核心方法：canvas.toDataURL
单纯按比例压缩

// 压缩文件
        async compressImg (file, fileName) {
          let reader = new FileReader()
          // 将读取到的文件编码成DataURL
          reader.readAsDataURL(file)
          // 压缩图片
          return new Promise((resolve, reject) => {
            reader.onload = function (ev) {
              try {
                // 读取图片来获得上传图片的宽高
                let img = new Image()
                img.src = ev.target.result
                img.onload = function (ev) {
                  // 将图片绘制到canvas画布上进行压缩
                  let canvas = document.createElement("canvas")
                  let context = canvas.getContext("2d")
                  let imgwidth = img.width
                  let imgHeight = img.height
                  // ?按比例缩放后图片宽高;
                  let targetwidth = imgwidth
                  let targetHeight = imgHeight
                  // 如果原图宽大于最大宽度
                  if (targetwidth > targetHeight) {
                    // 原图宽高比例
                    let scale = targetHeight / 1280
                    targetHeight = 1280
                    targetwidth = targetwidth / scale
                  } else {
                    // 原图宽高比例
                    let scale = targetwidth / 1280
                    targetwidth = 1280
                    targetHeight = targetHeight / scale
                  }
                  // ?缩放后高度仍然大于最大高度继续按比例缩小
                  canvas.width = targetwidth //canvas的宽=图片的宽
                  canvas.height = targetHeight //canvas的高=图片的高
                  context.clearRect(0, 0, canvas.width, canvas.height)
                  context.drawImage(this, 0, 0, canvas.width, canvas.height)
                  let imageDataURL = ""
                  // 如果图片小于0.2Mb，不进行压缩，并返回二进制流
                  if (file.size <= 200 * 1024) {
                    imageDataURL = canvas.toDataURL("image/jpeg")
                    resolve(file)
                  } else {
                    // todo 压缩文件大小比例
                    imageDataURL = canvas.toDataURL("image/jpeg", 0.5)
                    // 去掉URL的头,并转换为byte
                    const imageBytes = window.atob(imageDataURL.split(',')[1])
                    // 处理异常,将ascii码小于0的转换为大于0
                    const arrayBuffer = new ArrayBuffer(imageBytes.length)
                    const uint8Array = new Uint8Array(arrayBuffer)
                    for (let i = 0; i < imageBytes.length; i++) {
                      uint8Array[i] = imageBytes.charCodeAt(i)
                    }
                    let mimeType = imageDataURL.split(',')[0].match(/:(.*?);/)[1]
                    let newFile = new File([uint8Array], fileName, { type: mimeType || 'image/jpeg' })
                    resolve(newFile)
                  }
                }
              } catch (error) {
                console.log("压缩错误", error)
              }
            }
          })
        },

如需压缩到指定大小，请循环压缩。