let obj = {};
if (!obj[3]) {
console.log('obj[3] is not existed')
}
if (typeof obj[3] === 'undefined') {
console.log('obj[3] is not existed')
}
if (typeof(obj[3]) === 'undefined') {
console.log('obj[3] is not existed')
}
let arr = [];
if (!arr[3]) {
console.log('arr[3] is not existed')
}
if (typeof arr[3] === 'undefined') {
console.log('arr[3] is not existed')
}
if (typeof(arr[3]) === 'undefined') {
console.log('arr[3] is not existed')
}