to create a button in javascript dynamically

Use :

 button.setAttribute('text','Write');  

or

button.innerHTML = 'Write';

Try this

  var  button = document.createElement('input');
          button.type = 'button';
          button.value = 'Write';
          button.id ='Write';
          .........................
          .........................


document.getElementById("write_testi").appendChild(button);

You are getting stumped here.

var button = document.createElement('button');

Try:

var button = document.createElement('input');

Try using an input element (see it on jsFiddle):

var button = document.createElement('input');
button.setAttribute('name', 'write');
button.setAttribute('type', 'button');
button.setAttribute('class', 'ui-button_S40button');
button.setAttribute('style', 'width: 100px;');
button.setAttribute('value', 'Write');
document.getElementById("write_testi").appendChild(button);

Or assign the innerText DOM attribute (demo):

var button = document.createElement('button');
button.setAttribute('type', 'button');
button.setAttribute('name', 'write');
button.setAttribute('class', 'ui-button_S40button');
button.setAttribute('style', 'width: 100px;');
document.getElementById("write_testi").appendChild(button);
button.innerText="Write";

Try this:

var button = document.createElement('button');
    button.className = 'ui-button_S40button';
    button.style.width = '100px';
    button.innerHTML = 'Write';
    button.value = 'Write';
    button.name = 'Write';

document.getElementById("write_testi").appendChild(button);

See jsFiddle.

The other answers suggest switching to <input> (different Node) or using innerHTML (which deviates from your pure use of DOM methods so far).

You can do this in pure DOM methods as follows.

button.appendChild(document.createTextNode('Write'));

Try this:

function insertButton() {
    var button = document.createElement('input');
    button.type = 'button';
    button.setAttribute('name','write');
    button.setAttribute('class','ui-button_S40button');  
    button.setAttribute('style','width: 100px;');
    button.setAttribute('value','Write');  
    document.getElementById("write_testi").appendChild(button);
}

window.onload = insertButton;

Make sure write_testi is a div

If you have an existing function you call on window.onload, put code inside insertButton function in there.