fetchmap

Non-throwing Fetch API wrapper

Getting started

npm i fetchmap

Description

This is a simple wrapper for a fetch-like function that catches all possible exceptions and returns a 'success or failure' wrapped value. It takes an object that maps response.status to validating transform and standard fetch arguments.

Read the docs here and check how fetchmap infers types at the playground. Also there is a compatible result library - ts-railway.

Example

Server code:

import express from 'express'

express()
  .get('/data', (_, res) => {
    const rnd = Math.random()

    if (rnd < 0.34) {
      res.status(200).json({ some: 'data' })
    } else if (rnd < 0.67) {
      res.status(201).send('This is not JSON!')
    } else {
      res.status(500).send('Server error!')
    }
  })
  .listen(5005)

Client code:

import { createFetchmap } from 'fetchmap'
import nodeFetch, { Response } from 'node-fetch'
import { isRecord } from 'ts-is-record'

// fetchmap compatible result creators
const success = <T>(value: T) => ({ tag: 'success', success: value } as const)
const failure = <T>(error: T) => ({ tag: 'failure', failure: error } as const)

// wrap any fetch-like function
const fetchmap = createFetchmap(nodeFetch)

// data is expected to be JSON, so it has to be validated
const validateData = (body: unknown) =>
  isRecord(body) && 'some' in body && typeof body.some === 'string'
    ? success(body)
    : failure('data validation failed' as const)

// error is just a string, in this example no validation needed
const validateError = (body: string, { status }: Response) => success({ message: body, status })

const dataResult = await fetchmap(
  {
    // for any response with a status inside inclusive range 200..299
    // call 'json' method and validate its result with `validateData` function
    ok: { json: validateData },

    // for any response with a status outside inclusive range 200..299
    // call 'text' method and validate its result with `validateError` function
    notOk: { text: validateError }
  },

  // first argument for a wrapped fetch function
  'https://localhost:5005/data',

  // second argument for a wrapped fetch function
  {
    // request options: method, headers, body etc.
  }
)

expect([
  { tag: 'success', success: { some: 'data' } },
  { tag: 'failure', failure: { serverError: { message: 'Server error!', status: 500 } } },
  {
    tag: 'failure',
    failure: { mapError: new SyntaxError('Unexpected token T in JSON at position 0') }
  }
]).toContainEqual(dataResult)