[SalesForce] How tonsert/Update multiple records in a single Force.com REST API call

    <script src="/soap/ajax/29.0/connection.js"></script>
 <script>
 var contactId = "{!Contact_ID}";
    function initPage() {
       try{
           var contact = sforce.connection.retrieve("Phone",[contactId])[0];
           contact.Phone = "12398238";
           result = sforce.connection.update([account]);
      if (result[0].getBoolean("success")) {
         log("account with id " + result[0].id + " updated");
     } else {
      log("failed to update account " + result[0]);
     }

    }

  }

</script>

Please follow the above for how to approach using purely java script.Note with ajaz toolkit you don't need REST API at all .

I don't recommend using JavaScript or ajax toolkit for this requirement or using any REST API .A simple visualforce page with an extension controller would do the needful

You would typically send a POST request, rather than a GET, to insert/update a record, so you could pass your 100+ fields as JSON or XML in the POST body. Check what the external system team is expecting to see!