How to get one time alert inside JQuery post method within loop?

Try similar code

<script>

$('#save').on('click',function(){   
    var i =1;
            var cnt=0;
    //i have 4 item 
    $('.item').each(function() {        

        $.post( "example.php", { id: i })
        .done(function( data ) {
            cnt++;
                    if (cnt==4) {
                        alert(0); //for one time after all items post.  
                    }

        });

        i++;    
    });


       })
</script>

Hope this will help you!.

you can try by setting the counter like given below.

$('#save').on('click',function(){   
    var i =1;
    var counter = $('.item').length -1;
    //i have 4 item 
    $('.item').each(function() {        
        $.post( "example.php", { id: i })
        .done(function( data ) {
              if(counter == 0)
                   alert(0); //for one time after all items post.            
           counter = counter ==0 ? 0: counter -1;
        });
        i++;    
    });
});

By using jQuery.when() and deferred.resolve at jQuery.ajax complete event

you could gather all requests and see which one succeed and which one failed.

Try running the code below, but please note there are no example.php on stackoverflow, so you'll see all 4 requests failed.

jQuery(function($) {

  // make all requests
  var requests = $('.item').map(function(idx, elm) {
    var deferred = $.Deferred();

    $.ajax({
      method: 'post',
      url: 'example.php',
      data: { id: idx + 1 },
      complete: deferred.resolve
    });

    return deferred;
  });

  // when all requests are done.
  $.when.apply($, requests).done(function() {

    var stats = { success: 0, error: 0 };

    // count how many of them succeed or failed.
    for (var i = 0; i < arguments.length; i++) {
      stats[arguments[i][1]]++;
    }

    // print resullt.
    console.log(
      arguments.length + ' Requests completed, ' +
      stats.success + ' Succeeded, ' +
      stats.error + ' Failed.'
    );

  });

});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<button id="save">Save</button>

<div class="item">Item 1</div>
<div class="item">Item 2</div>
<div class="item">Item 3</div>
<div class="item">Item 4</div>