This is a simple reference:

// this is the id of the form
$("#idForm").submit(function(e) {

    e.preventDefault(); // avoid to execute the actual submit of the form.

    var form = $(this);
    var actionUrl = form.attr('action');
    
    $.ajax({
        type: "POST",
        url: actionUrl,
        data: form.serialize(), // serializes the form's elements.
        success: function(data)
        {
          alert(data); // show response from the php script.
        }
    });
    
});

Another similar solution using attributes defined on the form element:

<form id="contactForm1" action="/your_url" method="post">
    <!-- Form input fields here (do not forget your name attributes). -->
</form>

<script type="text/javascript">
    var frm = $('#contactForm1');

    frm.submit(function (e) {

        e.preventDefault();

        $.ajax({
            type: frm.attr('method'),
            url: frm.attr('action'),
            data: frm.serialize(),
            success: function (data) {
                console.log('Submission was successful.');
                console.log(data);
            },
            error: function (data) {
                console.log('An error occurred.');
                console.log(data);
            },
        });
    });
</script>

There are a few things you need to bear in mind.

1. There are several ways to submit a form

• using the submit button
• by pressing enter
• by triggering a submit event in JavaScript
• possibly more depending on the device or future device.

We should therefore bind to the form submit event, not the button click event. This will ensure our code works on all devices and assistive technologies now and in the future.

2. Hijax

The user may not have JavaScript enabled. A hijax pattern is good here, where we gently take control of the form using JavaScript, but leave it submittable if JavaScript fails.

We should pull the URL and method from the form, so if the HTML changes, we don't need to update the JavaScript.

3. Unobtrusive JavaScript

Using event.preventDefault() instead of return false is good practice as it allows the event to bubble up. This lets other scripts tie into the event, for example analytics scripts which may be monitoring user interactions.

Speed

We should ideally use an external script, rather than inserting our script inline. We can link to this in the head section of the page using a script tag, or link to it at the bottom of the page for speed. The script should quietly enhance the user experience, not get in the way.

Code

Assuming you agree with all the above, and you want to catch the submit event, and handle it via AJAX (a hijax pattern), you could do something like this:

$(function() {
  $('form.my_form').submit(function(event) {
    event.preventDefault(); // Prevent the form from submitting via the browser
    var form = $(this);
    $.ajax({
      type: form.attr('method'),
      url: form.attr('action'),
      data: form.serialize()
    }).done(function(data) {
      // Optionally alert the user of success here...
    }).fail(function(data) {
      // Optionally alert the user of an error here...
    });
  });
});

You can manually trigger a form submission whenever you like via JavaScript using something like:

$(function() {
  $('form.my_form').trigger('submit');
});

Edit:

I recently had to do this and ended up writing a plugin.

(function($) {
  $.fn.autosubmit = function() {
    this.submit(function(event) {
      event.preventDefault();
      var form = $(this);
      $.ajax({
        type: form.attr('method'),
        url: form.attr('action'),
        data: form.serialize()
      }).done(function(data) {
        // Optionally alert the user of success here...
      }).fail(function(data) {
        // Optionally alert the user of an error here...
      });
    });
    return this;
  }
})(jQuery)

Add a data-autosubmit attribute to your form tag and you can then do this:

HTML

<form action="/blah" method="post" data-autosubmit>
  <!-- Form goes here -->
</form>

JS

$(function() {
  $('form[data-autosubmit]').autosubmit();
});

You can also use FormData (But not available in IE):

var formData = new FormData(document.getElementsByName('yourForm')[0]);// yourForm: form selector        
$.ajax({
    type: "POST",
    url: "yourURL",// where you wanna post
    data: formData,
    processData: false,
    contentType: false,
    error: function(jqXHR, textStatus, errorMessage) {
        console.log(errorMessage); // Optional
    },
    success: function(data) {console.log(data)} 
});

This is how you use FormData.

Simple version (does not send images)

<form action="/my/ajax/url" class="my-form">
...
</form>
<script>
    (function($){
        $("body").on("submit", ".my-form", function(e){
            e.preventDefault();
            var form = $(e.target);
            $.post( form.attr("action"), form.serialize(), function(res){
                console.log(res);
            });
        });
    )(jQuery);
</script>