How to check if a "leave" member exists without testing existence of its parents

Question

Imagine this code:

var obj = {};
obj.x = {};
obj.x.y = {};

if (obj.x.y.z) {
  //the test work, I know obj.x.z.z do not exist
}

if (obj.a.b) {
  //fail because "cannot read property 'b' of undefined
}

The question is : How I can check the existence of a "leave" (if "b" exist, it must inside "a") without making the test of each its parents before ?

Check my answer this way you no need to figure what leave and parent are there. you can figure out passing params. — aviboy2006, May 30 '20 at 19:25

score 0 · Answer 1 · answered May 30 '20 at 19:18

0

Use to check if key exist or not.

var obj = {};
obj.x = {};
obj.x.y = {};

if (obj.x.y.z) {
  //the test work, I know obj.x.z.z do not exist
}

function isExist(arg){
   try{
      return arg();
   }catch(e){
      return false;
   }
}

console.log(isExist(()=>obj.x.y))
console.log(isExist(()=>obj.x.y.z))
console.log(isExist(()=>obj.a.b))

Reference : Test for existence of nested JavaScript object key

answered May 30 '20 at 19:18

aviboy2006

7,334
5
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1

Please flag the questions as duplicates instead of taking the answers from other questions and posting them again – adiga May 30 '20 at 19:24
Sure. Thanks will do. – aviboy2006 May 30 '20 at 19:26
Once you have 3k reputation, you can [vote to close](https://stackoverflow.com/help/privileges/close-questions) directly – adiga May 30 '20 at 19:27
Right now dont have that. Trying to get that. – aviboy2006 May 30 '20 at 19:28

score 0 · Answer 2 · answered May 30 '20 at 19:19

0

You can use elvis operator.

if (obj.a?.b) { // no error thrown
  console.log('error'); // 'error' isn't be logged
}

answered May 30 '20 at 19:19

vadimk7

3,389
1
11
14

I think you mean optional chaining – richytong May 30 '20 at 19:22
this way to should know before a is exist or not. Everything you have to add if case. – aviboy2006 May 30 '20 at 19:24
No, you don't. In this case `a` is undefined so we skip the code inside if case – vadimk7 May 30 '20 at 19:27

score 0 · Answer 3 · answered May 30 '20 at 19:21

There is a proposal in the works for an operator that looks like ?. called optional chaining

You would then use it on your example like

obj.a?.b

For now though, you'll have to write more to safely access nested properties

var obj = {};
obj.x = {};
obj.x.y = {};

if (obj.x.y.z) {
  //the test work, I know obj.x.z.z do not exist
}

if (obj && obj.a && obj.a.b) {
  // will not throw
} else {
  console.log('didn\'t throw!')
}

How to check if a "leave" member exists without testing existence of its parents

3 Answers3

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