Setting textbox to readonly not working in Javascript [closed]

Question

I have tried the following codes to set a textbox property to readonly but none of them is working. Can anyone tell me how to do it?

document.getElementById("phone_numbers").disabled=true;
document.getElementById("phone_numbers").readonly=true;
document.getElementById("phone_numbers").readOnly=true;
document.getElementById("phone_numbers").readOnly=readOnly;
document.getElementById("phone_numbers").readOnly=readonly;
document.getElementById("phone_numbers").readonly=readonly;
document.getElementById("phone_numbers").readonly=readOnly;
document.getElementById("phone_numbers").setAttribute('readonly', 'readonly');
document.getElementById("phone_numbers").setAttribute("readonly", "true");

EDIT
While I was editing my question to write more detail I realized what the problem was. I made a silly mistake. This was my original HTML code.

<input name="phone_numbers">

And I changed it to

<input id="phone_numbers">

And it worked!

By the way following are the functions that are actually working.

document.getElementById("phone_numbers").disabled=true;
document.getElementById("phone_numbers").readOnly=true;
document.getElementById("phone_numbers").setAttribute('readonly', 'readonly');
document.getElementById("phone_numbers").setAttribute("readonly", "true");

Answer 1

Your third line worked for me: JSFiddle

HTML

<input id="phone_numbers" type="textbox"></input>

Javascript

document.getElementById("phone_numbers").readOnly=true;