Gatsby Plugin for creating Fields On Nodes
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README.md

gatsby-plugin-node-fields

gatsby-plugin-node-fields offers you a simple, consistent way to manage the creation of fields on your nodes, with support for default values, transformations and validation of values. It is well tested and uses helpful error messages to guide you away from the rocks.

Quickstart

Install

yarn add gatsby-plugin-node-fields

Plugin vs Function

You can use gatsby-plugin-node-fields either as a standard Gatsby plugin, or you can use it as standalone function. Both will perform the same task, but you might prefer to keep node manipulation in your gatsby-node.js file instead of via a plugin defined in your gatsby-config.js.

Plugin

If you want to use it as a plugin, add it as the last plugin. You can place it anywhere, but bear in mind that it will only see changes made by plugins that come before it. Unless you have a good reason not to, place it last.

// gatsby-config.js

const plugins = [
 …
 {
  resolve: `gatsby-plugin-node-fields`,
  options: {
    // Your list of descriptors
    descriptors: [
      …
    ],
  },
 }
]

module.exports = {
  plugins,
}

Function

If you'd prefer to use it as a function in your gatsby-node.js file, you need to hook into Gatsby's onCreateNode yourself, passing in the three arguments it expects:

// gatsby-node.js

const { attachFields } = require(`gatsby-plugin-node-fields`)

// Your list of descriptors
const descriptors = [
  …
]

exports.onCreateNode = ({ node, boundActionCreators }) => {
  const { createNodeField } = boundActionCreators
  attachFields(node, createNodeField, descriptors)
}

Some Examples

Default value for a field

Imagine you have a series of Markdown Articles. They are mainly written by the same person with an occasional guest author. You support an author field in your article pages' front matter, but you don't want your regular author to have to add their own name to every article. Effectively you need a default value for author. To implement this you would create the following descriptor:

[
  {
    predicate: isArticleNode,
    fields: [
      {
        name: 'author',
        getter: node => node.frontmatter.author,
        defaultValue: 'John Doe',
      },
    ]
  }
]

This would result in node.fields.author being populated with either the author field of the node's front matter, or with a default value of 'John Doe'. In reality you'd probably want to pull the default value from a config object or similar.

Transforming a value

Imagine you allow an author to add a keywords field to an article's front matter which you use for the keywords metadata of the page (which will be a comma-separated list), and also extract for use as tags. To make these tags easier to work with you want to convert then to an array of strings.

[
  {
    predicate: isArticleNode,
    fields: [
      {
        name: 'tags',
        getter: node => node.frontmatter.keywords,
        defaultValue: '',
        transformer: value => isEmptyString(value) ? [] : cslToArray(value),
      },
    ]
  }
]

Here we again use getter to pull the value we want from the node's frontmatter. We set it's defaultValue to '', then we use the transformer to either transform the value to an empty array, or to an array of strings using a helper function cslToArray. Note that it might seem strange to set a default value, then immediately check it and convert it to an array, but I've found that treating each stage discretely makes for much cleaner code. This way the transformer knows it will receive a string, making it more focused.

Validating a value

It's often better to handle invalid values at compile time rather than trying to handle these values in the UI. Imagine you allow an author to add a title keyword to an article's frontmatter. Obviously without a title, an Article shouldn't be valid and it doesn't make sense to set a default title, so you add a validator to ensure the article's title is a non-empty string.

[
  {
    predicate: isArticleNode,
    fields: [
      {
        name: 'title',
        getter: node => node.frontmatter.title,
        validator: isNonEmptyString
      },
    ]
  }
]

This will result in an error with a useful message if an article is encountered that doesn't have a title set. For obvious reasons it's better to set a sensible default in most cases, but in instances like this, a useful compile-time error is better than a potentially obscure runtime error, or invalid data being displayed.

Overview

I have found that mixing queries for values stored in a node's frontmatter with queries for values stored in generated fields is uneven and confusing, so I now transfer all values that I will use in the UI over to fields. This transfer gives us the opportunity to do a number of important things:

  • set a default value
  • validate a value
  • transform a value

Validation

Your descriptors will be validated against a schema when first used with useful error messages if you have added an invalid field or the value of required field is invalid or missing.

Descriptors

The plugin automatically hooks into Gatsby's onCreateNode life cycle hook and will check the node it receives against an array of descriptors you provide. Each descriptor must provide a predicate - a function that will be passed the node and decides whether the descriptor should be used to transform it. For example we might want to check if the node is Markdown node, or if it represents a file from a particular directory.

If a descriptor's predicate returns true, the descriptor will be used to create new fields on that node using the contents of the descriptor's fields array. Each item in the fields array represents the creation of one or more fields and describes a series of steps

Here is an example of a descriptor that will be run for all markdown nodes, validating that a title exists, then running is through a function called preventOrphans before saving it as a title field:

[
  // Descriptor
  {
    predicate: isMarkdownNode,
    fields: [
      {
        name: 'title',
        getter: node => node.frontmatter.title,
        validator: isString,
        transformer: preventOrphans,
      },
    ]
  },
]

predicate [function]

A function that receives the newly created node as its single argument and returns true if the descriptor should apply to that node and false if it doesn't. Multiple descriptors can be applied to the same node if their predicates return true.

fields [array]

An array of objects representing fields that will be created on the node. Each object comprises of a set of keys and values that describe the creation process of a new field. You can use as few or as many keys as needed. For example if all you want to do is set a default value you could use only the name and defaultValue keys:

{
  name: 'example',
  defaultValue: 'Unknown',
}

Fields

A field can contain one or more of the following keys that describe which fields it should add, and how it should obtain and transform the data that will populate them. You can perform a one-to-one, many-to-one, or one-to-many mapping between values and fields. The order in which these fields are used is

  • getter or name
  • defaultValue
  • validator
  • transformer
  • setter or name

name [string]

A name field represents the name of the field that will be created. If no getter field is present on the descriptor, it will also be used to access a value on the node. For example if the name is 'alpha', it will create a field on the node called 'alpha'. If no getter field is present on the descriptor is will try and get the value for this field from node.alpha.

getter [function(node, context)]

A getter is a function that gets the value or values from the node. If a getter is not defined and a name is defined, node[name] will be used in its place.

A simple getter might look like:

node => node.frontmatter.title

You could also pull the value from a config object or anywhere else you like.

defaultValue [* | function(node, context)]

A defaultValue supplies a value in instances where no value exists on the node (the value is undefined), or no means of getting a value has been defined on the descriptor. In the following cases defaultValue will be used:

  • Only a name was defined and there is no prop on the node with that name.
  • A prop of name exists with but has a value of undefined
  • A getter was defined but returned undefined,
  • Neither a name nor a getter were defined.

If defaultValue is a function it should return a default value. If the value of defaultValue is not a function, that value will be used as the default value.

For example by using a function, you could use supply a default value using another property of the node:

node => node.someOtherValue

validator [function(value, node, context)]

A validator is just a predicate that receives the value and returns true or false, depending if it deems it to be valid or not. For example we might have a descriptor that has looked up node.Front Matter.slug, but there is no slug defined, we use a sanitised version of the title instead:

value => isValidDate(value)

transformer [function(value, node, context)]

A transformer transforms the value in some way. For example it might run the value through a function that cleans it up or formats it. A transformer function will be called with three arguments: the value, the node and the context, if defined.

value => preventOrphans(value)

setter [function(value, node, context, createNodeField)]

A setter defines how the value(s) are translated to fields. If no setter is defined, the name field will be used to create a field of that name using Gatsby's createNodeField , however using a setter function allows more flexibility. For example a value might be an object and we might want to transfer its values to multiple fields. A setter will receive three arguments: the value, createNodeField and any context. If you define a setter, that setter is responsible for using createNodeField to create fields.

(value, node) => {
  createNodeField({
    node,
    name: 'alpha',
    value: value.beta
  })
}

Context

To keep things as functional as possible and prevent the need for you to reach out to external data sources from within your functions, you can pass in a context. Context can be anything you like, but will probably be an object. getter, defaultValue, validator, transformer, and setter functions all receive the context as their second argument.

If you are using the plugin, pass the context as an option:

{
  resolve: `gatsby-plugin-node-fields`,
  options: {
    descriptors: [
      …
    ],
    context: {
      …
    }
  },
}

If you are using the function, pass it in as the fourth argument:

attachFields(node, createNodeField, descriptors, context)

Maintenance

Gatsby doesn't support ES6 imports, so we need to compile our ./src to ./lib, then reference the compiled file from gatsby-node.js.

Tests

Tests are written with Jest:

yarn test