var event = new Event('mousedown');
   document.dispatchEvent(event);

通过 new Event 来实现

例：

js代码 :

var btn1 = document.getElementById('btn1')

btn1.onclick = function(ele) {
    var div1 = document.getElementById('div1')
    var event = new Event('click');
    div1.dispatchEvent(event);
}

html ：

<div id="div1" onclick="alert('div clicked')">click via button</div>
<button id="btn1">click div</button>