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// I generally use the typeof operator:

if (typeof obj.foo !== 'undefined') {
  // your code here
}
// It will return "undefined" either if the property doesn't exist or its value is undefined.


// There are other ways to figure out if a property exists on an object, like the hasOwnProperty method:

if (obj.hasOwnProperty('foo')) {
  // your code here
}

// And the in operator:

if ('foo' in obj) {
  // your code here
}

// The difference between the last two is that the hasOwnProperty method will check if 
// the property exist physically on the object (the property is not inherited).

// The in operator will check on all the properties reachable up in the prototype chain, e.g.:

var obj = { foo: 'bar'};

obj.hasOwnProperty('foo'); // true
obj.hasOwnProperty('toString'); // false
'toString' in obj; // true

// As you can see, hasOwnProperty returns false and the in operator returns true when checking the toString 
// method, this method is defined up in the prototype chain, because obj inherits form Object.prototype.