file文件上传后 添加水印 并且生成file文件 使用formData上传

function hecheng (){
            
            // 创建一个canvas
            const d2 = testCanvas.getContext('2d');
            
            
            // 准备图片1
            const url = URL.createObjectURL(file.files[0]);
            var img = document.createElement("img");
            img.src = url;
            
            // 准备图片2
            const url1 = URL.createObjectURL(file1.files[0]);
            var img1 = document.createElement("img");
            img1.src = url1;
            
            setTimeout(()=>{
                d2.drawImage(img, 10, 10);
                
                d2.drawImage(img1, 10, 10);
                
                const base64 = testCanvas.toDataURL();
                
                let data = atob(base64.substring("data:image/png;base64,".length)),
                    asArray = new Uint8Array(data.length);
                
                for( let i = 0, len = data.length; i < len; ++i ) {
                    asArray[i] = data.charCodeAt(i);    
                }
                
                var file0 = new File(
                  [asArray.buffer],
                  'foo.png',
                  {
                    type: 'image/png',
                  }
                );
                
                const url = URL.createObjectURL(file0);
                
                var img2 = document.createElement("img");
                img2.src = url;
                document.body.appendChild(img2);
                
            }, 1000)
            
                
        }