hasOwnProperty()
方法,另一个使用in
关键字。如果我们有一个具有Alan
属性的对象users
,我们可以通过以下任一方式检查其存在: users.hasOwnProperty( '艾伦');
'艾伦'在用户;
//都返回true
users
,其中包含一些用户和一个函数isEveryoneHere
,我们将users
对象作为参数传递给它。完成编写此函数,以便仅当users
对象包含所有四个名称( Alan
, Jeff
, Sarah
和Ryan
)作为键时才返回true
,否则返回false
。 users
对象仅包含Alan
, Jeff
, Sarah
和Ryan
testString: 'assert("Alan" in users && "Jeff" in users && "Sarah" in users && "Ryan" in users && Object.keys(users).length === 4, "The users
object only contains the keys Alan
, Jeff
, Sarah
, and Ryan
");'
- text: 如果Alan
, Jeff
, Sarah
和Ryan
是users
对象的属性,则函数isEveryoneHere
返回true
testString: 'assert(isEveryoneHere(users) === true, "The function isEveryoneHere
returns true
if Alan
, Jeff
, Sarah
, and Ryan
are properties on the users
object");'
- text: 如果Alan
, Jeff
, Sarah
和Ryan
不是users
对象的属性,则函数isEveryoneHere
返回false
testString: 'assert((function() { delete users.Alan; delete users.Jeff; delete users.Sarah; delete users.Ryan; return isEveryoneHere(users) })() === false, "The function isEveryoneHere
returns false
if Alan
, Jeff
, Sarah
, and Ryan
are not properties on the users
object");'
```