docsify/src/core/fetch/ajax.js

import progressbar from '../render/progressbar'
import {noop, hasOwn} from '../util/core'

const cache = {}

/**
 * Simple ajax get
 * @param {string} url
 * @param {boolean} [hasBar=false] has progress bar
 * @return { then(resolve, reject), abort }
 */
export function get(url, hasBar = false, headers = {}) {
  const xhr = new XMLHttpRequest()
  const on = function () {
    xhr.addEventListener.apply(xhr, arguments)
  }
  const cached = cache[url]

  if (cached) {
    return {then: cb => cb(cached.content, cached.opt), abort: noop}
  }

  xhr.open('GET', url)
  for (const i in headers) {
    if (hasOwn.call(headers, i)) {
      xhr.setRequestHeader(i, headers[i])
    }
  }
  xhr.send()

  return {
    then: function (success, error = noop) {
      if (hasBar) {
        const id = setInterval(
          _ =>
            progressbar({
              step: Math.floor(Math.random() * 5 + 1)
            }),
          500
        )

        on('progress', progressbar)
        on('loadend', evt => {
          progressbar(evt)
          clearInterval(id)
        })
      }

      on('error', error)
      on('load', ({target}) => {
        if (target.status >= 400) {
          error(target)
        } else {
          const result = (cache[url] = {
            content: target.response,
            opt: {
              updatedAt: xhr.getResponseHeader('last-modified')
            }
          })

          success(result.content, result.opt)
        }
      })
    },
    abort: _ => xhr.readyState !== 4 && xhr.abort()
  }
}