Can I extend the console object (for rerouting the logging) in javascript?

Question

Is it possible to extend the console object?

I tried something like:

Console.prototype.log = function(msg){
    Console.prototype.log.call(msg);
    alert(msg);
}

But this didn't work. I want to add additional logging to the console object via a framework like log4javascript and still use the standard console object (in cases where log4javascript is not available) in my code.

Thanks in advance!

why are you extending the prototype object instead of extending the `Console` object itself? Do you ever call `new Console()`? — biziclop, Feb 14 '12 at 13:43

score 44 · Accepted Answer · answered Feb 14 '12 at 13:52

44

Try following:

(function() {
    var exLog = console.log;
    console.log = function(msg) {
        exLog.apply(this, arguments);
        alert(msg);
    }
})()

answered Feb 14 '12 at 13:52

Sergey Ilinsky

29,849
9
51
56

score 4 · Answer 2 · answered Dec 12 '16 at 04:54

4

You Can Also add log Time in This Way :

added Momentjs or use New Date() instead of moment.

var oldConsole = console.log;
console.log = function(){
    var timestamp = "[" + moment().format("YYYY-MM-DD HH:mm:ss:SSS") + "] ";
    Array.prototype.unshift.call(arguments, timestamp);
    oldConsole.apply(this, arguments);
};

answered Dec 12 '16 at 04:54

nitesh singh

796
9
21

This doesn't work as it generates a "moment is not defined" error. – nbsp Apr 06 '20 at 08:49
you have to install moment using npm install moment – nitesh singh May 07 '20 at 04:30
1

ok, I guess, I'd prefer a native solution though. For example NPM is yet "another" thing that needs to be present – nbsp Jun 02 '20 at 20:41

score 3 · Answer 3 · answered Dec 07 '18 at 15:29

3

It's really the same solution some others have given, but I believe this is the most elegant and least hacky way to accomplish this. The spread syntax (...args) makes sure not a single argument is lost.

var _console={...console}

console.log = function(...args) {
    var msg = {...args}[0];
    //YOUR_CODE
    _console.log(...args);
}

answered Dec 07 '18 at 15:29

eds1999

818
2
9
25

nice use of the spread/rest syntax, but its not needed, theres a 1 line smaller way that supports IE an doesnt overwrite protos above. – Mister SirCode Jan 29 '20 at 16:14

ekerner · Answer 4 · 2017-04-06T08:29:29.153

// console aliases and verbose logger - console doesnt prototype
var c = console;
c.l = c.log,
c.e = c.error,
c.v = c.verbose = function() {
    if (!myclass || !myclass.verbose) // verbose switch
        return;
    var args = Array.prototype.slice.call(arguments); // toArray
    args.unshift('Verbose:');
    c.l.apply(this, args); // log
};

// you can then do
var myclass = new myClass();
myclass.prototype.verbose = false;
// generally these calls would be inside your class
c.v('1 This will NOT log as verbose == false');
c.l('2 This will log');
myclass.verbose = true;
c.v('3 This will log');

I noted that the above use of Array.prototype.unshift.call by nitesh is a better way to add the 'Verbose:' tag.

score 1 · Answer 5 · answered Mar 06 '19 at 15:06

For ECMAScript 2015 and later

You can use the newer Proxy feature from the ECMAScript 2015 standard to "hijack" the global console.log.

Source-Code

'use strict';

class Mocker {
  static mockConsoleLog() {
    Mocker.oldGlobalConsole = window.console;

    window.console = new Proxy(window.console, {
      get(target, property) {
        if (property === 'log') {
          return function(...parameters) {
            Mocker.consoleLogReturnValue = parameters.join(' ');
          }
        }

        return target[property];
      }
    });
  }

  static unmockConsoleLog() {
    window.console = Mocker.oldGlobalConsole;
  }
}

Mocker.mockConsoleLog();

console.log('hello'); // nothing happens here

Mocker.unmockConsoleLog();

if (Mocker.consoleLogReturnValue === 'hello') {
  console.log('Hello world!'); // Hello world!
  alert(Mocker.consoleLogReturnValue);
  // anything you want to do with the console log return value here...
}

Online Demo

Repl.it.

Node.js users...

... I do not forget you. You can take this source-code and replace window.console by gloabl.console to properly reference the console object (and of course, get rid of the alert call). In fact, I wrote this code initially and tested it on Node.js.

this is enormous compared to the answer above with a 6 line overwriter, I think this is overkill for a simple function rewrite — Mister SirCode, Jan 29 '20 at 16:06

score 0 · Answer 6 · answered Nov 21 '20 at 17:25

You can override the default behavior of the console.log function using the below approach, the below example demonstrates to log the line number using the overridden function.

let line = 0;
const log = console.log;
console.log = (...data) => log(`${++line} ===>`, ...data)

console.log(11, 1, 2)
console.log(11, 1, 'some')

Can I extend the console object (for rerouting the logging) in javascript?

6 Answers6

For ECMAScript 2015 and later

Source-Code

Online Demo

Node.js users...

Linked

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