如何防止网页刷新JAVASCRIPT？ [重复]

Question

This question already has an answer here:

• Javascript - Stop form submit depending on ajax response [duplicate] 2 answers

I've been creating a form to submit values using AJAX and PHP. but every time the value sent the return value of AJAX = 0. i figured it out the problem he page was refreshed.

AJAX

var xhttp = new XMLHttpRequest();
xhttp.open(//code to the php);
xhttp.setRequestHeader("content-type", "application/...");
xhttp.open(data);

In this case I dont want to use jQuery. I want to finish this with Javascript or PHP.

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