# 🏁 Final Form Calculate

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Decorator for [🏁 Final Form](https://github.com/final-form/final-form) that
allows you to define calculations that happen between fields, i.e. "When field X
changes, update field Y."

---

## Installation

```bash
npm install --save final-form-calculate
```

or

```bash
yarn add final-form-calculate
```

## Usage

```js
import { createForm, getIn } from 'final-form'
import createDecorator from 'final-form-calculate'

// Create Form
const form = createForm({ onSubmit })

// Create Decorator
const decorator = createDecorator(
  // Calculations:
  {
    field: 'foo', // when the value of foo changes...
    updates: {
      // ...set field "doubleFoo" to twice the value of foo
      doubleFoo: (fooValue, allValues) => fooValue * 2
    }
  },
  {
    field: 'bar', // when the value of bar changes...
    updates: {
      // ...set field "foo" to previous value of bar
      foo: (fooValue, allValues, prevValues) => prevValues.bar
    }
  },
  {
    field: /items\[\d+\]/, // when a field matching this pattern changes...
    updates: {
      // ...sets field "total" to the sum of all items
      total: (itemValue, allValues) =>
        (allValues.items || []).reduce((sum, value) => sum + value, 0)
    }
  },
  {
    field: 'foo', // when the value of foo changes...
    updates: {
      // ...asynchronously set field "doubleFoo" to twice the value using a promise
      doubleFoo: (fooValue, allValues) =>
        new Promise(resolve => {
          setTimeout(() => resolve(fooValue * 2), 100)
        })
    }
  },
  {
    field: /\.timeFrom/, // when a deeper field matching this pattern changes...
    updates: (value, name, allValues) => {
      const toField = name.replace('timeFrom', 'timeTo')
      const toValue = getIn(allValues, toField)
      if (toValue && value > toValue) {
        return {
          [toField]: value
        }
      }

      return {}
    }
  }
)

// Decorate form
const undecorate = decorator(form)

// Use form as normal
```

## Table of Contents

<!-- START doctoc generated TOC please keep comment here to allow auto update -->

<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->

<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->

- [Example](#example)
  - [Calculated Fields Example](#calculated-fields-example)
- [API](#api)
  - [`createDecorator: (...calculations: Calculation[]) => Decorator`](#createdecorator-calculations-calculation--decorator)
- [Types](#types)
  - [`Calculation: { field: FieldPattern, updates: Updates }`](#calculation--field-fieldpattern-updates-updates-)
  - [`FieldName: string`](#fieldname-string)
  - [`FieldPattern: FieldName | RegExp`](#fieldpattern-fieldname--regexp)
  - [`Updates: { [FieldName]: (value: any, allValues: Object, prevValues: Object) => any }`](#updates--fieldname-value-any-allvalues-object--any-)

<!-- END doctoc generated TOC please keep comment here to allow auto update -->

## Example

### [Calculated Fields Example](https://codesandbox.io/s/oq52p6v96y)

Example using
[🏁 React Final Form](https://github.com/final-form/react-final-form#-react-final-form).

## API

### `createDecorator: (...calculations: Calculation[]) => Decorator`

A function that takes a set of calculations and returns a 🏁 Final Form
[`Decorator`](https://github.com/final-form/final-form#decorator-form-formapi--unsubscribe).

## Types

### `Calculation: { field: FieldPattern, isEqual?: (any, any) => boolean, updates: Updates }`

A calculation to perform, with an optional `isEqual` predicate to determine if a value has really changed (defaults to `===`).

### `FieldName: string`

### `FieldPattern: FieldName | RegExp | (FieldName | RegExp)[]`

A pattern to match a field with.

### `Updates: UpdatesByName | UpdatesForAll`

Either an object of updater functions or a function that generates updates for multiple fields.

### `UpdatesByName: { [FieldName]: (value: any, allValues: Object, prevValues: Object) => Promise | any }`

Updater functions for each calculated field.

### `UpdatesForAll: (value: any, field: string, allValues: Object, prevValues: Object) => Promise | { [FieldName]: any }`

Takes the value and name of the field that just changed, as well as all the values, and returns an object of fields and new values.