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问题:

On site there is code like that (its site on LAN)

<script language="JavaScript" type="text/javascript">         
    alert("ble");
</script>

I try to disable that alert using GM. I was trying to do this

unsafeWindow.alert=function() {};

but I see the alert and get this error

Error: uncaught exception: [Exception... "Component is not available"  nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)"  location: "JS frame :: file:///C:/Documents%20and%20Settings/arokitnicki/Dane%20aplikacji/Mozilla/Firefox/Profiles/sm4bsods.default/extensions/%7Be4a8a97b-f2ed-450b-b12d-ee082ba24781%7D/components/greasemonkey.js :: anonymous :: line 377"  data: no]

How to disable that alert?

P.S. this is NOT Javascript question, but greasemonkey question.

EDIT:

Its company's website, so I can't paste the real code

<head>
    <script>    
        dojo.require("dojo.back");
        dojo.back.init(); 
    </script>
</head>
<body onload="someMethod()">
    <iframe></iframe>
    <script>         
        alert("bla");
    </script>
</body>

There are also some scripts and css declarations in header.

回答1:

Update: For modern versions of Greasemonkey:
You can intercept alert() in most cases by using @run-at document-start. For example, load this script and then visit the test page:

// ==UserScript==
// @name            Overwrite Alert
// @description     Overwrites alert()
// @include         http://output.jsbin.com/*
// @grant           none
// @run-at          document-start
// ==/UserScript==

unsafeWindow.alert=function (str) {
    console.log ("Greasemonkey intercepted alert: ", str);
};

If your script requires GM_ functions, it must set @grant other than none. In that case use exportFunction() like so:

// ==UserScript==
// @name            Overwrite Alert
// @description     Overwrites alert()
// @include         http://output.jsbin.com/*
// @grant           GM_addStyle
// @run-at          document-start
// ==/UserScript==

function myAlert (str) {
    console.log ("Greasemonkey intercepted alert: ", str);
}
unsafeWindow.alert   = exportFunction (myAlert, unsafeWindow);

Old answer, for Greasemonkey before August 2011:

unsafeWindow.alert=function() {}; works fine in select situations.

But, if that really is the code on the page, then you will not be able to stop that alert using Greasemonkey.

This is because that alert will fire during the page load and before the DOMContentLoaded event -- which is when Greasemonkey is fired.

Load this GM script:

// ==UserScript==
// @name            Overwrite Alert
// @description     Overwrites alert()
// @include         http://jsbin.com/*
// ==/UserScript==

unsafeWindow.alert=function() {};

Then visit: http://jsbin.com/ajeqe4/6 .

Inspecting the code (http://jsbin.com/ajeqe4/6/edit), You will see 3 alerts. Greasemonkey is only able to stop the alerts that fire on load (usually).

Other factors can block GM's ability to stop the alert... The page loads too fast or closures, perhaps.

Paste the source of that page, unedited if at all possible, at pastebin.com. There may be something else you can do. Maybe block the script via adblock?

Otherwise, you'll have to write an extension/add-on.

回答2:

If you use Scriptish then the following will always work:

// ==UserScript==
// @id              alert-killer-test@erikvold.com
// @name            Overwrite Alert
// @description     Overwrites alert()
// @include         *
// @run-at          document-start
// ==/UserScript==

unsafeWindow.alert=function() {};

You can get the user script here.