Test if something is not undefined in JavaScript

Question

I\'m checking if(response[0].title !== undefined), but I get the error:

Uncaught TypeError: Cannot read property \'title\' of undefined.<

Answer 1

typeof:

var foo;
if (typeof foo == "undefined"){
  //do stuff
}

Answer 2

You must first check whether response[0] is undefined, and only if it's not, check for the rest. That means that in your case, response[0] is undefined.

Answer 3

Actually you must surround it with an Try/Catch block so your code won't stop from working. Like this:

try{
    if(typeof response[0].title !== 'undefined') {
        doSomething();
    }
  }catch(e){
    console.log('responde[0].title is undefined'); 
  }

Answer 4

I had trouble with all of the other code examples above. In Chrome, this was the condition that worked for me:

typeof possiblyUndefinedVariable !== "undefined"

I will have to test that in other browsers and see how things go I suppose.

Answer 5

Just check if response[0] is undefined:

if(response[0] !== undefined) { ... }

If you still need to explicitly check the title, do so after the initial check:

if(response[0] !== undefined && response[0].title !== undefined){ ... }

Answer 6

Check if you're response[0] actually exists, the error seems to suggest it doesn't.