FormKit uses a unique hierarchical configuration system that is well suited for forms. To understand how this configuration works, there are 4 questions we need to answer:

  1. What are core nodes?
  2. What are node options?
  3. What are node props?
  4. What is node config?

What are core nodes?

Every <FormKit> component has its own instance of a core node. This node is responsible for almost all of the component’s functionality. There are 3 types of core nodes: inputs, lists, and groups (forms are just a type of group!).

There is no global FormKit instance that controls the application. Instead, you can think of each node as its own little application — complete with its own configuration.

One last thing about nodes — they can all have parent nodes. Groups and lists can also have children. For example, a login form might have two children — the email and password inputs. You can draw this relationship as a simple tree diagram:

Hover over each node to see its initial options.

What are node options?

When creating one of these "mini applications" that we call core nodes, some options can be passed in. Except in extremely advanced use cases, you won't be creating your own core nodes — this is normally done for you by the <FormKit> component. However, it can be useful to define some of the node options globally. This is done with the @formkit/vue plugin — 💡 core node options are the same as the @formkit/vue plugin options.

For example, in a typical FormKit Vue registration, we use defaultConfig which is just a function that returns core node options:

Available node options

The following is a list of all the available options that can be used when registering FormKit or creating a node individually. Options that are passed to the @formkit/vue plugin will be applied to every <FormKit> component’s core node when it is created.

What is defaultConfig?

Developers familiar with FormKit will notice that the above list of node options differs slightly from the values that can be passed into the defaultConfig function.

Many of FormKit’s features, like validation, inputs, and Vue support are provided courtesy of first-party plugins. The defaultConfig function configures many of these plugins before they are given over to the Vue plugin as node options. So, defaultConfig can accept any of the above node options, but also a few extras:

What are node props?

All core nodes have a props object (node.props). FormKit core, and any third-party plugins or code can read and write values to this object. In fact, nearly every feature of FormKit references node.props to determine how it should operate.

For example, the validation plugin looks at node.props.validation to determine if there are any rules it needs to run. So the real question is — how are these props being set? There are 3 primary mechanisms for setting props:

  • Direct assignment
  • Component props
  • Vue plugin options

Let’s see how we can set the validation rules of an input (node.props.validation) these three ways:

1. Direct assignment

If you have a node instance, you can directly assign it a prop value:

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2. Component props

Any props passed to the <FormKit> input are assigned to the node.props object (you know the drill).

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3. Vue plugin options

When registering the @formkit/vue plugin, you can provide prop values to be injected into to all <FormKit> components.

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What is node config?

Props are pretty powerful, but in addition to node.props, core nodes all have a config object node.config. This is where configuration hierarchy comes in. The node.config object acts like initial values for node.props. If a given prop is requested, like node.props.validation, and that property is not explicitly set using any of the methods discussed above, then FormKit will check the node.config object to see if it has a value. If it does not have a value, then it recursively checks the node parent's config object — then the grandparent's config object — and so on — until a value is found or it reaches a node with no parent.

This diagram describes how a request for property of node.props resolves.

What does this mean in practice? When you combine the tree like structure of forms (and their corresponding core nodes) and this hierarchical configuration you can do some pretty exciting things. For example, here we set the validation visibility of an entire form:

Load Live Example

It’s worth noting that plugins have their own inheritance model which differs from config and props, and is described in more detail in the core documentation.