Understanding Vue.js Reactive Properties

Vue.js is a widely used progressive JavaScript framework that builds user interfaces. It is crucial to understand how to correctly declare a reactive property that is not part of the data object in a Vue.js component. The correct method, according to the question, is this.$set(this, 'newProp', value).

This operation is essential for situations where you may need to add a new property to an object after it was created, especially when Vue's reactivity system won't recognize the property immediately.

Applying the Correct Method - this.$set(this, 'newProp', value)

In the Vue.js framework, the $set instance method is used to add a property to a reactive object and ensure that the new property is also reactive - just like properties that were part of the object when it was initially created. Here's an example:

this.$set(this, 'newProp', value);

In the above syntax, this points to the Vue instance, 'newProp' is the name of the new property you're adding, and value is the value of the property.

It's also important to note that $set can be used to add properties to nested objects as well. You would do this by passing in the nested object, the new property name, and its value.

Best Practices & Additional Insights

While $set is the correct way to add reactive properties to a Vue instance or nested object, it's important to consider good design and coding practices. If you find yourself needing to add a lot of properties after the instance is created, it could signify that the initial structure of your data is not well thought out.

Remember, Vue's reactivity system only monitors properties that exist when the Vue instance or the data object is created. If you know a property will be needed later, but you don't have a value for it yet, you can inform Vue about the forthcoming property by setting it to null in the data object.

This will allow Vue to track changes to the property even before it has a real value. It's a more maintainable and intuitive way of managing your data and interacting with Vue's reactivity system.

Nevertheless, for those times when you do need to add properties to an instance or nested object after it's created, this.$set(this, 'newProp', value) is the way to go.