The question has been answered, but for your future coding reference you might like to consider this.
In your HTML, add the name as an attribute to the button and remove the onclick reference.
<button id="button" data-name="Mathew" type="button">click</button>
In your JavaScript, grab the button using its ID, assign the function to the button's click event, and use the function to display the button's data-name attribute.
var button = document.getElementById('button');
button.onclick = myfunction;
function myfunction() {
var name = this.getAttribute('data-name');
alert(name);
}