onclick 点击不生效，但是每次刷新就能触发一次

<html>
<head>
    <title>var that = this</title>
</head>
<body>
    <p id="demo">1234</p>
    <button id="btn">随机点名</button>
</body>
<script>
    let demo = document.getElementById("demo");
    let btn = document.getElementById("btn");
    let names = ['小明', '张强', '迪子鄂', '车兮', '乌镇可', '髭须', '貔貅', '饕餮', '魍魉', '魑魅'];
    let ranarr = function(arr) {
        let rannum = Math.ceil(Math.random() * 10);
        return arr[rannum - 1];
    }

    let dm = function(tdemo, arr, callback) {
        demo.innerHTML = ranarr(names);
    }

    let dj = function() {
        console.log('按钮被点击')
    }
    btn.onclick = dm(demo, names, ranarr);
</script>
</html>