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  3. Conditionally Add Properties to an Object in JavaScript or Node.js

Conditionally Add Properties to an Object in JavaScript or Node.js

You can dynamically compose objects in JavaScript. You may add a property depending on a configuration or not.

For example, when creating a database model you may allow users to configure whether to add or skip timestamps like createdOn and updatedOn. Model instances won’t have timestamps if a user configures the model to skip timestamps. Otherwise the model instances will have the timestamps.

This tutorial shows you how to conditionally add properties to a JavaScript object.

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Optionally Add Properties to an Object in JavaScript

Let’s follow the example from the introduction: you’re supporting a configuration setting which determines whether to add the createdOn and updatedOn timestamps to an object.

We’re showing you two ways adding properties to an object.

Add Properties Using the Spread Operator

You may use JavaScript’s spread operator to merge another object into an existing one. This way requires you to compose the data to be added before spreading the properties into the target object.

For example, you may use a createUser method that composes the user object and fills that object with data. You must pass the user’s data to the function and have all properties ready at the time you’re calling that function.

The spread operator also works with undefined and null and won’t add any properties when receiving a data object that is nullish.

Here’s the mentioned createUser function that adds or overrides the given data to the user object:

/**
 * Returns a user object with merged `data`.
 * 
 * @param {Object} data
 * 
 * @returns {Object}
 */
function createUser(data) {  
  return {
    name: 'Future Studio',
    ...data
  }
}

You may then compose the user objects like this:

createUser()  
// { name: 'Future Studio' }

createUser(null)  
// { name: 'Future Studio' }

createUser({ name: 'Marcus' })  
// { name: 'Marcus', createdAt: …, updatedAt: … }

createUser({ createdAt: new Date(), updatedAt: new Date() })  
// { name: 'Future Studio', createdAt: …, updatedAt: … }

Conditionally Adding Properties Using a Boolean Check

Another approach is to support settings that may add properties to the target object. You may accept the options as an argument in a method and then determine whether to add a property.

The following code snippet illustrates a createUser function that accepts and options object. The only option supported is shouldAddTimestamps. You can combine JavaScript’s spread operator and a boolean check which defines whether to add properties to the target object.

Here’s a sample createUser function conditionally adding the createdOn and updatedOn timestamps when the options.shouldAddTimestamps property evaluates to “true”:

/**
 * Returns a user object with our without timestamps.
 * 
 * @param {Object} options
 * 
 * @returns {Object}
 */
function createUser(options = {}) {  
  return {
    name: 'Future Studio',
    ...(options.shouldAddTimestamps && { 
      createdAt: new Date(),
      updatedAt: new Date(),
    })
  }
}

You may then call the createUser function with provided options defining whether to add timestamps:

createUser({ shouldAddTimestamps: false })  
// { name: 'Future Studio' }

createUser({ shouldAddTimestamps: true })  
// { name: 'Future Studio', createdAt: …, updatedAt: … }

Enjoy adding properties to JavaScript objects!

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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