JavaScriptCore console.log

Question

I've put together a very simple program that uses JavaScriptCore to evaluate JS:

#import <CoreFoundation/CoreFoundation.h>
#import <JavaScriptCore/JavaScriptCore.h>

int main(int argc, const char * argv[])
{
    JSGlobalContextRef ctx = JSGlobalContextCreate(NULL);

    FILE *f = fopen(argv[1],"r");
    char * buffer = malloc(10000000);
    fread(buffer,1,10000000,f);

    CFStringRef strs = CFStringCreateWithCString(NULL, buffer, kCFStringEncodingASCII);

    JSStringRef jsstr = JSStringCreateWithCFString(strs);
    JSValueRef result = JSEvaluateScript(ctx, jsstr, NULL, NULL, 0, NULL);

    double res  = JSValueToNumber(ctx, result, NULL);
    JSGlobalContextRelease(ctx);

    printf("%lf\n", res);
    return 0;
}

The idea here is that the last value is expected to be a Number, and that value is printed. This works for valid javascript code, such as

var square = function(x) { return x*x; }; square(4)

However, if the code tries to perform a console.log, the program segfaults. Is there a log function available in JSC or do I have to roll my own?

Answer 1

You do have to provide your own console log if using the JavaScriptCore framework from Mac or IOS.

Here is some code that worked for me (sorry it is Objective-C rather than standard C as per your code above):

JSContext *javascriptContext  = [[JSContext alloc] init];
javascriptContext[@"consoleLog"] = ^(NSString *message) {
    NSLog(@"Javascript log: %@",message);
};

Then you use it from Javascript by:

consoleLog("My debug message");

Note that I have tried to define a vararg version (log taking multiple parameters) but I couldn't get this to work correctly across the framework api.

Note that this solution uses features introduced with the new Objective-C API for the JavaScriptCore.framework introduced at the same time as IOS 7. If you are looking for an intro to this well-integrated bridge between Objective-C and Javascript, check out the 2013 WWDC introduction "Integrating JavaScript into Native Apps" session on Apple's developer network: https://developer.apple.com/videos/wwdc/2013/?id=615

Update to answer:

For those of you wanting to maximise your javascript code reuse without refactoring, I've managed to get a version working that declares a log of the form console.log() :

JSContext *javascriptContext  = [[JSContext alloc] init];
[javascriptContext evaluateScript:@"var console = {}"];
javascriptContext[@"console"][@"log"] = ^(NSString *message) {
    NSLog(@"Javascript log: %@",message);
};

Then you use it from Javascript by:

console.log("My debug message");

Answer 2

Swift 3.0

let javascriptContext = JSContext()
javascriptContext?.evaluateScript("var console = { log: function(message) { _consoleLog(message) } }")
let consoleLog: @convention(block) (String) -> Void = { message in
    print("console.log: " + message)
}
javascriptContext?.setObject(unsafeBitCast(consoleLog, to: AnyObject.self), forKeyedSubscript: "_consoleLog" as (NSCopying & NSObjectProtocol)!)

Swift 2.1

let javascriptContext = JSContext()
javascriptContext.evaluateScript("var console = { log: function(message) { _consoleLog(message) } }")
let consoleLog: @convention(block) String -> Void = { message in
    print("console.log: " + message)
}
javascriptContext.setObject(unsafeBitCast(consoleLog, AnyObject.self), forKeyedSubscript: "_consoleLog")

Then you use it from Javascript by:

console.log("My debug message");

Answer 3

self.jsContext = JSContext()
self.jsContext.evaluateScript(...)       
let logFunction: @convention(block) (String) -> Void  = { (string: String) in
    print(string)
}

self.jsContext.setObject(logFunction, forKeyedSubscript: "consoleLog" as NSCopying & NSObjectProtocol)

Answer 4

You can debug JS file attached to context in Safari.

Steps:

1) Start Safari

2) In Safari, enable the Develop menu by going to "Preferences" -> "Advanced" -> "Show Develop menu in menu bar"

3) Go to Develop menu -> "Simulator" or name of your computer -> select "Automatically show web inspector for JSContexts" and "Automatically pause connecting to JSContexts"

4) Re-run your project and Safari should auto-show the web inspector

Answer 5

Swift 5.0

The other suggestions didn't work for me, so I found a web post that explains how to do it now.Essentially

let logFunction: @convention(block) (String) -> Void = { string in
    print("JS_Console:", string)
}
if let console = context.objectForKeyedSubscript("console") {
    console.setObject(logFunction, forKeyedSubscript: "log") // works for me
    // is this needed? "console.setObject(unsafeBitCast(logFunction, to: AnyObject.self), forKeyedSubscript: "log")
}

log.console is variadic, but I could find no way to utilize it even though the link above suggests it's possible. What I did discover though is that you can use JavaScript interpolation to get values, for example:

console.log(`getCombinedFrameYaw: ${frameYaw} rot=${pathRotation}`)