Test for existence of nested JavaScript object key

You have to do it step by step if you don't want a TypeError because if one of the members is null or undefined, and you try to access a member, an exception will be thrown.

You can either simply catch the exception, or make a function to test the existence of multiple levels, something like this:

function checkNested(obj /*, level1, level2, ... levelN*/) {
  var args = Array.prototype.slice.call(arguments, 1);

  for (var i = 0; i < args.length; i++) {
    if (!obj || !obj.hasOwnProperty(args[i])) {
      return false;
    }
    obj = obj[args[i]];
  }
  return true;
}

var test = {level1:{level2:{level3:'level3'}} };

checkNested(test, 'level1', 'level2', 'level3'); // true
checkNested(test, 'level1', 'level2', 'foo'); // false

ES6 UPDATE:

Here is a shorter version of the original function, using ES6 features and recursion (it's also in proper tail call form):

function checkNested(obj, level,  ...rest) {
  if (obj === undefined) return false
  if (rest.length == 0 && obj.hasOwnProperty(level)) return true
  return checkNested(obj[level], ...rest)
}

However, if you want to get the value of a nested property and not only check its existence, here is a simple one-line function:

function getNested(obj, ...args) {
  return args.reduce((obj, level) => obj && obj[level], obj)
}

const test = { level1:{ level2:{ level3:'level3'} } };
console.log(getNested(test, 'level1', 'level2', 'level3')); // 'level3'
console.log(getNested(test, 'level1', 'level2', 'level3', 'length')); // 6
console.log(getNested(test, 'level1', 'level2', 'foo')); // undefined
console.log(getNested(test, 'a', 'b')); // undefined

The above function allows you to get the value of nested properties, otherwise will return undefined.

UPDATE 2019-10-17:

The optional chaining proposal reached Stage 3 on the ECMAScript committee process, this will allow you to safely access deeply nested properties, by using the token ?., the new optional chaining operator:

const value = obj?.level1?.level2?.level3 

If any of the levels accessed is null or undefined the expression will resolve to undefined by itself.

The proposal also allows you to handle method calls safely:

obj?.level1?.method();

The above expression will produce undefined if obj, obj.level1, or obj.level1.method are null or undefined, otherwise it will call the function.

You can start playing with this feature with Babel using the optional chaining plugin.

Since Babel 7.8.0, ES2020 is supported by default

Check this example on the Babel REPL.

🎉🎉UPDATE: December 2019 🎉🎉

The optional chaining proposal finally reached Stage 4 in the December 2019 meeting of the TC39 committee. This means this feature will be part of the ECMAScript 2020 Standard.

Here is a pattern I picked up from Oliver Steele:

var level3 = (((test || {}).level1 || {}).level2 || {}).level3;
alert( level3 );

In fact that whole article is a discussion of how you can do this in javascript. He settles on using the above syntax (which isn't that hard to read once you get used to it) as an idiom.