A code that detects if a value start with something (?)

[bladefinor]

A textfield has the value "http://www.google.com/".
I want to have a javascript that detects if the value starts with "http://", because then it will do "alert('The value starts with http://')". Or if the textfield's value is something else, it should do "alert('Nothing special...')"

How to do this?

[Twey]

String.prototype.startsWith = function(s) {
    return this.substr(0, s.length) === s;
};

"http://www.google.com/".startsWith("http://") // true
"http://www.google.com/".startsWith("google") // false

[bladefinor]

I don't get it.
Where do I put this?

Or should i do like this?

function goButton(){
	String.prototype.startsWith = function(s){
		return this.substr(0, s.length) === s;
	}
	if(document.getElementById('addressBar').value.startsWith == "http://"){
		alert('The value starts with http://');
	}
}

I don't think so... But I don't understand what you mean by just posting the code :S
But i'm appreciating your help

[rangana]

Implementation from Twey's prototype:

<script type="text/javascript">
String.prototype.startsWith = function(s) {
    return this.substr(0, s.length) === s;
};
function goButton(el,string)
{
alert(document.getElementById(el).value.startsWith(string)?'The value starts with '+string:'It doesn\'t starts with '+string);
}
</script>
<input type="text" id="inp"><input type="button" onclick="goButton('inp','http://')" value="Check Start">

[Twey]

String.prototype.startsWith = function(s){
    return this.substr(0, s.length) === s;
};

function goButton() {
    if (document.getElementById("addressBar").value.startsWith("http://"))
        alert("The value starts with http://");
}

[bladefinor]

yeah thx, i figured it out myself
thx guys!