graphql-typed-client/src/render/typeGuards/renderTypeGuards.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 { GraphQLSchema, isInterfaceType, isObjectType, isUnionType } from 'graphql'
import { excludedTypes } from '../common/excludedTypes'
import { RenderContext } from '../common/RenderContext'

const renderTypeGuard = (target: string, possible: string[]) => `
  const ${target}_possibleTypes = [${possible.map(t => `'${t}'`).join(',')}]
  export const is${target} = (obj: { __typename: String }): obj is ${target} => {
    if (!obj.__typename) throw new Error('__typename is missing')
    return ${target}_possibleTypes.includes(obj.__typename)
  }
`

export const renderTypeGuards = (schema: GraphQLSchema, ctx: RenderContext) => {
  for (const name in schema.getTypeMap()) {
    if (excludedTypes.includes(name)) continue

    const type = schema.getTypeMap()[name]

    if (isUnionType(type)) {
      const types = type.getTypes().map(t => t.name)
      ctx.addCodeBlock(renderTypeGuard(type.name, types))
    } else if (isInterfaceType(type)) {
      const types = schema.getPossibleTypes(type).map(t => t.name)
      ctx.addCodeBlock(renderTypeGuard(type.name, types))
    } else if (isObjectType(type)) {
      ctx.addCodeBlock(renderTypeGuard(type.name, [type.name]))
    }
  }
}