javascript使用链接跨域下载图片

/
  事件触发器
  @Author: wujiang
  @Date: 2018-11-11 22:50:43
  @Last Modified by: wujiang
  @Last Modified time: 2018-11-11 23:39:40
 /
const EventEmitter = require('events')
class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter()

export default myEmitter

import eventEmitter from './mon/eventEmitter'
// 图片下载对象
let downloadedImg = null
// 待下载文件名
let fileName = null

// 监听是否获取图片地址
eventEmitter.addListener('getHref', href => {
 imatateDownloadByA(href, fileName)
})

/
  base64下载图片（可跨域）
  @param {} img
  @param {String} filename 文件名
 /
function downloadImg (imgUrl, filename) {
 fileName = filename
 startDownloadImg(imgUrl, getImgDataUrl)
}

/
  开始下载图片之前
  @param {String} imgUrl
  @param {Function} callback
 /
function startDownloadImg (imgUrl, callback) {
 let imageURL = imgUrl
 downloadedImg = new Image()
 downloadedImg.crossOrigin = 'Anonymous'
 downloadedImg.src = imageURL
 downloadedImg.addEventListener('load', callback, false)
}

/
  获取图片下载base64字符串
  @param {DOMimg} domimg
 /
function getImgDataUrl () {
 const canvas = document.createElement('canvas')
 const context = canvas.getContext('2d')
 const width = downloadedImg.width || downloadedImg.naturalWidth
 const height = downloadedImg.height || downloadedImg.naturalHeight
 const imgSrc = downloadedImg.src
 let type = imgSrc.substring(imgSrc.lastIndexOf('.') + 1)
 let dtype = type === 'jpg' ? 'jpeg' : 'png'
 canvas.width = width
 canvas.height = height
 context.drawImage(downloadedImg, 0, 0)
 eventEmitter.emit('getHref', canvas.toDataURL(`image/${dtype}`))
}

/
  通过a标签模拟下载
  @param {String} href
  @param {String} filename
 /
function imatateDownloadByA (href, filename) {
 const a = document.createElement('a')
 a.download = filename
 a.style.display = 'none'
 a.href = href
 document.body.appendChild(a)
 a.click()
 a.remove()
 window.URL.revokeObjectURL(href)
}

downloadImg('https://img.alicdn./imgextra/i2/54199406/O1CN012JLzreAxzn1xhNP_!!54199406.jpg', '测试')