Detecting an undefined object property

Striding through undefined: your toolset

To navigate the undefined landscape effectively, here are some tried and tested tools you should always keep at your disposal.

hasOwnProperty - your property owner investigator

Use obj.hasOwnProperty('prop') when you need to confirm whether a property exists on an object without considering the prototype chain.

if (obj.hasOwnProperty('someProperty')) {
  // Property is owned by obj, but its value could still be undefined
  console.log('Egad! We may well be onto something.');
}


The untouchable undefined in ES5 and beyond

Post ES5, undefined is an immutable standard value. However pre-ES5, void 0 was used frequently as a secure way to get the undefined value; today it serves as a historical reminder of JavaScript's dynamic evolution.

Undefined, defined? typeof is your answer

Whether a variable is declared or not, typeof will safely return 'undefined' if it holds no value. If in doubt, typeof is your scout.

if (typeof notSureThisExists === 'undefined') {
  // Variable either is undeclared or holds undefined value
  console.log('A variable walked into a bar… only to realize it was undefined.');
}


JavaScript scope: named undefined variables

Be careful out there, because JavaScript allows local variables to be named undefined, which can end up hiding the global undefined. Keep a keen eye on the scope!

Playing "hide-and-seek" with properties using in

The in operator does a thorough job when checking for property existence, diving into the prototype chain too, unlike hasOwnProperty.

if ('someProperty' in obj) {
  // Property found in obj or within its prototype chain
  console.log('Jackpot! This property plays hard to get.');
}


Keep those traps at bay: common pitfalls and solutions

Let's go through some common mistakes you could make while dealing with undefined properties and learn how to avoid them.

That falsy booby trap

Beware, if(obj.prop) checks for other falsy values—'', 0, null, not just undefined. Explicitness is your best bet in JavaScript.

The undeniable power of triple equals

A direct obj.prop === undefined check is clean when looking for explicit undefined properties. It's like stating loudly, "I want to see your ID".

if (typeof obj !== 'undefined' && obj.someProperty === undefined) {
  // obj exists, and obj.someProperty is explicitly undefined
  console.log('I found undefined! Oh wait, it was hiding in plain sight.');
}


null vs undefined: the battle of missing values

Remember, null and undefined are almost twins in JavaScript—null == undefined. So when a property's null, it's a significant absence of value.

if (person.name == undefined) {
  // person.name is either null or undefined
  console.log('Hello, Null or Undefined. Nice to meet... Hmm, who exactly?');
}
