verifica l'esempio.

codice:
<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<button>doc1</button>
<button>doc2</button>
<div id="result"></div>

<body>
    <script>
        // creo l'array dei button
        const buttons = document.getElementsByTagName("button")
        const result = document.getElementById('result')
        // programmo i buttons
        for (let i = 0; i < buttons.length; i++) { buttons[i].onclick = carica; }

        function carica(e) {
            const httpRequest = new XMLHttpRequest()
            httpRequest.responseType = 'text'
            httpRequest.open("GET", e.target.textContent + ".html", true)
            httpRequest.send()
            httpRequest.onreadystatechange = function () {
                if (this.readyState == 4 && this.status == 200) {
                    const DOM = new DOMParser()
                    const html = DOM.parseFromString(httpRequest.response, "text/html")
                    result.textContent = html.getElementById('qui').textContent
                }
            }
        }

    </script>
</body>

</html>