有些项目，如果服务器图片资源不能互相访问时，需要下载后并上传到另一个服务器，网上很多都是转 base64 字符串后上传到服务器，下面我们试一下下载为 blob 大文件并转为 file 对象进行上传。

// url 文件地址转为文件对象
function urlToFile(url, fileName) {
    return new Promise((resolve, reject) => {
        fetch(url).then(res => res.blob()).then(blob => {
            let file = new window.File([blob], fileName);
            resolve(file);
        }).catch(error => {
            console.log("error", error);
            reject(error)
        });
    });
}

// 获取文件对象数组
function getFileList(urls) {
    return new Promise(async (resolve) => {
        let fileList = [];
        for (let i = 0; i < urls.length; i++) {
            let fileName = urls[i].substring(urls[i].lastIndexOf('/') + 1);
            let file = await this.urlToFile(urls[i], fileName);
            if(file instanceof window.File){
                fileList.push(file);
            }
        }
        resolve(fileList);
    });
}

// 上传文件对象
function uploadFile(urls){
    return new Promise(async (resolve) => {
        let fileList = await this.getFileList(urls);
        let formData = new FormData();
        for (let i = 0; i < fileList.length; i++) {
            formData.append("files", fileList[i]);
        }
        fetch("http://localhost:8080/upload/img/list", {
            method: "POST",
            body: formData
        }).then(result => result.json()).then(json => {
            console.log("result", json);
            resolve(json);
        })
    });
}