阻止表单重定向或提交时刷新？

It looks like you're missing a return false.

Just handle the form submission on the submit event, and return false:

$('#contactForm').submit(function () {
sendContactForm();
return false;
});

You don't need any more the onclick event on the submit button:

<input class="submit" type="submit" value="Send" />

Here:

function submitClick(e)
{
e.preventDefault();
$("#messageSent").slideDown("slow");
setTimeout('$("#messageSent").slideUp();
$("#contactForm").slideUp("slow")', 2000);
}


$(document).ready(function() {
$('#contactSend').click(submitClick);
});

Instead of using the onClick event, you'll use bind an 'click' event handler using jQuery to the submit button (or whatever button), which will take submitClick as a callback. We pass the event to the callback to call preventDefault, which is what will prevent the click from submitting the form.

An alternative solution would be to not use form tag and handle click event on submit button through jquery. This way there wont be any page refresh but at the same time there is a downside that "enter" button for submission wont work and also on mobiles you wont get a go button( a style in some mobiles). So stick to use of form tag and use the accepted answer.

In the opening tag of your form, set an action attribute like so:

<form id="contactForm" action="#">

If you want to see the default browser errors being displayed, for example, those triggered by HTML attributes (showing up before any client-code JS treatment):

<input name="o" required="required" aria-required="true" type="text">

You should use the submit event instead of the click event. In this case a popup will be automatically displayed requesting "Please fill out this field". Even with preventDefault:

$('form').on('submit', function(event) {
event.preventDefault();
my_form_treatment(this, event);
}); // -> this will show up a "Please fill out this field" pop-up before my_form_treatment

As someone mentioned previously, return false would stop propagation (i.e. if there are more handlers attached to the form submission, they would not be executed), but, in this case, the action triggered by the browser will always execute first. Even with a return false at the end.

So if you want to get rid of these default pop-ups, use the click event on the submit button:

$('form input[type=submit]').on('click', function(event) {
event.preventDefault();
my_form_treatment(this, event);
}); // -> this will NOT show any popups related to HTML attributes

Unlike most of the previous answers, the solution below demonstrates how to prevent the page from refreshing/redirecting on <form> submission using pure Javascript, instead of JQuery.

The HTML form

No need for the onclick event (which fires when the user uses the mouse to click on a button) or the onsubmit event (which fires when the user hits the enter key) on the submit button.

<form id="myForm">
<input type="text" name="contactName" id="contactName">
<input class="submit" type="submit" value="Submit">
</form>

The Javascript code

Below is the Javascript code to handle the form submission on the submit event. The preventDefault() method of the Event interface tells the user agent that if the event does not get explicitly handled, its default action should not be taken as it normally would be.

Note: Make sure to register the event handler after the HTML element thas been added to the DOM tree; otherwise, a runtime error will be caused, as you'll be trying to set a property (an event handler) of a non-existent object. One way to ensure this is to simply place the script after the element in question (i.e., <form>), but as this might be a bit dangerous—since you are relying on how you assume a browser works—you can assign the event handler after the initial HTML document has been completely loaded and parsed, using the DOMContentLoaded event. Example:

document.addEventListener('DOMContentLoaded', (event) => {
document.getElementById("myForm").addEventListener("submit", function(e) {
e.preventDefault() // Cancel the default action
sendContactForm();
});
});

All together

<!DOCTYPE html>
<html>
<head>
<script>
document.addEventListener('DOMContentLoaded', (event) => {
document.getElementById("myForm").addEventListener("submit", function(e) {
e.preventDefault() // Cancel the default action
sendContactForm();
});
});
</script>
</head>
<body>
<form id="myForm">
<input type="text" name="contactName" id="contactName">
<input class="submit" type="submit" value="Submit">
</form>
</body>
</html>