terminated or contain stuff like ' which you don't want in your JavaScript. So it may as well crash when you try to create your string.

The problematic part is using the %s formatter here:

NSString *strForEvaluate = [NSString stringWithFormat:@"PDFViewerApplication.open(new Uint8Array('%s'));",myArray];

(Also, uint8_t myArray[len]; may cause a stack overflow [ha!] for large data.)

What you want instead is to create a string that represents a JavaScript array of numbers, like [25, 243, 0, 123]. Here's a way that might work (untested):

NSMutableArray<NSString *> *bytes = [NSMutableArray array];
const uint8_t *rawBytes = data.bytes;
for (NSUInteger i = 0; i < data.length; i++) {
    [bytes addObject:[NSString stringWithFormat:@"%d", (int)rawBytes[i]]];
}

NSString *javaScriptArray = [bytes componentsJoinedByString:@","];

NSString *strForEvaluate = [NSString stringWithFormat:
    @"PDFViewerApplication.open(new Uint8Array([%@]));", javaScriptArray];