HTML
<input type="checkbox" name="others" önclick="enable_text(this.checked)" />
<input type="text" id="other_text" />
JavaScript
function enable_text(status)
{
document.getElementById("other_text").disabled = !status;
}
DEMO[
^]
EDIT
if (document.getElementById("Other").checked && isBlank(document.getElementsByName('other_text').value)) {
alert("your message......... ");
return false;
}
use below function to check blank input
function isBlank(str) {
return (!str || /^\s*$/.test(str));
}