Usage example of final-form-calculate

final-form-calculate-usage.js

import { createForm } 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: /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)
    }
  }
)

// Decorate form
const undecorate = decorator(form)

// Use form as normal