create-redirects.js

import { HEADER_COMMENT } from "./constants"
import { appendFile, exists, readFile, writeFile } from "fs-extra"

export default async function writeRedirectsFile(
  pluginData,
  redirects,
  rewrites
) {
  const { publicFolder } = pluginData

  if (!redirects.length && !rewrites.length) return null

  const FILE_PATH = publicFolder(`_redirects`)

  // Map redirect data to the format Netlify expects
  // https://www.netlify.com/docs/redirects/
  redirects = redirects.map(redirect => {
    const {
      fromPath,
      isPermanent,
      redirectInBrowser, // eslint-disable-line no-unused-vars
      toPath,
      ...rest
    } = redirect

    // The order of the first 3 parameters is significant.
    // The order for rest params (key-value pairs) is arbitrary.
    const pieces = [
      fromPath,
      toPath,
      isPermanent ? 301 : 302, // Status
    ]

    for (let key in rest) {
      const value = rest[key]

      if (typeof value === `string` && value.indexOf(` `) >= 0) {
        console.warn(
          `Invalid redirect value "${value}" specified for key "${key}". ` +
            `Values should not contain spaces.`
        )
      } else {
        pieces.push(`${key}=${value}`)
      }
    }

    return pieces.join(`  `)
  })

  rewrites = rewrites.map(
    ({ fromPath, toPath }) => `${fromPath}  ${toPath}  200`
  )

  let appendToFile = false

  // Websites may also have statically defined redirects
  // In that case we should append to them (not overwrite)
  // Make sure we aren't just looking at previous build results though
  const fileExists = await exists(FILE_PATH)
  if (fileExists) {
    const fileContents = await readFile(FILE_PATH)
    if (fileContents.indexOf(HEADER_COMMENT) < 0) {
      appendToFile = true
    }
  }

  const data = `${HEADER_COMMENT}\n\n${[...redirects, ...rewrites].join(`\n`)}`

  return appendToFile
    ? appendFile(FILE_PATH, `\n\n${data}`)
    : writeFile(FILE_PATH, data)
}