如何检查对象是否存在并具有X属性？

Question

I am trying to check if an object exists and has X property.

First I try it like this:

let object = {
    foo: 1
}

console.log('foo' in object)
console.log(object.hasOwnProperty('foo'))
console.log(typeof(object.foo) !== undefined)

But I realized that if the object is undefined all of them return errors.

I know I can use:

let object = undefined

if (object) {
    console.log('foo' in object)
    console.log(object.hasOwnProperty('foo'))
    console.log(typeof (object.foo) !== undefined)
}

To check if an object exists and has X property, but I would like to know if I can resume all of this in one line. Something like:

typeof(object) !== undefined && ('foo' in object)

Answer 1

You can use it just like you were doing in your multi line example, by just testing object .

 function testObject(object) { if (object && ('foo' in object)) { console.log(true); } else { console.log(false); } } testObject(undefined); testObject({foo: 'bar'}); 

Answer 2

只需首先使用检查对象是否存在：

object && 'foo' in object

Answer 3

these examples works fine:

 let object = { foo: 1 }; if (object && object.hasOwnProperty('foo')) { console.log(object['foo']); } else { console.log('object has no foo key'); } let obj2 = {}; if (obj2 && obj2.hasOwnProperty('foo')) { console.log(obj2['foo']); } else { console.log('obj2 has no foo key'); } let obj3; if (obj3 && obj3.hasOwnProperty('foo')) { console.log(obj2['foo']); } else { console.log('obj3 has no foo key'); } 

For this one I don't have any idea

 if (obj3 && obj3.hasOwnProperty('foo')) { console.log(obj2['foo']); } else { console.log('obj3 has no foo key'); }