length = 0时，布尔值为false
比如在if判断时

	const arr = []
	if(arr.length){
		console.log(1)
	}
	if(!arr.length){
		console.log(2)
	}

最后输出2

        arr1 = []
        console.log(arr1.length);
        console.log(arr1.length === false);
        console.log(arr1.length == false);
        console.log(0 == false);
        console.log(1 == true);
        console.log(2 == true);
        if(!arr1.length) console.log('!');

可见，0判断为false，1为true，但是===就不行了