PHP JSON header causes error on JSON.parse (using jQuery)

I am getting some JSON data in JavaScript from a PHP file.
To use the data, I am using JSON.parse(json_response) which works except if I use a JSON header in my PHP:

header('Content-Type: application/json');

When used, I get the following message in console:

Uncaught SyntaxError: Unexpected token o in JSON at position 1

At the moment I just choose between using the PHP header() or the JS JSON.parse() and found this useful question.
It looks like setting a JSON header "automatically parses" the JSON for my JS script.

• Is it a normal behaviour ? What may cause this ?
• What should I do ? Randomly choosing between header() and JSON.parse() probably isn't a good idea.

Actual code:
index.chart.php:

<?php

header('Content-Type: application/json');
// [...] <- config lines, no output


// Dummy data for Chart.js
$data = [
    'labels'   => ['test', 'a', 'z', 'e', 'r', 't'],
    'datasets' => [
        [
            'label'           => 'First',
            'backgroundColor' => 'rgb(63, 123, 249)',
            'borderColor'     => 'rgb(31, 117, 219)',
            'data'            => [
                20, 10, 30, 45, 51.2, 5
            ],
            'fill' => false
        ]
    ]
];

echo json_encode($data);

?>

index.chart.js:

window.addEventListener('DOMContentLoaded', function () {

    // jQuery Ajax
    $.get('assets/inc/index.chart.php').done(function (json) {

        var response = JSON.parse(json);
        console.log(response);

    }).fail(function (error) {
        window.console.log(error);
    });

});