How does Facebook disable the browser's integrated Developer Tools?


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Besides redefining console._commandLineAPI, there are some other ways to break into InjectedScriptHost on WebKit browsers, to prevent or alter the evaluation of expressions entered into the developer's console.


Chrome has fixed this in a past release. - which must have been before February 2015, as I created the gist at that time

So here's another possibility. This time we hook in, a level above, directly into InjectedScript rather than InjectedScriptHost as opposed to the prior version.

Which is kind of nice, as you can directly monkey patch InjectedScript._evaluateAndWrap instead of having to rely on InjectedScriptHost.evaluate as that gives you more fine-grained control over what should happen.

Another pretty interesting thing is, that we can intercept the internal result when an expression is evaluated and return that to the user instead of the normal behavior.

Here is the code, that does exactly that, return the internal result when a user evaluates something in the console.

var is;
       return this._lR;
       if (typeof this._commandLineAPIImpl=="object") is=this;
   var ev=is._evaluateAndWrap;
       var res=ev.apply(is,arguments);
       if (arguments[2]==="completion") {
           //This is the path you end up when a user types in the console and autocompletion get's evaluated

           //Chrome expects a wrapped result to be returned from evaluateAndWrap.
           //You can use `ev` to generate an object yourself.
           //In case of the autocompletion chrome exptects an wrapped object with the properties that can be autocompleted. e.g.;
           //{iGetAutoCompleted: true}
           //You would then go and return that object wrapped, like
           //return (is, '', '({test:true})', 'completion', true, false, true);
           //Would make `test` pop up for every autocompletion.
           //Note that syntax as well as every Object.prototype property get's added to that list later,
           //so you won't be able to exclude things like `while` from the autocompletion list,
           //unless you wou'd find a way to rewrite the getCompletions function.
           return res; //Return the autocompletion result. If you want to break that, return nothing or an empty object
       } else {
           //This is the path where you end up when a user actually presses enter to evaluate an expression.
           //In order to return anything as normal evaluation output, you have to return a wrapped object.

           //In this case, we want to return the generated remote object. 
           //Since this is already a wrapped object it would be converted if we directly return it. Hence,
           //`return result` would actually replicate the very normal behaviour as the result is converted.
           //to output what's actually in the remote object, we have to stringify it and `evaluateAndWrap` that object again.`
           //This is quite interesting;
           return (is, null, '(' + JSON.stringify (res) + ')', "console", true, false, true)

It's a bit verbose, but I thought I put some comments into it

So normally, if a user, for example, evaluates [1,2,3,4] you'd expect the following output:

enter image description here

After monkeypatching InjectedScript._evaluateAndWrap evaluating the very same expression, gives the following output:

enter image description here

As you see the little-left arrow, indicating output, is still there, but this time we get an object. Where the result of the expression, the array [1,2,3,4] is represented as an object with all its properties described.

I recommend trying to evaluate this and that expression, including those that generate errors. It's quite interesting.

Additionally, take a look at the is - InjectedScriptHost - object. It provides some methods to play with and get a bit of insight into the internals of the inspector.

Of course, you could intercept all that information and still return the original result to the user.

Just replace the return statement in the else path by a console.log (res) following a return res. Then you'd end up with the following.

enter image description here

End of Edit

This is the prior version which was fixed by Google. Hence not a possible way anymore.

One of it is hooking into

Chrome evaluates the entered expression by calling its eval function with InjectedScriptHost as thisArg

var result =, expression);

Given this, you can listen for the thisArg of call being evaluate and get a reference to the first argument (InjectedScriptHost)

if (window.URL) {
    var ish, _call =; = function () { //Could be wrapped in a setter for _commandLineAPI, to redefine only when the user started typing.
        if (arguments.length > 0 && === "evaluate" && arguments [0] === "InjectedScriptHost") { //If thisArg is the evaluate function and the arg0 is the ISH
            ish = arguments[0];
            ish.evaluate = function (e) { //Redefine the evaluation behaviour
                throw new Error ('Rejected evaluation of: \n\'' + e.split ('\n').slice(1,-1).join ("\n") + '\'');
   = _call; //Reset the
            return _call.apply(this, arguments);  

You could e.g. throw an error, that the evaluation was rejected.

enter image description here

Here is an example where the entered expression gets passed to a CoffeeScript compiler before passing it to the evaluate function.