不确定此JavaScript“未定义”检查出了什么问题

Question

I thought I had correctly followed the recommendations in this question for checking undefined :

if(typeof window.session.location.address.city != "undefined")       
    console.log(window.session.location.address.city);

But the code above generates this error:

 Uncaught TypeError: Cannot read property 'city' of undefined

What's the correct way to perform this check?

Answer 1

address is undefined so you can't read its property city

You'll have to check that address is defined first.

Answer 2

Check for the existence of each property:

if (window.session && session.location && session.location.address)
    console.log(session.location.address.city);

That may log undefined , but will not result in an error. If you only want to log city if it is not undefined , just add in a && typeof session.location.address.city != "undefined" . We use typeof in this case because if city contains an empty string or null , it will also evaluate to false (ie, it is "falsey"). Of course, if you only want to log city if it has a value, leave off the typeof and just check to see if it evaluates to true (ie, it is "truthy") the same way as the others.

Answer 3

if(typeof window.session.location.address === 'undefined')
    alert("address is undefined!");
else
    console.log(window.session.location.address.city);