This is possible to do in javascript. I would suggest using jQuery Ajax method to accomplish this:
http://api.jquery.com/jQuery.post/
You should be able to do something like this:
$.ajax({
type: "POST",
url: url,
data: data,
success: success,
dataType: xml
});
where data is the XML document you want to write to zoho. For example to write to the Lead table it would look like:
<Leads>
<row no="1">
<FL val="Lead Source">My Lead Source</FL>
<FL val="Last Name">Smith</FL>
<FL val="First Name">James</FL>
<FL val="Email">name@mydomain.com</FL>
<FL val="Phone">555-1234</FL>
<FL val="Website">www.mydomain.com</FL>
<FL val="Description">Some Informative Description</FL>
</row>
</Leads>
All of this would get posted to
crm.zoho.com/crm/private/xml/Leads/insertRecords along with your auth token and, scope=crmapi and newFormat=1.
More information can be found here:
http://www.zoho.com/crm/help/api/insertrecords.html
All that being said, using javascript might not be advisable, as you would have to expose your AUTH key to the world. I don't know for sure, but with this auth key, I would imagine anyone could read, write, or update your Zoho CRM without your knowledge or permission. I'd suggest doing some research on the security implications of doing this in Javascript. I personally haven't ever done it in JS; only in PHP and C#.
James