1、在config/utils文件封装通用的图片压缩方法

// 图片压缩
export const imgPreview = (file, callback) => {
    //将base64转换为文件
    function dataURLtoFile(dataurl, file) {
        var arr = dataurl.split(","),
            bstr = atob(arr[1]),
            n = bstr.length,
            u8arr = new Uint8Array(n);
        while (n--) {
            u8arr[n] = bstr.charCodeAt(n);
        }
        return new File([u8arr], file.name, {
            type: file.type
        });
    }
    // 压缩图片
    function compress(img) {
        let canvas = document.createElement("canvas");
        let ctx = canvas.getContext("2d");
        //瓦片canvas
        let tCanvas = document.createElement("canvas");
        let tctx = tCanvas.getContext("2d");
        // let initSize = img.src.length;
        let width = img.width;
        let height = img.height;
        //如果图片大于四百万像素，计算压缩比并将大小压至400万以下
        let ratio;
        if ((ratio = (width * height) / 4000000) > 1) {
            // console.log("大于400万像素");
            ratio = Math.sqrt(ratio);
            width /= ratio;
            height /= ratio;
        } else {
            ratio = 1;
        }
        canvas.width = width;
        canvas.height = height;
        //    铺底色
        ctx.fillStyle = "#fff";
        ctx.fillRect(0, 0, canvas.width, canvas.height);
        //如果图片像素大于100万则使用瓦片绘制
        let count;
        if ((count = (width * height) / 1000000) > 1) {
            // console.log("超过100W像素");
            count = ~~(Math.sqrt(count) + 1); //计算要分成多少块瓦片
            //      计算每块瓦片的宽和高
            let nw = ~~(width / count);
            let nh = ~~(height / count);
            tCanvas.width = nw;
            tCanvas.height = nh;
            for (let i = 0; i < count; i++) {
                for (let j = 0; j < count; j++) {
                    tctx.drawImage(
                        img,
                        i * nw * ratio,
                        j * nh * ratio,
                        nw * ratio,
                        nh * ratio,
                        0,
                        0,
                        nw,
                        nh
                    );
                    ctx.drawImage(tCanvas, i * nw, j * nh, nw, nh);
                }
            }
        } else {
            ctx.drawImage(img, 0, 0, width, height);
        }
        //进行最小压缩
        let ndata = canvas.toDataURL("image/jpeg", 0.3);
        tCanvas.width = tCanvas.height = canvas.width = canvas.height = 0;
        return ndata;
    }
    // 看支持不支持FileReader
    if (!file || !window.FileReader) {
        return;
    }
    if (/^image/.test(file.type)) {
        // 创建一个reader
        let reader = new FileReader();
        // 将图片转成 base64 格式
        reader.readAsDataURL(file);
        // 读取成功后的回调
        reader.onloadend = function() {
            let result = this.result;
            let img = new Image();
            img.src = result;
            //判断图片是否大于500K,是就直接上传，反之压缩图片
            if (this .result.length <= 100 * 1024) {
                // 上传图片
                let imageFile = dataURLtoFile(this.result, file);
                callback(imageFile);
            } else {
                img.onload = function() {
                    let data = compress(img);
                    // 上传图片
                    let imageFile = dataURLtoFile(data, file);
                    callback(imageFile);
                };
            }
        };
    }
};

2、用法

import { imgPreview } from "@/config/utils.js";
//afterRead方法是vant的van-uploader提供的文件加载后方法
afterRead(file, detail) {
      let that = this;
      let index = detail.index;
      //上传的图片可能分为单张上传和多张上传
      if (file instanceof Array) {
        for (let i = 0; i < file.length; i++) {
          (function (arg) {
            let formData = new FormData();
            imgPreview(file[arg].file, async (files) => {
              formData.append("file", files);
              formData.append("kind", "indexPic");
              await check.fileUpload2(formData).then((res) => {
                if (res) {
                  let data = {};
                  data.jdlj = res.data.data[0].absUrl;
                  data.xdlj = res.data.data[0].url;
                  data.mc = res.data.data[0].name;
                  data.url = that.globalAddress + res.data.data[0].url;
                  that.tempList[index + i] = data;
                }
              });
            });
          })(i);
        }
      } else {
        let formData = new FormData();
        imgPreview(file.file, (files) => {
          formData.append("file", files);
          formData.append("kind", "indexPic");
          check.fileUpload2(formData).then((res) => {
            if (res) {
              let data = {};
              data.jdlj = res.data.data[0].absUrl;
              data.xdlj = res.data.data[0].url;
              data.mc = res.data.data[0].name;
              data.url = that.globalAddress + res.data.data[0].url;
              that.tempList[index] = data;
            }
          });
        });
      }
    },

解释：上传的图片可能是单张可能是多张，所以需要根据file是数组还是对象来执行不同的流程，如果是数组中进行图片压缩完成后，再调接口，可能会出现for循环中的异步操作这个经典问题，导致变量始终为最后一项，所以利用了闭包配合async/await来按顺序调取接口