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How to Automatically Add Properties to an Object that is Undefined with JavaScript?

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Sometimes, we want to automatically add properties to an object that is undefined with JavaScript.

In this article, we’ll look at how to automatically add properties to an object that is undefined with JavaScript.

Automatically Add Properties to an Object that is Undefined with JavaScript

To automatically add properties to an object that is undefined with JavaScript, we can use the hasOwnProperty method to check if a property exists.

It’ll return true if it exists and false otherwise.

If it returns false , then we can set the property value to the value we want.

For instance, we can write:

const test = {}  
if (!test.hasOwnProperty('hello')) {  
  test.hello = {};  
}  
test.hello.world = "Hello World!"

We have the test object that we want to add the hello property to if it doesn’t exist.

We check that it doesn’t exist with !test.hasOwnProperty(‘hello’) .

And if that’s true , we set test.hello to an empty object.

Then we set test.hello.world to “Hello World!” .

The hasOwnProperty method can be overridden easily since it’s inherited from the Object constructor.

Therefore to make sure we always call the right hasOwnProperty method, we can write:

const test = {}  
if (!Object.prototype.hasOwnProperty.call(test, 'hello')) {  
  test.hello = {};  
}  
test.hello.world = "Hello World!"

We call Object.prototype.hasOwnProperty.call with test to do the same thing as test.hasOwnProperty , but we make sure that we always call the right one from the Object constructor.

Conclusion

To automatically add properties to an object that is undefined with JavaScript, we can use the hasOwnProperty method to check if a property exists.

It’ll return true if it exists and false otherwise.

If it returns false , then we can set the property value to the value we want.

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