PHP ReactJS/Axios Ajax POST to codeigniter problem

On the codeigniter side the post data isn't being received.  PHP is however receiving it.

Here is the ajax call

   
    var postData = {
        'partNumber' : '45367'
    }
axios.post('http://localhost/stuff/index.php/admin/reactTestaroo', postData)
        .then(response => renderTheTestData(response.data))

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Here is what is in my PHP/CI controller:

    function reactTestaroo(){
        $entityBody = file_get_contents('php://input'); // just as a test I used this and it returns "{'partNumber':'45367'}" which seems to indicate it's being passed fine
        $pastNumber = $this->input->post('partNumber'); // returns NULL, this is the problem.. it should show the part number value.  
    }

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I can use jQuery ajax all day long and it works fine like this:

    $.ajax({
        url: 'http://localhost/stuff/index.php/admin/reactTestaroo',
        data: postData,
        type: 'POST',
        success: function(response) {
        var parsedJSON = $.parseJSON(response);
        renderTheTestData(parsedJSON.part_name)
        },
        error: function(response){
            alert('nope');
        }
    });

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With jQuery it works every time but I want to ditch jQuery and use react and a ajax plugin like axios but clearly I'm having an issue with it.  Can someone help me get the ajax via axios working?