yuizho/checker.js

## checker.js
(function() {
    var click = function() {
        alert("入れ替え後のイベント");
    };
    var load = function() {
        // htmlの書き換え
        var h1 = document.createElement('h3');
        h1.setAttribute("class", "message");
        h1.innerHTML = "checkerが有効です";
        document.body.insertBefore(h1, document.body.firstChild);
        // イベントの挿入
        var btns = document.querySelectorAll('button');
        for(var i=0, len=btns.length; i < len; i++) {
            btns[i].addEventListener("click", click, false);
        }
    };
    document.addEventListener("DOMContentLoaded", load, false);
}());

## index.html
<html>
<head>
    <script>
    (function() {
        var click = function() {
            alert("元のイベント");
        }
        var load = function() {
            var btns = document.querySelectorAll('button');
            for(var i=0, len=btns.length; i < len; i++) {
                btns[i].addEventListener("click", click, false);
            }
        };
        document.addEventListener("DOMContentLoaded", load, false);
    }());
    </script>
</head>
<body>
    <button>OK</button>
</body>
</html>

## manifest.json
{
  "manifest_version": 2,
  "name": "checker",
  "description": "checker",
  "version": "0.1",
  "content_scripts": [{
    "matches": [
        "http://*/*",
        "https://*/*",
        "file://*/*"
    ],
    "js": ["checker.js"],
    "run_at": "document_start"
  }]
}
	(function() {
	var click = function() {
	alert("入れ替え後のイベント");
	};
	var load = function() {
	// htmlの書き換え
	var h1 = document.createElement('h3');
	h1.setAttribute("class", "message");
	h1.innerHTML = "checkerが有効です";
	document.body.insertBefore(h1, document.body.firstChild);
	// イベントの挿入
	var btns = document.querySelectorAll('button');
	for(var i=0, len=btns.length; i < len; i++) {
	btns[i].addEventListener("click", click, false);
	}
	};
	document.addEventListener("DOMContentLoaded", load, false);
	}());
	<html>
	<head>
	<script>
	(function() {
	var click = function() {
	alert("元のイベント");
	}
	var load = function() {
	var btns = document.querySelectorAll('button');
	for(var i=0, len=btns.length; i < len; i++) {
	btns[i].addEventListener("click", click, false);
	}
	};
	document.addEventListener("DOMContentLoaded", load, false);
	}());
	</script>
	</head>
	<body>
	<button>OK</button>
	</body>
	</html>
	{
	"manifest_version": 2,
	"name": "checker",
	"description": "checker",
	"version": "0.1",
	"content_scripts": [{
	"matches": [
	"http:///",
	"https:///",
	"file:///"
	],
	"js": ["checker.js"],
	"run_at": "document_start"
	}]
	}