当你调用 join 而没有传递任何参数时，,(comma) 被作为默认值，并且toString 在不传递任何参数的情况下内部调用 join.

When you call join without any argument being passed, ,(comma) is taken as default and toString internally calls join without any argument being passed.

所以，传递你自己的分隔符.

So, pass your own separator.

var str = array.join(' '); //'apple tree'
// separator ---------^

Array.join 上的 MDN