Test if an object exists JavaScript

Question

I can't believe this question would not have been asked before, but I searched as much as I could and couldn't find it so here it goes.

I want to test if a dynamically built JavaScript object exists. However when I test this:

var myObject = {};
var dynamicName = "customName";

if(myObject[dynamicName].object == undefined){
  myObject[dynamicName].object = "something"; // Make an entry in the dynamic object
  alert("The object didn't exist, so we populated it");
}else{
  alert("The object already exist");
}

If the object doesn't exist and I try to run the above code I get an error saying "myObject[dynamicName] is undefined" and all javascript execution halts.

Is there a way to check if this object exists I want without causing such an error?

Possible duplicate of [Check if object exists in JavaScript](https://stackoverflow.com/questions/4186906/check-if-object-exists-in-javascript) — Joseph Cho, Jul 26 '18 at 02:31
Because that alert saying it's not defined is in the same if that's defining it. So the order of the if is: 1. test and find not defined, 2. define it, 3.alert (not defined) — Mark, Jul 26 '18 at 02:31
@MarkMeyer How would that explain the error? I understand the alerts aren't perfect but that's just psudeo code. I really want to get rid of the error — YAHsaves, Jul 26 '18 at 02:32
I don't get an error when I run your code (other than the syntax errors), but I did make a `myObject` object and populate `dynamicName` with a string. It would help if you made a runnable snippet with the (<>) button that showed the error. — Mark, Jul 26 '18 at 02:33
@MarkMeyer Sorry I posted this question too quick without testing it myself, I edited the code to reflect more my true circumstances and this does cause the error — YAHsaves, Jul 26 '18 at 02:36

score 2 · Accepted Answer · answered Jul 26 '18 at 02:44

2

If myObject[dynamicName] doesn't exist, you can't access its properties. Trying to access or assign a property of undefined causes an error.

You need to create the object, then you can create the property

if (myObject && myObject[dynamicName] && myObject[dynamicName].object !== undefined) {
    alert("The object already exists");
} else {
    myObject = myObject || {};
    myObject[dynamicName] = myObject[dynamicName] || {};
    myObject[dynamicName].object = something;
    alert("The object didn't exist, so we populated it");
}

answered Jul 26 '18 at 02:44

Barmar

741,623
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Thank you that worked! Just to make sure I understand what's going on, are we creating the object piece by piece as we go along? – YAHsaves Jul 26 '18 at 02:47
Yes, that's what it's doing. It uses `||` so if that level of the object nesting already exists, it's reused. – Barmar Jul 26 '18 at 14:41

amankkg · Answer 2 · 2018-07-26T03:34:56.357

1

There is a shorter version, (almost) without clumsy if's

const myObject = {};
const dynamicName = 'customName';

const { [dynamicName]: obj = {} } = myObject;
if (obj.object == null) {
  obj.object = 'something'
  console.log('populated');
} else {
  console.log('existed');
}
myObject[dynamicName] = obj;

console.log(myObject)

edited Jul 26 '18 at 03:34

answered Jul 26 '18 at 02:55

amankkg

4,503
1
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How can you tell if the object was made before? If the object is already made I want to take another path of logic – YAHsaves Jul 26 '18 at 03:29
yeah, if there are different statements required then we have to use `if` :( – amankkg Jul 26 '18 at 03:36

Dinesh Ghule · Answer 3 · 2018-07-26T02:53:33.240

0

Check typeof == object

var userObject={
"name":"abc",
"kyc":true
};

var dynamicName="name";

if (typeof userObject == 'object' && userObject[dynamicName] != undefined) {
  console.log("UserObject Exists and valid");
}else{
console.log("UserObject does not Exists");
}

edited Jul 26 '18 at 02:53

answered Jul 26 '18 at 02:42

Dinesh Ghule

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4
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Test if an object exists JavaScript

3 Answers3