CODEWITHSUNDEEP
javascript
Zorak
Zorak

Reputation: 709

Read console.log() output form javascript

I am writing an overlay UI for one application using Greasemonkey, injecting JS to a page.

Now I have issue to hook on some ajax calls that runs on the webpage. However, the page is generating console logs, informing that content is fully loaded. My question is:

In plain javascript, is there a possibility of reading console output generated by completely different script?

function blackBox(){
   //this is generating come console output.
   //this runs on code layer I have no access to.
}

function readConsole(){
   //read console record by record
   if(console.msg == "something I want"){
      doSomething();
   }

}

Thanks in advance :)

Upvotes: 3

Views: 10944

Answers (4)

Roy
Roy

Reputation: 471

Directly there's no way to access console outputs, but what you can do is that you can override console.log in a way where it checks for you condition first and then outputs the content, below is a sample code for that

console.stdlog = console.log.bind(console);
console.log = function(msg){
    if(msg == "something I want"){
    ...
    }
    console.stdlog.apply(console, arguments);
}

Although you'll need to be very careful with this since if you add any console.log in that condition, it will create an infinite loop.

Upvotes: 1

Drakinite
Drakinite

Reputation: 362

Even if it were possible, that would be bad practice. The console is for human eyes to see, when debugging. It usually takes several milliseconds to execute console.log, so executing it in production code is not a good idea.

I would say programming in booleans (like ifDidX and ifDidY) would be better. But if you really had to do this message-history thing, a better alternative would be to store messages in some other array. Here is an example:

var messagesLog = [];
//Since console.log takes several milliseconds to execute, you should tick this to false in production code.
const logToConsole = true;

function logMessage( message ){

  //adds message to JS log.
  messagesLog.push(message);
  
  //performs console.log if not production code
  if(logToConsole){
    console.log(message);
  }
}

logMessage("pizza");
logMessage("second to last message");
logMessage("last message");

//checks last message and this will be true
if( messagesLog[messagesLog.length - 1] == "last message" ){
  logMessage("The last message in the log was 'last message'");
}
//checks last message and this will be false
if( messagesLog[messagesLog.length - 1] == "pizza" ){
  logMessage("This will not occur because the last message was 'last message'");
}
//searches through log to find certain message
for(let i = 0; i < messagesLog.length; i++){
  if(messagesLog[i] == "pizza"){
    logMessage("one of the messages in the log was pizza");
  }
}

Upvotes: 2

Charles Cavalcante
Charles Cavalcante

Reputation: 1618

Redefine console.log and storage messages on a array:

var consoleStorage = [];

console.log = function(msg){
   consoleStorage.push(msg);
   console.warn(msg); // if you need to print the output
}

Now you can read all logs:

function readConsole(){
   //read console record by record
   consoleStorage.forEach(msg => {
      doSomething(msg);
   });
}

Upvotes: 1

Blue
Blue

Reputation: 22921

As others have stated, you can override the console.log() function with your own, and implement your own implementation:

var oldLog = unsafeWindow.console.log;
var messages = [];

unsafeWindow.console.log = function(msg) {
    messages.push(msg);
    oldLog.apply(null, arguments);
}

// Access the entire console.log history in messages

Upvotes: 2

Related Questions