JavaScript Answers

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

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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 = {};  
} = "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 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 (!, 'hello')) {  
  test.hello = {};  
} = "Hello World!"

We 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.

By John Au-Yeung

Web developer specializing in React, Vue, and front end development.

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