js前端下载文件，利用download.js或者，纯js下载文件，图片，视频，pdf等

目前，我在百度上，找到了三种方式，下载

第一种

这个方法，用的是FileReader然后readAsDataURL，我自己测试，会比方法二，下载速度要快一点

下面的代码段，可以替换，方法三种 download(blob, fileName)这一行

const a = document.createElement('a')
        document.body.appendChild(a)
        a.style.display = 'none'
        FileReader主要用于将文件内容读入内存
        var reader = new FileReader();
        reader.readAsDataURL(blob);
        // onload当读取操作成功完成时调用
        reader.onload = function(e) {
          var a = document.createElement('a');
          // 获取文件名fileName
          a.download = fileName;
          a.href = e.target.result;
          document.body.appendChild(a);
          a.click();
          document.body.removeChild(a);
        }

第二种

这是最早我百度到的方法，后来，发现，大文件在下，需要很长的请求，因为转化成blob前，需要fecth去加载，我们提供的下载地址，加载的过程中，文件越大，越耗时间。所以转化blob也就耗时间，所以可以加loading

下面的代码段，可以替换，方法三种 download(blob, fileName)这一行

const a = document.createElement('a')
        document.body.appendChild(a)
        a.style.display = 'none'
const url = window.URL.createObjectURL(blob)
        a.href = url
        a.download = fileName
        a.click()
        document.body.removeChild(a)
        window.URL.revokeObjectURL(url)

第三种（融合了方法一和方法二，是一个npm包download.js）fetch是浏览器原生提供的一个ajax请求，类似于我们用的axios。我也用axios了。但是跨域了。

如果真的想用axios那用get方法吧,替换方法三种的fetch，post方法，我测试的时候，跨域.

axios.get(url,{responseType: 'blob'}).then(res => {
	const blob =  res.data
	 download(blob, fileName)
 })

// js处理特殊文件下载（pdf、mp3、mp4、图片）
    jsDownload (url, fileName) {
    // 请求进来，就可以加在loading
      fetch(url).then(res => {
        return res.blob()
      }).then(blob => {
      // 这是一个外部引用的download.js文件，这个文件，在下面，可以粘贴到你的项目使用
        download(blob, fileName)
        // 下载结束，loading结束
      }).catch(err => {
        console.log(err)
        return false
      }).finally(res => {
      // 这里也可以结束loading。不管是成功失败，都会结束loading
        return true
      })
    },

download.js文件，可以复制，保存到项目里，直接import 就行

download.js的github地址。膜拜大佬

//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() */
}));