How to add Custom Action Menu In SharePoint ListItem using Jquery

 The below code is used to add Custom Action Menu in SharePoint ListItem using JSOM.

Add a CEWP/Script Editor webpart in the "/sites/TeamSite/List/EmployeeDetails/AllItems.aspx" Page and paste the below code.

JSOM Code:

<script language="javascript" type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/1.8.1/jquery.min.js"></script>
<script language="javascript" type="text/javascript">
    $(document).ready(function() {
        SP.SOD.executeFunc('sp.js', 'SP.ClientContext', AddCustomUserActionToListItem);
    });

function AddCustomUserActionToListItem() {
    var clientContext = new SP.ClientContext();
    var oWeb = clientContext.get_web();
    var oList = oWeb.get_lists().getByTitle('EmployeeDetails');
    var userCustomActionColl = oList.get_userCustomActions();
    clientContext.load(oList, 'UserCustomActions', 'Title');
    clientContext.executeQueryAsync(function() {
        var customActionEnumerator = userCustomActionColl.getEnumerator();
        var foundAction = 0;
        while (customActionEnumerator.moveNext()) {
            var oUserCustomAction = customActionEnumerator.get_current();
            if (oUserCustomAction.get_title() == 'Custom Edit Page') {
                foundAction = 1;
                break;
            }
        }
        if (foundAction == 0) {
            var oUserCustomAction = userCustomActionColl.add();
            oUserCustomAction.set_location('EditControlBlock');
            oUserCustomAction.set_sequence(100);
            oUserCustomAction.set_title("Custom Edit Page");
            oUserCustomAction.set_url("/sites/TeamSite/Lists/Customer/dispform.aspx?ID={1}&Source=/sites/TeamSite/Lists/EmployeeDetails/AllItems.aspx");
            oUserCustomAction.update();
            clientContext.load(userCustomActionColl);
            clientContext.executeQueryAsync();
        }
    }, function(sender, args) {
        console.log('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
    });
}
</script>