js中关键字in

`let opo = ['a', 'b', 'c', 'd']

console.log('arr1', 'b' in opo);

console.log('arr2', 3 in opo);

let opj = { aa: '0', bb: '1', cc: '2', dd: '3' }

console.log('obj1', 'bb' in opj);

console.log('obj2', 2 in opj); `

结论：in：查询数组中的下标， 查询对象中的属性名