var haha="<br />";
haha=haha.replace('<br />','\n');
var showMsg=document.getElementById("showMsg");
showMsg.value="后面是换行"+haha+"第二行";
haha=haha.replace('<br />','\n');
var showMsg=document.getElementById("showMsg");
showMsg.value="后面是换行"+haha+"第二行";