压缩图片实例

function changeUpload(e) {
  for (let i = 0, j = e.target.files.length; i < j; i ++) {
    const file = e.target.files[i]
    let reader = new FileReader()
    reader.readAsDataURL(file)
    let self = this
    reader.onload = function() {
    compressImg(this.result, file.size, i)
    }
  }
}

function compressImg(data, size, index) {
  let img = new Image()
  img.src = data
  img.onload = () => {
    let width = img.width
    let height = img.height
    let canvas = document.createElement('canvas')
    canvas.width = width
    canvas.height = height
    let ctx = canvas.getContext('2d')
    ctx.drawImage(img, 0, 0, width, height)

    let ratio = 1
    let max = 200 * 1024 // 200k举例
    if(size > max) ratio = max / size

    let key = 'imgData' + index
    return canvas.toDataURL("image/jpeg", ratio)
  }
}