Check if part of an object is defined

Question

I'm trying to set a variable:

latestPostId = posts[latestPost].post_id

But in one scenario, it is not defined yet. What is the best way to check?

I have tried these:

if (data.post_id !== undefined) {
if (data.post_id !== 'undefined') {
if (typeof data.post_id != 'undefined') {

But none seem to work. What is the best way to check if posts[latestPost].post_id is defined?

`if (posts && latestPost in posts && 'post_id' in posts[latestPost])` — adeneo, May 06 '14 at 21:49
Check out this question and explanation - http://stackoverflow.com/questions/519145/how-can-i-check-whether-a-variable-is-defined-in-javascript — WillardSolutions, May 06 '14 at 21:50
possible duplicate of [javascript test for existence of nested object key](http://stackoverflow.com/questions/2631001/javascript-test-for-existence-of-nested-object-key) — Felix Kling, May 06 '14 at 22:58

score 0 · Answer 1 · answered May 06 '14 at 21:49

0

What is the best way to check if posts[latestPost].post_id is defined?

Does posts[latestPost].post_id !== undefined work?

I would probably do posts[latestPost] && posts[latestPost].post_id.

answered May 06 '14 at 21:49

djechlin

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score 0 · Answer 2 · answered May 06 '14 at 21:50

0

You can do it in multiple ways (your last one is ok):

   if (typeof data.post_id != 'undefined')

Or simpler

   if (data.post_id)

BUT: This may generate an error, if data does not exist at all. So you should do:

   if (data && data.post_id)

For an object like: a.b.c.d.e.f, you should do:

   if (a && a.b && a.b.c && a.b.c.d && etc)

answered May 06 '14 at 21:50

zozo

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typeof is not a function – adeneo May 06 '14 at 21:50
check it in your console – zozo May 06 '14 at 21:53
but you are right... js doc says is operator. my bad. => edited answer. – zozo May 06 '14 at 21:54
I'm not saying it's not working, but you wrote "your last one was close", and it wasn't close, it was correct, yours is wrong, `typeof` is still not a function. – adeneo May 06 '14 at 21:54

score 0 · Answer 3 · answered May 06 '14 at 21:58

You could use a helper to avoid long if statements and improve re-usability, something like:

/**
 * @param {string} key
 * @param {object} obj
 */
function keyExists(key, obj) {
  var res = key.split('.').reduce(function(acc,x) {
    return acc[x];    
  },obj);
  return res != null || false;
}

And use it like so:

var obj = {a: {b: {c:'foo'}}};

console.log(keyExists('a.b.c', obj)); //=> true
console.log(keyExists('a.b.c.d', obj)); //=> false

if (keyExists('a.b.c', obj)) {
  // it exists
}

score 0 · Answer 4 · answered May 06 '14 at 22:06

0

The in operator checks an object for a property.

'post_id' in posts[latestPost]`

Some people prefer hasOwnProperty. in traverses an object's prototype chain, while hasOwnProperty doesn't.

answered May 06 '14 at 22:06

guest

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I should point out that you can define a property to hold the value known as `undefined`. JavaScript, yeah! – guest May 06 '14 at 22:16

Check if part of an object is defined

4 Answers4