创建一个type="file"的input标签，用于文件上传。

<input type="file" name="" id="upload" value="" />

通过js实现图片压缩

 window.onload = function () {
   const upload = document.getElementById("upload");
   upload.onchange = uploadImage;
 };

 // 图片上传
 function uploadImage(event) {
   const file = event.target.files[0];
   console.log(file, "未压缩的图片file")
   createImage(file, async function (img) {
     createCanvas(img, 1200, file, async function (newFile) {
       console.log(newFile, "压缩后的图片")
     });
   });
 }

 // 生成图片副本
 // 将file通过FileReader 转换为base64格式
 async function createImage(file, callback) {
   const reader = new FileReader();
   reader.readAsDataURL(file);
   reader.onload = function () {
     const img = new Image();
     img.src = reader.result;
     callback(img);
   };
 }

 // 通过canvas绘制图片
 function createCanvas(img, max, file, callback) {
   const cvs = document.createElement("canvas");
   const ctx = cvs.getContext("2d");
   let width = img.naturalWidth || 400;
   let height = img.naturalHeight || 400;
   const ratio = width / height;
   if (width > max) {
     width = max;
     height = width / ratio;
   }
   cvs.width = width;
   cvs.height = height;
   img.onload = async function () {
     // 降低图片质量
     const base64 = await compressImage(cvs, ctx, img, width, height, max);
     // base64转成blod格式文件
     const BlobData = await convertBase64UrlToBlob(base64);
     // blod格式文件转成file格式并返回
     const this_file = new File([BlobData], file.name, {
       type: file.type,
     });
     callback(this_file);
   };
 }

 // 图片质量压缩 
 // 通过canvas.toDataUR()降低图片质量
 async function compressImage(cvs, ctx, img, width, height, max) {
 	// 图像质量  quality值越小，所绘制出的图像越模糊
   var quality = 0.7;
   var anw = document.createAttribute("width");
   anw.nodeValue = width;
   var anh = document.createAttribute("height");
   anh.nodeValue = height;
   cvs.setAttributeNode(anw);
   cvs.setAttributeNode(anh);
   ctx.drawImage(img, 0, 0, width, height);
   
   var base64 = cvs.toDataURL("image/jpeg", quality);
   return base64;
 }

 // base64转成blod格式文件
 async function convertBase64UrlToBlob(urlData) {
   var arr = urlData.split(","),
     mime = arr[0].match(/:(.*?);/)[1],
     bstr = atob(arr[1]),
     n = bstr.length,
     u8arr = new Uint8Array(n);
   while (n--) {
     u8arr[n] = bstr.charCodeAt(n);
   }
   return new Blob([u8arr], { type: mime });
 }

最终实现的效果如图所示