Add, Modify and Read Data from SharePoint List using Cient Side Object Model REST API

<script src="/Style%20Library/jquery-1.7.1.min.js"></script>
<script type="text/javascript">
jQuery(document).ready(function($) {

 //getItems(url);
var url ="/_api/Web/Lists/GetByTitle('TESTList')/Items";
var data = {
    __metadata: { 'type': 'SP.Data.TESTListItem' },
    Title: 'ASD',
    Employee_x0020_Name1: 'RAM',
    Employee_x0020_ID1: '67',
    Employee_x0020_Age1: '30',
    Department:'Accounts'
};
addNewItem(url,data);

});


function addNewItem(url, data) {

    $.ajax({
        url: _spPageContextInfo.webAbsoluteUrl + url,
        type: "POST",
        headers: {
            "accept": "application/json;odata=verbose",
            "X-RequestDigest": $("#__REQUESTDIGEST").val(),
            "content-Type": "application/json;odata=verbose"
        },
        data: JSON.stringify(data),
        success: function (data) {
            alert("inserted");
        },
        error: function (error) {
            alert(JSON.stringify(error));
        }
    });
}


</script>

Modify Data

<script src="/Style%20Library/jquery-1.7.1.min.js"></script>
<script type="text/javascript">

    var requestUri = _spPageContextInfo.webAbsoluteUrl + '/_api/Web/Lists/getByTitle(\'TESTList\')/items(2)';
    var requestHeaders = { 'Accept': 'application/json;odata=verbose',
        'X-RequestDigest': $('#__REQUESTDIGEST').val(),
        'X-HTTP-Method': 'MERGE',
        'If-Match': '*'
   };
    var customerData = { __metadata: { 'type': 'SP.Data.TESTListItem' }, Title: 'NewName', Department: 'Commerce'   };
    requestBody = JSON.stringify(customerData);
    $.ajax({ url: requestUri, type: 'POST',
        contentType: 'application/json;odata=verbose', headers: requestHeaders,
        data: JSON.stringify(customerData)
    ,
    success: function (response)
    {
      alert('Success');
    },
    error: function(error)
    {
        alert(JSON.stringify(error));
    }
    });


</script>

<script src="/Style%20Library/jquery-1.7.1.min.js"></script>
<script type="text/javascript">

var url ="/_api/Web/Lists/GetByTitle('TESTList')/items";

jQuery(document).ready(function($) {
 getItems(url);
});

function getItems(url) {

    $.ajax({
        url: _spPageContextInfo.webAbsoluteUrl + url,

        type: "GET",
        headers: {
            "accept": "application/json;odata=verbose",
        },
        success: function (data) {
            for( i=0; i<=data.d.results.length; i++)
   {
   alert("Hi");
   alert(data.d.results[i].Employee_x0020_Name);
   }
        },
        error: function (error) {
            alert(JSON.stringify(error));
        }
    });
}
</script>