方法说明：
将多个参数组合成一个 path （详细请看例子）
语法：
path.join([path1], [path2], [...])
由于该方法属于path模块，使用前需要引入path模块（var path= require(“path”) ）
例子：
path.join('/foo', 'bar', 'baz/asdf', 'quux', '..')
// returns
'/foo/bar/baz/asdf'
path.join('foo', {}, 'bar')
// throws exception
TypeError: Arguments to path.join must be strings
源码：
// windows version
exports.join = function() {
function f(p) {
if (!util.isString(p)) {
throw new TypeError('Arguments to path.join must be strings');
}
return p;
}
var paths = Array.prototype.filter.call(arguments, f);
var joined = paths.join('\\');
// Make sure that the joined path doesn't start with two slashes, because
// normalize() will mistake it for an UNC path then.
//
// This step is skipped when it is very clear that the user actually
// intended to point at an UNC path. This is assumed when the first
// non-empty string arguments starts with exactly two slashes followed by
// at least one more non-slash character.
//
// Note that for normalize() to treat a path as an UNC path it needs to
// have at least 2 components, so we don't filter for that here.
// This means that the user can use join to construct UNC paths from
// a server name and a share name; for example:
// path.join('//server', 'share') -> '\\\\server\\share\')
if (!/^[\\\/]{2}[^\\\/]/.test(paths[0])) {
joined = joined.replace(/^[\\\/]{2,}/, '\\');
}
return exports.normalize(joined);
};