Builder DSL for SvelteKit responses

builder.js

/*
  In SvelteKit, endpoints return an object, like:
  
   {
     status: 200,
     headers: {...},
     body: JSON.stringify({a: 1})
   }
   
   This is an exploration into how it feels using a DSL instead. Like express and others use.
   From svelte-kits' perspective it's the same data. The DSL is returning an object that exposes a `.then()`, so it acts like a `Promise`
   
   Examples:

   // in src/routes/example.js
   import { res } from '$lib/builder'

   export function get(req) {
     //return res().status(200)
     //return res().type('application/json')
     //return res().type('json')
     //return res().json({test: 1234})
     //return res().text('hello world')
     //return res().redirect('http://google.com')
     //return res().error('whoops')
     //return res().cookie(name, value)
     //return res().clearCookie(name)
     //return res().attachment('foo.csv').body('hello,world')
     //return res().download('src/routes/index.svelte')
  }
*/

import mime from 'mime-types'
import path from 'path'
import fs from 'fs'

class Response {
  constructor() {
    this._body = null
    this._status = 200
    this._headers = []
    this._cookies = []
  }

  json(data) {
    this.type('application/json')
    this._body = JSON.stringify(data)

    return this
  }

  text(data) {
    this.type('text/plain')
    this._body = data

    return this
  }

  type(value) {
    value = mime.lookup(value) || value
    this.header('content-type', value)

    return this
  }

  header(name, value) {
    this._headers[name] = value

    return this
  }

  headers(entries) {
    Object
      .entries(entries)
      .map(this.header.bind(this))

    return this
  }

  status(code) {
    this._status = code

    return this
  }

  redirect(url, code = 302) {
    return this
      .status(code)
      .header('location', url)
  }

  error(message) {
    this.status(500)
    this._error = message
    return this
  }

  cookie(name, value, options = {}) {
    this._cookies.push({name, value, options})
    return this
  }

  clearCookie(name) {
    return this.cookie(name, '', {expires: new Date(2000, 0, 1)})
  }

  attachment(filename=null) {
    if (filename) {
      this
        .header('content-disposition', `attachment; filename="${filename}"`)
        .header('content-type', mime.lookup(filename))

    } else {
      this.header('content-disposition', 'attachment')
    }

    return this
  }

  download(filepath, name=null) {
    const data = fs.readFileSync(filepath)

    return this
      .attachment(name || path.basename(filepath))
      .body(data)
  }

  body(data) {
    this._body = data
    return this
  }

  then(resolve) {
    console.log(this)

    const cookieStrings = this._cookies.map(formatCookie)
    const headers = Object.assign({}, this._headers, {'set-cookie': cookieStrings})

    resolve({
      headers,
      status: this._status,
      body: this._body,
      redirection: this._redirect,
      error: this._error
    })
  }
}

// missing encoding and and optional encode function parameter
// missing value signing
function formatCookie({name, value, options = {}}) {
  let string = `${name}=${value}`

  if (options.expires) {
    string += `;Expires=${options.expires}`
  }
  if (options.maxAge) {
    string += `;Max-Age=${options.maxAge}`
  }
  if (options.domain) {
    string += `;Domain=${options.domain}`
  }
  if (options.path) {
    string += `;Path=${options.path}`
  }
  if (options.secure) {
    string += `;Secure`
  }
  if (options.httpOnly) {
    string += `;HttpOnly`
  }
  if (options.sameSite) {
    string += `;SameSite=${options.sameSite}`
  }

  return string
}

export function res() {
  return new Response()
}