項目中的附件列表，通常情況都需要提供下載、刪除的功能，功能本身沒有什么要說的，都是基本功能，使用瀏覽器的的下載功能，也都是用window.open(url)，或者window.location.href=url的方式，url即為附件下載接口，瀏覽器自動解析，如果是圖片、視頻、txt等格式的會直接預覽文件，而不是像docx、xlsx一樣直接下載，項目中為了功能明確，將預覽單獨加了一個查看按鈕，下載按鈕要實現(xiàn)無論什么文件格式都直接下載。

有兩種方式處理：

一.后臺接口層面處理：（比較常見）

響應頭中添加標識：
Response.AddHeader("Content-Disposition", "attachment; filename=" +
HttpUtility.UrlEncode(annotation.FileName, System.Text.Encoding.UTF8));

添加后，瀏覽器就會全都直接下載，不區(qū)分文件格式

二.前端JS使用XMLHttpRequest結(jié)合download.js處理

1）首先頁面中需要引用download.js

download官方：http://danml.com/download.html

http://danml.com/js/download2.js

2）公共方法

/*
????* 使用download.js 強制瀏覽器下載圖片、視頻等文件
????* @param {any} url url鏈接地址
????* @param {any} strFileName 文件名
????* @param {any} strMimeType 文件類型
????* dzl
????* 2020年5月8日
?????*/
????function downloadfile(url, strFileName, strMimeType) {
??????var xmlHttp = null;
??????if (window.ActiveXObject) {
????????// IE6, IE5 瀏覽器執(zhí)行代碼
????????xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
??????} else if (window.XMLHttpRequest) {
????????// IE7+, Firefox, Chrome, Opera, Safari 瀏覽器執(zhí)行代碼
????????xmlHttp = new XMLHttpRequest();
??????}
??????//2.如果實例化成功，就調(diào)用open（）方法：
??????if (xmlHttp != null) {
????????xmlHttp.open("get", url, true);
????????xmlHttp.responseType = 'blob';//關鍵
????????xmlHttp.send();
????????xmlHttp.onreadystatechange = doResult; //設置回調(diào)函數(shù)
??????}
??????function doResult() {
????????if (xmlHttp.readyState == 4) { //4表示執(zhí)行完成
??????????if (xmlHttp.status == 200) { //200表示執(zhí)行成功
????????????download(xmlHttp.response, strFileName, strMimeType);
??????????}
????????}
??????}
????}

注意： xmlHttp.responseType = 'blob';是關鍵，必須要設置

3）下載按鈕點擊事件中調(diào)用公共方法

downloadfile(url, filename, getFileType(filename))

附上根據(jù)文件名的尾綴 返回文件類型的公共方法:

/*
?* 根據(jù)文件名的尾綴 返回文件類型
?* @param {any} fileName 文件名
?* dzl
?* 2020年5月9日
?*/
function getFileType(fileName) {
??// 后綴獲取
??let suffix = '';
??// 獲取類型結(jié)果
??let result = '';
??try {
????const flieArr = fileName.split('.');
????suffix = flieArr[flieArr.length - 1];
??} catch (err) {
????suffix = '';
??}
??// fileName無后綴返回 false
??if (!suffix) { return false; }
??suffix = suffix.toLocaleLowerCase();
??// 圖片格式
??const imglist = ['png', 'jpg', 'jpeg', 'bmp', 'gif'];
??// 進行圖片匹配
??result = imglist.find(item => item === suffix);
??if (result) {
????return 'image';
??}
??// 匹配txt
??const txtlist = ['txt'];
??result = txtlist.find(item => item === suffix);
??if (result) {
????return 'txt';
??}
??// 匹配 excel
??const excelist = ['xls', 'xlsx'];
??result = excelist.find(item => item === suffix);
??if (result) {
????return 'excel';
??}
??// 匹配 word
??const wordlist = ['doc', 'docx'];
??result = wordlist.find(item => item === suffix);
??if (result) {
????return 'word';
??}
??// 匹配 pdf
??const pdflist = ['pdf'];
??result = pdflist.find(item => item === suffix);
??if (result) {
????return 'pdf';
??}
??// 匹配 ppt
??const pptlist = ['ppt', 'pptx'];
??result = pptlist.find(item => item === suffix);
??if (result) {
????return 'ppt';
??}
??// 匹配 視頻
??const videolist = ['mp4', 'm2v', 'mkv', 'rmvb', 'wmv', 'avi', 'flv', 'mov', 'm4v'];
??result = videolist.find(item => item === suffix);
??if (result) {
????return 'video';
??}
??// 匹配 音頻
??const radiolist = ['mp3', 'wav', 'wmv'];
??result = radiolist.find(item => item === suffix);
??if (result) {
????return 'radio';
??}
??// 其他 文件類型
??return 'other';
}

download.js源碼

//download.js v4.21, by dandavis; 2008-2018. [MIT] see http://danml.com/download.html for tests/usage
// v1 landed a FF+Chrome compatible way of downloading strings to local un-named files, upgraded to use a hidden frame and optional mime
// v2 added named files via a[download], msSaveBlob, IE (10+) support, and window.URL support for larger+faster saves than dataURLs
// v3 added dataURL and Blob Input, bind-toggle arity, and legacy dataURL fallback was improved with force-download mime and base64 support. 3.1 improved safari handling.
// v4 adds AMD/UMD, commonJS, and plain browser support
// v4.1 adds url download capability via solo URL argument (same domain/CORS only)
// v4.2 adds semantic variable names, long (over 2MB) dataURL support, and hidden by default temp anchors
// https://github.com/rndme/download
?
(function (root, factory) {
????if (typeof define === 'function' && define.amd) {
????????// AMD. Register as an anonymous module.
????????define([], factory);
????} else if (typeof exports === 'object') {
????????// Node. Does not work with strict CommonJS, but
????????// only CommonJS-like environments that support module.exports,
????????// like Node.
????????module.exports = factory();
????} else {
????????// Browser globals (root is window)
????????root.download = factory();
?}
}(this, function () {
?
????return function download(data, strFileName, strMimeType) {
?
????????var self = window, // this script is only for browsers anyway...
????????????defaultMime = "application/octet-stream", // this default mime also triggers iframe downloads
????????????mimeType = strMimeType || defaultMime,
????????????payload = data,
????????????url = !strFileName && !strMimeType && payload,
????????????anchor = document.createElement("a"),
????????????toString = function(a){return String(a);},
????????????myBlob = (self.Blob || self.MozBlob || self.WebKitBlob || toString),
????????????fileName = strFileName || "download",
????????????blob,
????????????reader;
????????????myBlob= myBlob.call ? myBlob.bind(self) : Blob ;
??????
????????if(String(this)==="true"){ //reverse arguments, allowing download.bind(true, "text/xml", "export.xml") to act as a callback
????????????payload=[payload, mimeType];
????????????mimeType=payload[0];
????????????payload=payload[1];
????????}
?
?
????????if(url && url.length< 2048){ // if no filename and no mime, assume a url was passed as the only argument
????????????fileName = url.split("/").pop().split("?")[0];
????????????anchor.href = url; // assign href prop to temp anchor
????????????if(anchor.href.indexOf(url) !== -1){ // if the browser determines that it's a potentially valid url path:
????????????var ajax=new XMLHttpRequest();
????????????ajax.open( "GET", url, true);
????????????ajax.responseType = 'blob';
????????????ajax.onload= function(e){ 
?????????????????download(e.target.response, fileName, defaultMime);
????????????????};
????????????setTimeout(function(){ ajax.send();}, 0); // allows setting custom ajax headers using the return:
??????????????return ajax;
????????????} // end if valid url?
????????} // end if url?
?
?
????????//go ahead and download dataURLs right away
????????if(/^data:([\w+-]+\/[\w+.-]+)?[,;]/.test(payload)){
?????????
????????????if(payload.length > (1024*1024*1.999) && myBlob !== toString ){
????????????????payload=dataUrlToBlob(payload);
????????????????mimeType=payload.type || defaultMime;
????????????}else{????????? 
????????????????return navigator.msSaveBlob ? // IE10 can't do a[download], only Blobs:
????????????????????navigator.msSaveBlob(dataUrlToBlob(payload), fileName) :
????????????????????saver(payload) ; // everyone else can save dataURLs un-processed
????????????}
?????????????
????????}else{//not data url, is it a string with special needs?
????????????if(/([\x80-\xff])/.test(payload)){?????????? 
????????????????var i=0, tempUiArr= new Uint8Array(payload.length), mx=tempUiArr.length;
????????????????for(i;i<mx;++i) tempUiArr[i]= payload.charCodeAt(i);
????????????????payload=new myBlob([tempUiArr], {type: mimeType});
????????????}??????? 
????????}
????????blob = payload instanceof myBlob ?
????????????payload :
????????????new myBlob([payload], {type: mimeType}) ;
?
?
????????function dataUrlToBlob(strUrl) {
????????????var parts= strUrl.split(/[:;,]/),
????????????type= parts[1],
????????????indexDecoder = strUrl.indexOf("charset")>0 ? 3: 2,
????????????decoder= parts[indexDecoder] == "base64" ? atob : decodeURIComponent,
????????????binData= decoder( parts.pop() ),
????????????mx= binData.length,
????????????i= 0,
????????????uiArr= new Uint8Array(mx);
?
????????????for(i;i<mx;++i) uiArr[i]= binData.charCodeAt(i);
?
????????????return new myBlob([uiArr], {type: type});
?????????}
?
????????function saver(url, winMode){
?
????????????if ('download' in anchor) { //html5 A[download]
????????????????anchor.href = url;
????????????????anchor.setAttribute("download", fileName);
????????????????anchor.className = "download-js-link";
????????????????anchor.innerHTML = "downloading...";
????????????????anchor.style.display = "none";
????????????????anchor.addEventListener('click', function(e) {
????????????????????e.stopPropagation();
????????????????????this.removeEventListener('click', arguments.callee);
????????????????});
????????????????document.body.appendChild(anchor);
????????????????setTimeout(function() {
????????????????????anchor.click();
????????????????????document.body.removeChild(anchor);
????????????????????if(winMode===true){setTimeout(function(){ self.URL.revokeObjectURL(anchor.href);}, 250 );}
????????????????}, 66);
????????????????return true;
????????????}
?
????????????// handle non-a[download] safari as best we can:
????????????if(/(Version)\/(\d+)\.(\d+)(?:\.(\d+))?.*Safari\//.test(navigator.userAgent)) {
????????????????if(/^data:/.test(url))? url="data:"+url.replace(/^data:([\w\/\-\+]+)/, defaultMime);
????????????????if(!window.open(url)){ // popup blocked, offer direct download:
????????????????????if(confirm("Displaying New Document\n\nUse Save As... to download, then click back to return to this page.")){ location.href=url; }
????????????????}
????????????????return true;
????????????}
?
????????????//do iframe dataURL download (old ch+FF):
????????????var f = document.createElement("iframe");
????????????document.body.appendChild(f);
?
????????????if(!winMode && /^data:/.test(url)){ // force a mime that will download:
????????????????url="data:"+url.replace(/^data:([\w\/\-\+]+)/, defaultMime);
????????????}
????????????f.src=url;
????????????setTimeout(function(){ document.body.removeChild(f); }, 333);
?
????????}//end saver
?
?
?
?
????????if (navigator.msSaveBlob) { // IE10+ : (has Blob, but not a[download] or URL)
????????????return navigator.msSaveBlob(blob, fileName);
????????}
?
????????if(self.URL){ // simple fast and modern way using Blob and URL:
????????????saver(self.URL.createObjectURL(blob), true);
????????}else{
????????????// handle non-Blob()+non-URL browsers:
????????????if(typeof blob === "string" || blob.constructor===toString ){
????????????????try{
????????????????????return saver( "data:" + mimeType? + ";base64," + self.btoa(blob) );
????????????????}catch(y){
????????????????????return saver( "data:" + mimeType? + "," + encodeURIComponent(blob) );
????????????????}
????????????}
?
????????????// Blob but not URL support:
????????????reader=new FileReader();
????????????reader.onload=function(e){
????????????????saver(this.result);
????????????};
????????????reader.readAsDataURL(blob);
????????}
????????return true;
????}; /* end download() */
}));

在前端想要生成txt或者其他格式文件來保存一些數(shù)據(jù)，或者在數(shù)據(jù)請求服務器成功后想要將數(shù)據(jù)生成文件并下載，這些需求還是比較多的，這里使用到的download.js。既可以滿足這些需求。js文件下載插件介紹和使用方法 download.js

第一步：下載

1.下載可以在官網(wǎng)上進行下載 原網(wǎng)址:download2.js

2.也可以使用我上傳的資源(下載與上述官網(wǎng)) 免費哦！ 下載

第二步：引用

1.將下載的文件放在項目中，并使用 <script src="./download2.js"></script> 進行引用 (速度快)

2.使用網(wǎng)絡引用 <script src="http://danml.com/js/download2.js"></script>

第三步：開始使用

//生成一個txt的文件，講字符串變量 str 的內(nèi)容放在里面，并完成下載
var str = "hello world可以中文嗎？答案：可以的";
download(str, "Text.txt", "text/plain");
?
//也將生成一個txt文件，和上面的結(jié)果是一摸一樣
str = "data:text/plain,hello%20world";
download(str, "dlDataUrlText.txt", "text/plain");
?
//將數(shù)組或者json對象寫入到文件中
var debug = {hello : "world"};
var blob = new Blob([JSON.stringify(debug, null, 2)]);
download(blob, "dlTextBlob.txt", "text/plain");
?
var str= "hello world", arr= new Uint8Array(str.length);
str.split("").forEach(function(a,b){
??arr[b]=a.charCodeAt();
});
download( arr, "textUInt8Array.txt", "text/plain" );
?
//將html頁面中的一部分生成單獨的 html文檔
download(document.body.outerHTML, "dlHTML.html", "text/html");
?
//給字符串添加標簽，或者html片段直接給方法，將會生成對應內(nèi)容的文件
download(new Blob(["hello world".bold()]), "dlHtmlBlob.html", "text/html");
?
?//將ajax查詢回來的結(jié)果，賦值給文件，并下載
$.ajax({
??url: "/download.html",
??success: download.bind(true, "text/html", "dlAjaxCallback.html")
});
?
//將圖片的base64裝入到圖片中并下載。
download("data:image/gif;base64,R0lGODlhRgAVAIcAAOfn5+/v7/f39////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////yH5BAAAAP8ALAAAAABGABUAAAj/AAEIHAgggMGDCAkSRMgwgEKBDRM+LBjRoEKDAjJq1GhxIMaNGzt6DAAypMORJTmeLKhxgMuXKiGSzPgSZsaVMwXUdBmTYsudKjHuBCoAIc2hMBnqRMqz6MGjTJ0KZcrz5EyqA276xJrVKlSkWqdGLQpxKVWyW8+iJcl1LVu1XttafTs2Lla3ZqNavAo37dm9X4eGFQtWKt+6T+8aDkxUqWKjeQUvfvw0MtHJcCtTJiwZsmLMiD9uplvY82jLNW9qzsy58WrWpDu/Lp0YNmPXrVMvRm3T6GneSX3bBt5VeOjDemfLFv1XOW7kncvKdZi7t/S7e2M3LkscLcvH3LF7HwSuVeZtjuPPe2d+GefPrD1RpnS6MGdJkebn4/+oMSAAOw==", "dlDataUrlBin.gif", "image/gif");
?
//使用xhr獲取本域名下文件，并封裝到文件進行下載。這里的前提是xhr訪問的文件必須和該程序執(zhí)行在同一域名下，
//否則會報跨域的問題。比如此程序執(zhí)行在localhost下面，而這里的訪問地址使用的是http://danml.com，就會訪問不成功。
??var x=new XMLHttpRequest();
??x.open("GET", "http://danml.com/wave2.gif", true);
??x.responseType = 'blob';
??x.οnlοad=function(e){
????download(x.response, "dlBinAjax.gif", "image/gif" );
??};
??x.send();

擴展：還可以使用文件上傳再下載進行測試。

<input id="file" type="file" οnchange="download(this.files[0], this.files[0].name, this.files[0].type)">

以上就是JS端基于download.js實現(xiàn)圖片、視頻時直接下載而不是打開預覽的詳細內(nèi)容，