graphql-typed-client/src/client/getFieldFromPath.ts

A tool that generates a strongly typed client library for any GraphQL endpoint. The client allows writing GraphQL queries as plain JS objects (with type safety, awesome code completion experience, custom scalar type mapping, type guards and more)

import { LinkedField, LinkedType } from './linkTypeMap'
import { startsWith } from 'lodash'

export const getFieldFromPath = (root: LinkedType | undefined, path: string[]) => {
  let current: LinkedField | undefined

  if (!root) throw new Error('root type is not provided')

  if (path.length === 0) throw new Error(`path is empty`)

  path.forEach(f => {
    const type = current ? current.type : root

    if (!type.fields) throw new Error(`type \`${type.name}\` does not have fields`)

    const possibleTypes = Object.keys(type.fields)
      .filter(i => startsWith(i, 'on_'))
      .reduce(
        (types, fieldName) => {
          const field = type.fields && type.fields[fieldName]
          if (field) types.push(field.type)
          return types
        },
        [type],
      )

    let field: LinkedField | null = null

    possibleTypes.forEach(type => {
      const found = type.fields && type.fields[f]
      if (found) field = found
    })

    if (!field) throw new Error(`type \`${type.name}\` does not have a field \`${f}\``)

    current = field
  })

  return <LinkedField>current
}