Failed to get DOM in insert mode

[zxyqq]

I want to use the following map to trigger the specified DOM item clicked. but failed
while it works when execute the same code in console by pressing F12, why?
var a = document.getElementsByClassName("icon aui-icon aui-icon-small aui-iconfont-editor-list-number")

imapkey('', "#confluence list-number", function() {
alert("xxx1")
var a = document.getElementsByClassName("icon aui-icon aui-icon-small aui-iconfont-editor-list-number")
alert(a)
alert(a.length)
if(a.length) {
alert("xxx2")
a[0].click();
}
});