SharePoint 2013 REST api to update list item- Uncaught ReferenceError: etag is not defined

Question

I try to update my sharepoint list using REST api. But following code throws exception in browser "Uncaught ReferenceError: etag is not defined " Can someone help to resolve this issue?

 // update an item 
    var requestUri = _spPageContextInfo.webAbsoluteUrl + '/_api/Web/Lists/getByTitle(\'Suppliers\')/items(1)';
    var requestHeaders = { 'Accept': 'application/json;odata=verbose', 
        'X-RequestDigest': $('#__REQUESTDIGEST').val(), 
        'X-HTTP-Method': 'MERGE', 
        'If-Match': etag }; 
    var customerData = { __metadata: { 'type': 'SP.Data.ContactsListItem' }, Title: 'NewName', }; 
    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('error');
    } 
    });

Answer 1

This error occurs since tag variable is not defined.

About eTag

When updating or deleting items within SharePoint lists via REST you must specify the Entity Tag (eTag) value that was returned with the item during the initial query. This enables SharePoint to determine if the item has changed since it was requested. Alternatively you can tell SharePoint to perform the operation regardless by specifying * as the eTag value. For example:

• If-Match: item.__metadata.etag can be used to specify the actual eTag value (‘item’ is the object returned from SharePoint containing
  the list item in JSON format).
• If-Match: * can be used to match any eTag value resulting in the operation being performed regardless of the actual value.

How to update List Item via SharePoint REST interface

function updateJson(endpointUri,payload, success, error) 
{
    $.ajax({       
       url: endpointUri,   
       type: "POST",   
       data: JSON.stringify(payload),
       contentType: "application/json;odata=verbose",
       headers: { 
          "Accept": "application/json;odata=verbose",
          "X-RequestDigest" : $("#__REQUESTDIGEST").val(),
          "X-HTTP-Method": "MERGE",
           "If-Match": "*"
       },   
       success: success,
       error: error
    });
}

function getItemTypeForListName(name) {
    return"SP.Data." + name.charAt(0).toUpperCase() + name.slice(1) + "ListItem";
}

function updateListItem(webUrl,listTitle,listItemId,itemProperties,success,failure)
{
     var listItemUri =  webUrl + "/_api/web/lists/getbytitle('" + listTitle + "')/items(" + listItemId + ")";
     var itemPayload = {
       '__metadata': {'type': getItemTypeForListName(listTitle)}
     };
     for(var prop in itemProperties){
           itemPayload[prop] = itemProperties[prop];
     }
     updateJson(listItemUri,itemPayload,success,failure);
}

Usage

var itemProperties = {'Title':'John Doe'};
updateListItem(_spPageContextInfo.webAbsoluteUrl,'Contacts',1,itemProperties,printInfo,logError);
function printInfo()
{
    console.log('Item has been created');
}
function logError(error){
    console.log(JSON.stringify(error));
}

References

Manipulating list items in SharePoint Hosted Apps using the REST API