$.ajax serialize() does not pass data to php file - ajax

Question

What is incorrect in the code? Can not pass data to _autosave.php
<script type="text/javascript">
$(document).ready(function(){
autosave();
});
function autosave() {
var t = setTimeout("autosave()", 5000);
var inputValues= $('.input_form').serialize();
$.ajax( {
type: "POST",
url: "_autosave.php",
data: inputValues,
} )
.done(function(data){
alert(data);
});
...
    
Input is this
<form id="input_form" autocomplete="off" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]) ?>"
method="post">
<input type="text" name="input" id="input">
_autosave.php is this
$input = $_POST['input'];
echo $input .' input<br>';
If I enter some value in input, get input<br> instead of entered value
Update
If for someone may be necessary here is working code
$.post("_autosave.php", $("#form1").serialize(), function(data) {
$('#load').html(data);
$('#is_row_changed1').val(0)
});

Answer 1

You have a trailing comma here resulting in invalid javascript:
data: inputValues,
Here's how you could fix (and improve your current code):
<script type="text/javascript">
$(document).ready(autosave);
function autosave() {
window.setTimeout(autosave, 5000);
var inputValues = $('.input_form').serialize();
$.ajax({
type: "POST",
url: "_autosave.php",
data: inputValues
})
.done(function(data) {
alert(data);
});
}
</script>
or if you prefer a shorthand:
<script type="text/javascript">
$(document).ready(autosave);
function autosave() {
window.setTimeout(autosave, 5000);
var inputValues = $('.input_form').serialize();
$.post("_autosave.php", inputValues, function(data) {
alert(data);
});
}
</script>

Answer 2

Have you tried with serializeArray() instead?
<script type="text/javascript">
$(document).ready(autosave);
function autosave() {
window.setTimeout(autosave, 5000);
$.post("_autosave.php", $('.input_form').serializeArray(),
function(data) {
alert(data);
});
}
</script>