What is the best practice for checking undefined property in chained object form?

Question

Let's say I'm implementing a.b.c.d on separate file.

So, I need to check a,b,c is defined and fallback into catch clause when those property is not defined.

Currently, I'm using this code. But it's too long, dirty and hard to maintain.

try{
  if(!("a" in window)) {
    throw new Error();
  }
  if(!("b" in a)) {
    throw new Error();
  }
  if(!("c" in a.b)) {
    throw new Error();
  }
} catch(e){ }

What is the best practice?

You might look at [*Access / process (nested) objects, arrays or JSON*](http://stackoverflow.com/questions/11922383/access-process-nested-objects-arrays-or-json/11922384#11922384). Define "best practice", it depends on your criteria for "best". — RobG, Apr 12 '16 at 02:08

score 1 · Accepted Answer · answered Apr 12 '16 at 01:06

1

You're manually throwing an error in each of those cases, but the behaviour when you try to access a property of undefined is to throw a ReferenceError anyway, so you can just catch that instead:

try{
  a.b.c.d
} catch(e){ // Either a, a.b, or a.b.c was undefined }

answered Apr 12 '16 at 01:06

Paul

139,544
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Since you caught the [*error*](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error), you can also look at the message, e.g. `Cannot read property 'd' of undefined`. ;-) – RobG Apr 12 '16 at 02:12

score -2 · Answer 2 · answered Apr 12 '16 at 01:06

-2

if !(a && a.b && a.b.c && a.b.c.d) {
  throw new Error();
}
// else a.b.c.d exists

answered Apr 12 '16 at 01:06

Wainage

4,892
3
12
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1

Except that if `a.b.c.d` exists and has a falsey value, this will throw an error (not my down vote BTW). – RobG Apr 12 '16 at 02:09

What is the best practice for checking undefined property in chained object form?

2 Answers2