快速和正确的路径工作

(filename.match(/[^\\\/]\.([^.\\\/]+)$/) || [null]).pop()

一些边缘情况

/path/.htaccess => null
/dir.with.dot/file => null

使用split的解决方案很慢，使用lastIndexOf的解决方案不能处理边缘情况。

我刚刚意识到，对p4bl0的回答发表评论是不够的，尽管Tom的回答显然解决了问题:

return filename.replace(/^.*?\.([a-zA-Z0-9]+)$/, "$1");

我已经晚了很多个月，但为了简单起见，我使用了这样的东西

var 文件名 = “I.Am.文件名.docx”; var nameLen = fileName.length; var lastDotPos = fileName.lastIndexOf（“.”）; var 文件名子 = 假; if（lastDotPos === -1） { 文件名子 = 假; } 还 { 如果希望“.”也保留，请删除 +1 fileNameSub = fileName.substr（lastDotPos + 1， nameLen）; } document.getElementById（“showInMe”）.innerHTML = fileNameSub; <div id=“showInMe”></div>

使用reduce和数组解构来获取文件名和扩展名:

Var STR = "filename.with_dot.png"; Var [filename, extension] = str.split('.')。Reduce ((acc, val, i, arr) => (i == arr。长度- 1)?(acc [0] .substring (1), val]: [[acc [0], val] . join ('.')], []) console.log({文件名,扩展});

有更好的缩进:

var str = "filename.with_dot.png";
var [filename, extension] = str.split('.')
   .reduce((acc, val, i, arr) => (i == arr.length - 1) 
       ? [acc[0].substring(1), val] 
       : [[acc[0], val].join('.')], [])


console.log({filename, extension});

// {
//   "filename": "filename.with_dot",
//   "extension": "png"
// }

以下解决方案足够快速和简短，可以用于批量操作并节省额外的字节:

 return fname.slice((fname.lastIndexOf(".") - 1 >>> 0) + 2);

下面是另一个单行非regexp通用解决方案:

 return fname.slice((Math.max(0, fname.lastIndexOf(".")) || Infinity) + 1);

对于没有扩展名(例如myfile)或以开头的名称，两者都可以正常工作。点(例如。htaccess):

 ""                            -->   ""
 "name"                        -->   ""
 "name.txt"                    -->   "txt"
 ".htpasswd"                   -->   ""
 "name.with.many.dots.myext"   -->   "myext"

如果你关心速度，你可以运行基准测试，检查提供的解决方案是否是最快的，而简短的解决方案是非常快的:

短的是如何工作的:

String.lastIndexOf method returns the last position of the substring (i.e. ".") in the given string (i.e. fname). If the substring is not found method returns -1. The "unacceptable" positions of dot in the filename are -1 and 0, which respectively refer to names with no extension (e.g. "name") and to names that start with dot (e.g. ".htaccess"). Zero-fill right shift operator (>>>) if used with zero affects negative numbers transforming -1 to 4294967295 and -2 to 4294967294, which is useful for remaining the filename unchanged in the edge cases (sort of a trick here). String.prototype.slice extracts the part of the filename from the position that was calculated as described. If the position number is more than the length of the string method returns "".

如果你想要更清晰的解决方案，将以同样的方式工作(加上额外的全路径支持)，请检查以下扩展版本。这个解决方案比以前的一行程序要慢，但是更容易理解。

function getExtension(path) {
    var basename = path.split(/[\\/]/).pop(),  // extract file name from full path ...
                                               // (supports `\\` and `/` separators)
        pos = basename.lastIndexOf(".");       // get last position of `.`

    if (basename === "" || pos < 1)            // if file name is empty or ...
        return "";                             //  `.` not found (-1) or comes first (0)

    return basename.slice(pos + 1);            // extract extension ignoring `.`
}

console.log( getExtension("/path/to/file.ext") );
// >> "ext"

这三种变体都可以在客户端的任何web浏览器中工作，也可以在服务器端NodeJS代码中使用。

function func() {
  var val = document.frm.filename.value;
  var arr = val.split(".");
  alert(arr[arr.length - 1]);
  var arr1 = val.split("\\");
  alert(arr1[arr1.length - 2]);
  if (arr[1] == "gif" || arr[1] == "bmp" || arr[1] == "jpeg") {
    alert("this is an image file ");
  } else {
    alert("this is not an image file");
  }
}