An example of getting the current user via SharePoint's client object model

getcurrentuser.html

<script type="text/javascript">
function CustomExecuteFunction() {
	var self = this;
	self.context = new SP.ClientContext.get_current();
	self.web = context.get_web();
	self.currentUser = web.get_currentUser();
	context.load(self.currentUser);
	
	self.asyncSuccess = function(sender, args) {
		var user = this.currentUser;
		console.log('asyncSuccess', user);
		document.getElementById('userLoginName').innerHTML = user.get_loginName(); 
		document.getElementById('userId').innerHTML = user.get_id();
		document.getElementById('userTitle').innerHTML = user.get_title();
		document.getElementById('userEmail').innerHTML = user.get_email();
		
		var url = '?userinfo=';
		url += '0[' + encodeURIComponent(user.get_loginName()) + ']';
		url += '1[' + encodeURIComponent(user.get_id()) + ']';
		url += '2[' + encodeURIComponent(user.get_title()) + ']';
		url += '3[' + encodeURIComponent(user.get_email()) + ']';
		document.getElementById('customButton').href += url;
	};
	
	self.asyncFailure = function(sender, args) {
		alert('Request failed. \nError: ' + args.get_message() + '\nStackTrace: ' + args.get_stackTrace());
	};
	
	// actually fires off the AJAX request
	// more info: http://msdn.microsoft.com/en-us/library/dn168907.aspx
	context.executeQueryAsync(
		Function.createDelegate(self,self.asyncSuccess), 
		Function.createDelegate(self,self.asyncSuccess)
	);
}

ExecuteOrDelayUntilScriptLoaded(CustomExecuteFunction,'sp.js');
</script>
<div>Current Logged User:
	<span id="userLoginName"></span>
	<span id="userId"></span>
	<span id="userTitle"></span>
	<span id="userEmail"></span>
	<a id="customButton" href="https://example.sharepoint.com/SitePages/Test.aspx">Click me!</a>
</div>