js中有五个false的值
console.log(Boolean(false)) console.log(Boolean(undefined)) console.log(Boolean("")) console.log(Boolean(0)) console.log(Boolean(null))
其余均为true 如下边几个都为true
console.log(Boolean("false")) console.log(Boolean("null")) console.log(Boolean(-1))
console.log(Boolean({}))
所以判断数组长度不为0 就可以用
if(arr.length)