lib/index.js
function getMeta(host, accessKey, secretKey, bucket, objectKey) {
    var expires = arguments.length <= 5 || arguments[5] === undefined ? 0 : arguments[5];
    var opts = arguments.length <= 6 || arguments[6] === undefined ? {} : arguments[6];

    var date = utcDate();
    if (opts.versionId) url += '?versionId=' + opts.versionId;
    var resource = genResource(bucket, objectKey, (0, _lodash.pick)(opts, ['versionId']));
    var authorization = authorize(accessKey, secretKey, 'HEAD', '', '', date, [], resource);
    var signature = (0, _urlencode2.default)(generateSignature(secretKey, 'HEAD', '', '', date, [], resource));
    var url = pub ? 'http://' + bucket + '.' + host + '/' + objectKey : 'http://' + host + '/' + bucket + '/' + objectKey + '?NOSAccessKeyId=' + accessKey + '&Expires=' + expires + '&Signature=' + signature;

    var headers = { Date: date };
    if (opts.modifiedSince) headers['If-Modified-Since'] = opts.modifiedSince;
    headers['url'] = url;

    return nosRequest({ method: 'head', uri: url, headers: headers }).then(function (res) {
        var contentType = res['content-type'];
        var lastModified = res['last-modified'];
        var etag = res['etag'];
        var requestId = res['x-nos-request-id'];
        return { contentType: contentType, lastModified: lastModified, etag: etag, requestId: requestId };
    });
}
function getMeta(host, accessKey, secretKey, bucket, objectKey) {
    var expires = arguments.length <= 5 || arguments[5] === undefined ? 0 : arguments[5];
    var opts = arguments.length <= 6 || arguments[6] === undefined ? {} : arguments[6];

    var date = utcDate();
    if (opts.versionId) url += '?versionId=' + opts.versionId;
    var resource = genResource(bucket, objectKey, (0, _lodash.pick)(opts, ['versionId']));
    var authorization = authorize(accessKey, secretKey, 'HEAD', '', '', date, [], resource);
    var signature = (0, _urlencode2.default)(generateSignature(secretKey, 'HEAD', '', '', date, [], resource));
    var url = pub ? 'http://' + bucket + '.' + host + '/' + objectKey : 'http://' + host + '/' + bucket + '/' + objectKey + '?NOSAccessKeyId=' + accessKey + '&Expires=' + expires + '&Signature=' + signature;

    var headers = { Date: date };
    if (opts.modifiedSince) headers['If-Modified-Since'] = opts.modifiedSince;
    headers['url'] = url;

    return nosRequest({ method: 'head', uri: url, headers: headers }).then(function (res) {
        var contentType = res['content-type'];
        var lastModified = res['last-modified'];
        var etag = res['etag'];
        var requestId = res['x-nos-request-id'];
        return { contentType: contentType, lastModified: lastModified, etag: etag, requestId: requestId };
    });
}
获取对象相关元数据信息
@param {String} accessKey
@param {String} secretKey
@param {String} bucket
@param {String} objectKey
@param {Object} [opts]
@param {String} [opts.modifiedSince]
@returns {Promise}