Making a simple form validator with AJAX and JSON but without jQuery

1.html : My goal is to make a simple form with name, password and password confirmation with AJAX and JSON but absolutely without jQuery.

2.javascript: make a XMLHttpRequest() with POST

3.JSON : make a php file for JSON.parse()

I want to check the availability of login and the equality of password

HTML:

<form>
<div>
    <label>Login</label>
    <input placeholder="Login" type="text" name="login" id="login"/>
</div>
<div>
    <label>Password</label>
    <input placeholder="password" type="password" name="password1" id="password1"/>
</div>
<div>
    <label>Confirm Password</label>
    <input placeholder="password" type="password" name="password2" id="password2"/>
</div>
<div>
    <button type="submit" name="submit" id="submit">Send</button>
</div>

<div id="response"> <h1>Response</h1> </div>

JAVASCRIPT:

(function () {
var bouton = document.getElementById('submit');
var reponse = document.getElementById('reponse');

bouton.addEventListener('click', ajax, false);

function ajax() {
    var xhr = new XMLHttpRequest();
    var $$$ = document.getElementById('$$$').value;
    var url = 'file.php?$$$=' + $$$;

    xhr.open(
            'POST',
            url
            );

    xhr.send(null);

    xhr.onreadystatechange = function () {


        switch (xhr.readyState) {
            case xhr.DONE:
                if (xhr.status === 200) {
                    var json = JSON.parse(xhr.responseText);
                    reponse.innerHTML += '<p>' + json + '</p>';

                } else {
                    console.log('STATUS (' + xhr.status + ') : ' + xhr.statusText);
                }

                break;

            default:
                console.log('DEFAULT CASE');

                break;
        }
    };
}
})();

PHP File:

I don't know ... with JSON

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Thank you for your great help !