Node.JS中的 path.join 非常方便，能直接按相對或絕對合并路徑，使用： path.join([path1], [path2], [...])，有時侯前端也需要這種方法，如何實現呢？

其實直接從 node.js 的 path.js 拿到源碼加工一下就可以了：

1. 將 const 等 es6 屬性改為 var，以便前端瀏覽器兼容
2. 添加一個判斷路戲分隔符的變量 sep，即左斜杠還是右斜杠，以第一個路戲分隔符為準
3. 將引用的變量和函數放到一個文件里就可以了：

Path 的源碼： https://github.com/nodejs/node/blob/master/lib/path.js

var CHAR_FORWARD_SLASH = 47var CHAR_BACKWARD_SLASH = 92var CHAR_DOT = 46function isPathSeparator(code) { return code === CHAR_FORWARD_SLASH || code === CHAR_BACKWARD_SLASH;}function isPosixPathSeparator(code) { return code === CHAR_FORWARD_SLASH;}function normalize(path) { if (path.length === 0)  return '.'; var isAbsolute = path.charCodeAt(0) === CHAR_FORWARD_SLASH; var trailingSeparator =  path.charCodeAt(path.length - 1) === CHAR_FORWARD_SLASH; // Normalize the path path = normalizeString(path, !isAbsolute, '/', isPosixPathSeparator); if (path.length === 0 && !isAbsolute)  path = '.'; if (path.length > 0 && trailingSeparator)  path += '/'; if (isAbsolute)  return '/' + path; return path;}function normalizeString(path, allowAboveRoot, separator, isPathSeparator) { var res = ''; var lastSegmentLength = 0; var lastSlash = -1; var dots = 0; var code; for (var i = 0; i <= path.length; ++i) {  if (i < path.length)   code = path.charCodeAt(i);  else if (isPathSeparator(code))   break;  else   code = CHAR_FORWARD_SLASH;  if (isPathSeparator(code)) {   if (lastSlash === i - 1 || dots === 1) {    // NOOP   } else if (lastSlash !== i - 1 && dots === 2) {    if (res.length < 2 || lastSegmentLength !== 2 ||      res.charCodeAt(res.length - 1) !== CHAR_DOT ||      res.charCodeAt(res.length - 2) !== CHAR_DOT) {     if (res.length > 2) {      const lastSlashIndex = res.lastIndexOf(separator);      if (lastSlashIndex !== res.length - 1) {       if (lastSlashIndex === -1) {        res = '';        lastSegmentLength = 0;       } else {        res = res.slice(0, lastSlashIndex);        lastSegmentLength = res.length - 1 - res.lastIndexOf(separator);       }       lastSlash = i;       dots = 0;       continue;      }     } else if (res.length === 2 || res.length === 1) {      res = '';      lastSegmentLength = 0;      lastSlash = i;      dots = 0;      continue;     }    }    if (allowAboveRoot) {     if (res.length > 0)      res += `${separator}..`;     else      res = '..';     lastSegmentLength = 2;    }   } else {    if (res.length > 0)     res += separator + path.slice(lastSlash + 1, i);    else     res = path.slice(lastSlash + 1, i);    lastSegmentLength = i - lastSlash - 1;   }   lastSlash = i;   dots = 0;  } else if (code === CHAR_DOT && dots !== -1) {   ++dots;  } else {   dots = -1;  } } return res;}function join() { if (arguments.length === 0)  return '.'; var sep = arguments[0].indexOf('/') > -1 ? '/' : '//' var joined; var firstPart; for (var i = 0; i < arguments.length; ++i) {  var arg = arguments[i];  if (arg.length > 0) {   if (joined === undefined)    joined = firstPart = arg;   else    joined += sep + arg;  } } if (joined === undefined)  return '.'; var needsReplace = true; var slashCount = 0; if (isPathSeparator(firstPart.charCodeAt(0))) {  ++slashCount;  var firstLen = firstPart.length;  if (firstLen > 1) {   if (isPathSeparator(firstPart.charCodeAt(1))) {    ++slashCount;    if (firstLen > 2) {     if (isPathSeparator(firstPart.charCodeAt(2)))      ++slashCount;     else {      // We matched a UNC path in the first part      needsReplace = false;     }    }   }  } } if (needsReplace) {  // Find any more consecutive slashes we need to replace  for (; slashCount < joined.length; ++slashCount) {   if (!isPathSeparator(joined.charCodeAt(slashCount)))    break;  }  // Replace the slashes if needed  if (slashCount >= 2)   joined = sep + joined.slice(slashCount); } return normalize(joined);}