有条件的ajax调用

This is perhaps very basic jquery syntax, but I am not very familiar with it. I want to make a jquery ajax call only when an html input with a specific id exists on the page. How do I do this?

You can check if an element with the id exists on page, using length on the selector.If yes, perform your request

if($("#yourId").length>0){
     //Your request
     $.post( "ajax/test.html", function( data ) {

     });
}

1. Make shure that your script is included after the markup to validate (eg just before the closing </body-tag)or wait for the document ready DOM-event.
2. Check for the DOM-elements existence.

$( document ).ready(function() {
    if ( $('#jepp').length ) { // 0 validates falsy and != 0 truthy
        $('#jepp').val('do your stuff eg ajax')
    }
});

// or a little bit more advanced
$(function() { // document ready
    if ( document.getElementById('jepp') ) { // vanilla JavaScript but still straight forward
        $('#jepp').val( $('#jepp').val() + ' | a bit more advanced')
    }
})

<input id="jepp" value="" style="width:100%">

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

</div>

<input name="foo" id="data">
<script type="javascript">
    ...
        var elementSize = $('#data').size();
        if(size > 0){
             $.ajax({
                ...
             });
             // Page contains id
        }else {
            //Page doesn't contains id
        }
    ...
</script>

You can do like that.

$('#element-id').size() or $('#element-id').length

is size of id element. Id value must be 1 and unique. you check size, and make ajax call