diff --git a/oauth-1.0a.d.ts b/oauth-1.0a.d.ts
index 9569bc1..9f2b29b 100644
--- a/oauth-1.0a.d.ts
+++ b/oauth-1.0a.d.ts
@@ -6,7 +6,6 @@ export as namespace OAuth;
 export = OAuth;
 
 declare class OAuth {
-
   body_hash_function: OAuth.BodyHashFunction;
   consumer: OAuth.Consumer;
   hash_function: OAuth.HashFunction;
@@ -22,17 +21,28 @@ declare class OAuth {
   /**
    * Sign a request.
    */
-  authorize(request: OAuth.RequestOptions, token?: OAuth.Token): OAuth.Authorization;
+  authorize(
+    request: OAuth.RequestOptions,
+    token?: OAuth.Token,
+    verifier?: string
+  ): OAuth.Authorization;
 
   /**
    * Generate the oauth signature (i.e. oauth_signature).
    */
-  getSignature(request: OAuth.RequestOptions, token_secret: string | undefined, oauth_data: OAuth.Data): string;
+  getSignature(
+    request: OAuth.RequestOptions,
+    token_secret: string | undefined,
+    oauth_data: OAuth.Data
+  ): string;
 
   /**
    * Generate the body signature (i.e. oauth_body_hash).
    */
-  getBodyHash(request: OAuth.RequestOptions, token_secret: string | undefined): string;
+  getBodyHash(
+    request: OAuth.RequestOptions,
+    token_secret: string | undefined
+  ): string;
 
   /**
    * Encode the request attributes.
@@ -44,7 +54,10 @@ declare class OAuth {
   /**
    * Encode the oauth data and the request parameter,
    */
-  getParameterString(request: OAuth.RequestOptions, oauth_data: OAuth.Data): string;
+  getParameterString(
+    request: OAuth.RequestOptions,
+    oauth_data: OAuth.Data
+  ): string;
 
   /**
    * Generate the signing key.
@@ -96,11 +109,12 @@ declare class OAuth {
   /**
    * Sort an object properties by keys.
    */
-  sortObject<O extends {[k: string]: any}, K extends string>(obj: O): Array<{key: keyof O, value: O[K]}>;
+  sortObject<O extends { [k: string]: any }, K extends string>(
+    obj: O
+  ): Array<{ key: keyof O; value: O[K] }>;
 }
 
 declare namespace OAuth {
-
   /**
    * OAuth data, including the signature.
    */
@@ -135,6 +149,7 @@ declare namespace OAuth {
     oauth_version: string;
     oauth_token?: string;
     oauth_body_hash?: string;
+    oauth_verifier?: string;
   }
 
   /**
@@ -188,5 +203,4 @@ declare namespace OAuth {
     key: string;
     secret: string;
   }
-
 }
diff --git a/oauth-1.0a.js b/oauth-1.0a.js
index 5c15dd4..c2af0b3 100644
--- a/oauth-1.0a.js
+++ b/oauth-1.0a.js
@@ -1,5 +1,5 @@
-if (typeof(module) !== 'undefined' && typeof(exports) !== 'undefined') {
-    module.exports = OAuth;
+if (typeof module !== 'undefined' && typeof exports !== 'undefined') {
+  module.exports = OAuth;
 }
 
 /**
@@ -7,45 +7,45 @@ if (typeof(module) !== 'undefined' && typeof(exports) !== 'undefined') {
  * @param {Object} opts consumer key and secret
  */
 function OAuth(opts) {
-    if(!(this instanceof OAuth)) {
-        return new OAuth(opts);
-    }
+  if (!(this instanceof OAuth)) {
+    return new OAuth(opts);
+  }
 
-    if(!opts) {
-        opts = {};
-    }
+  if (!opts) {
+    opts = {};
+  }
 
-    if(!opts.consumer) {
-        throw new Error('consumer option is required');
-    }
+  if (!opts.consumer) {
+    throw new Error('consumer option is required');
+  }
 
-    this.consumer            = opts.consumer;
-    this.nonce_length        = opts.nonce_length || 32;
-    this.version             = opts.version || '1.0';
-    this.parameter_seperator = opts.parameter_seperator || ', ';
-    this.realm               = opts.realm;
+  this.consumer = opts.consumer;
+  this.nonce_length = opts.nonce_length || 32;
+  this.version = opts.version || '1.0';
+  this.parameter_seperator = opts.parameter_seperator || ', ';
+  this.realm = opts.realm;
 
-    if(typeof opts.last_ampersand === 'undefined') {
-        this.last_ampersand = true;
-    } else {
-        this.last_ampersand = opts.last_ampersand;
-    }
+  if (typeof opts.last_ampersand === 'undefined') {
+    this.last_ampersand = true;
+  } else {
+    this.last_ampersand = opts.last_ampersand;
+  }
 
-    // default signature_method is 'PLAINTEXT'
-    this.signature_method = opts.signature_method || 'PLAINTEXT';
+  // default signature_method is 'PLAINTEXT'
+  this.signature_method = opts.signature_method || 'PLAINTEXT';
 
-    if(this.signature_method == 'PLAINTEXT' && !opts.hash_function) {
-        opts.hash_function = function(base_string, key) {
-            return key;
-        }
-    }
+  if (this.signature_method == 'PLAINTEXT' && !opts.hash_function) {
+    opts.hash_function = function (base_string, key) {
+      return key;
+    };
+  }
 
-    if(!opts.hash_function) {
-        throw new Error('hash_function option is required');
-    }
+  if (!opts.hash_function) {
+    throw new Error('hash_function option is required');
+  }
 
-    this.hash_function = opts.hash_function;
-    this.body_hash_function = opts.body_hash_function || this.hash_function;
+  this.hash_function = opts.hash_function;
+  this.body_hash_function = opts.body_hash_function || this.hash_function;
 }
 
 /**
@@ -59,34 +59,42 @@ function OAuth(opts) {
  * @param  {Object} key and secret token
  * @return {Object} OAuth Authorized data
  */
-OAuth.prototype.authorize = function(request, token) {
-    var oauth_data = {
-        oauth_consumer_key: this.consumer.key,
-        oauth_nonce: this.getNonce(),
-        oauth_signature_method: this.signature_method,
-        oauth_timestamp: this.getTimeStamp(),
-        oauth_version: this.version
-    };
+OAuth.prototype.authorize = function (request, token, verifier) {
+  var oauth_data = {
+    oauth_consumer_key: this.consumer.key,
+    oauth_nonce: this.getNonce(),
+    oauth_signature_method: this.signature_method,
+    oauth_timestamp: this.getTimeStamp(),
+    oauth_version: this.version
+  };
+
+  if (!token) {
+    token = {};
+  }
 
-    if(!token) {
-        token = {};
-    }
+  if (token.key !== undefined) {
+    oauth_data.oauth_token = token.key;
+  }
 
-    if(token.key !== undefined) {
-        oauth_data.oauth_token = token.key;
-    }
+  if (!request.data) {
+    request.data = {};
+  }
 
-    if(!request.data) {
-        request.data = {};
-    }
+  if (request.includeBodyHash) {
+    oauth_data.oauth_body_hash = this.getBodyHash(request, token.secret);
+  }
 
-    if(request.includeBodyHash) {
-      oauth_data.oauth_body_hash = this.getBodyHash(request, token.secret)
-    }
+  if (verifier) {
+    oauth_data.oauth_verifier = verifier;
+  }
 
-    oauth_data.oauth_signature = this.getSignature(request, token.secret, oauth_data);
+  oauth_data.oauth_signature = this.getSignature(
+    request,
+    token.secret,
+    oauth_data
+  );
 
-    return oauth_data;
+  return oauth_data;
 };
 
 /**
@@ -96,22 +104,28 @@ OAuth.prototype.authorize = function(request, token) {
  * @param  {Object} oauth_data   OAuth data
  * @return {String} Signature
  */
-OAuth.prototype.getSignature = function(request, token_secret, oauth_data) {
-    return this.hash_function(this.getBaseString(request, oauth_data), this.getSigningKey(token_secret));
+OAuth.prototype.getSignature = function (request, token_secret, oauth_data) {
+  return this.hash_function(
+    this.getBaseString(request, oauth_data),
+    this.getSigningKey(token_secret)
+  );
 };
 
 /**
  * Create a OAuth Body Hash
  * @param {Object} request data
  */
-OAuth.prototype.getBodyHash = function(request, token_secret) {
-  var body = typeof request.data === 'string' ? request.data : JSON.stringify(request.data)
+OAuth.prototype.getBodyHash = function (request, token_secret) {
+  var body =
+    typeof request.data === 'string'
+      ? request.data
+      : JSON.stringify(request.data);
 
   if (!this.body_hash_function) {
     throw new Error('body_hash_function option is required');
   }
 
-  return this.body_hash_function(body, this.getSigningKey(token_secret))
+  return this.body_hash_function(body, this.getSigningKey(token_secret));
 };
 
 /**
@@ -120,8 +134,14 @@ OAuth.prototype.getBodyHash = function(request, token_secret) {
  * @param  {Object} OAuth data
  * @return {String} Base String
  */
-OAuth.prototype.getBaseString = function(request, oauth_data) {
-    return request.method.toUpperCase() + '&' + this.percentEncode(this.getBaseUrl(request.url)) + '&' + this.percentEncode(this.getParameterString(request, oauth_data));
+OAuth.prototype.getBaseString = function (request, oauth_data) {
+  return (
+    request.method.toUpperCase() +
+    '&' +
+    this.percentEncode(this.getBaseUrl(request.url)) +
+    '&' +
+    this.percentEncode(this.getParameterString(request, oauth_data))
+  );
 };
 
 /**
@@ -134,43 +154,56 @@ OAuth.prototype.getBaseString = function(request, oauth_data) {
  * @param  {Object} OAuth data
  * @return {Object} Parameter string data
  */
-OAuth.prototype.getParameterString = function(request, oauth_data) {
-    var base_string_data;
-    if (oauth_data.oauth_body_hash) {
-        base_string_data = this.sortObject(this.percentEncodeData(this.mergeObject(oauth_data, this.deParamUrl(request.url))));
-    } else {
-        base_string_data = this.sortObject(this.percentEncodeData(this.mergeObject(oauth_data, this.mergeObject(request.data, this.deParamUrl(request.url)))));
-    }
+OAuth.prototype.getParameterString = function (request, oauth_data) {
+  var base_string_data;
+  if (oauth_data.oauth_body_hash) {
+    base_string_data = this.sortObject(
+      this.percentEncodeData(
+        this.mergeObject(oauth_data, this.deParamUrl(request.url))
+      )
+    );
+  } else {
+    base_string_data = this.sortObject(
+      this.percentEncodeData(
+        this.mergeObject(
+          oauth_data,
+          this.mergeObject(request.data, this.deParamUrl(request.url))
+        )
+      )
+    );
+  }
 
-    var data_str = '';
-
-    //base_string_data to string
-    for(var i = 0; i < base_string_data.length; i++) {
-        var key = base_string_data[i].key;
-        var value = base_string_data[i].value;
-        // check if the value is an array
-        // this means that this key has multiple values
-        if (value && Array.isArray(value)){
-          // sort the array first
-          value.sort();
-
-          var valString = "";
-          // serialize all values for this key: e.g. formkey=formvalue1&formkey=formvalue2
-          value.forEach((function(item, i){
-            valString += key + '=' + item;
-            if (i < value.length){
-              valString += "&";
-            }
-          }).bind(this));
-          data_str += valString;
-        } else {
-          data_str += key + '=' + value + '&';
-        }
+  var data_str = '';
+
+  //base_string_data to string
+  for (var i = 0; i < base_string_data.length; i++) {
+    var key = base_string_data[i].key;
+    var value = base_string_data[i].value;
+    // check if the value is an array
+    // this means that this key has multiple values
+    if (value && Array.isArray(value)) {
+      // sort the array first
+      value.sort();
+
+      var valString = '';
+      // serialize all values for this key: e.g. formkey=formvalue1&formkey=formvalue2
+      value.forEach(
+        function (item, i) {
+          valString += key + '=' + item;
+          if (i < value.length) {
+            valString += '&';
+          }
+        }.bind(this)
+      );
+      data_str += valString;
+    } else {
+      data_str += key + '=' + value + '&';
     }
+  }
 
-    //remove the last character
-    data_str = data_str.substr(0, data_str.length - 1);
-    return data_str;
+  //remove the last character
+  data_str = data_str.substr(0, data_str.length - 1);
+  return data_str;
 };
 
 /**
@@ -178,14 +211,18 @@ OAuth.prototype.getParameterString = function(request, oauth_data) {
  * @param  {String} token_secret Secret Token
  * @return {String} Signing Key
  */
-OAuth.prototype.getSigningKey = function(token_secret) {
-    token_secret = token_secret || '';
+OAuth.prototype.getSigningKey = function (token_secret) {
+  token_secret = token_secret || '';
 
-    if(!this.last_ampersand && !token_secret) {
-        return this.percentEncode(this.consumer.secret);
-    }
+  if (!this.last_ampersand && !token_secret) {
+    return this.percentEncode(this.consumer.secret);
+  }
 
-    return this.percentEncode(this.consumer.secret) + '&' + this.percentEncode(token_secret);
+  return (
+    this.percentEncode(this.consumer.secret) +
+    '&' +
+    this.percentEncode(token_secret)
+  );
 };
 
 /**
@@ -193,8 +230,8 @@ OAuth.prototype.getSigningKey = function(token_secret) {
  * @param  {String} url
  * @return {String}
  */
-OAuth.prototype.getBaseUrl = function(url) {
-    return url.split('?')[0];
+OAuth.prototype.getBaseUrl = function (url) {
+  return url.split('?')[0];
 };
 
 /**
@@ -202,33 +239,33 @@ OAuth.prototype.getBaseUrl = function(url) {
  * @param  {String} string
  * @return {Object}
  */
-OAuth.prototype.deParam = function(string) {
-    var arr = string.split('&');
-    var data = {};
-
-    for(var i = 0; i < arr.length; i++) {
-        var item = arr[i].split('=');
-
-        // '' value
-        item[1] = item[1] || '';
-
-        // check if the key already exists
-        // this can occur if the QS part of the url contains duplicate keys like this: ?formkey=formvalue1&formkey=formvalue2
-        if (data[item[0]]){
-          // the key exists already
-          if (!Array.isArray(data[item[0]])) {
-            // replace the value with an array containing the already present value
-            data[item[0]] = [data[item[0]]];
-          }
-          // and add the new found value to it
-          data[item[0]].push(decodeURIComponent(item[1]));
-        } else {
-          // it doesn't exist, just put the found value in the data object
-          data[item[0]] = decodeURIComponent(item[1]);
-        }
+OAuth.prototype.deParam = function (string) {
+  var arr = string.split('&');
+  var data = {};
+
+  for (var i = 0; i < arr.length; i++) {
+    var item = arr[i].split('=');
+
+    // '' value
+    item[1] = item[1] || '';
+
+    // check if the key already exists
+    // this can occur if the QS part of the url contains duplicate keys like this: ?formkey=formvalue1&formkey=formvalue2
+    if (data[item[0]]) {
+      // the key exists already
+      if (!Array.isArray(data[item[0]])) {
+        // replace the value with an array containing the already present value
+        data[item[0]] = [data[item[0]]];
+      }
+      // and add the new found value to it
+      data[item[0]].push(decodeURIComponent(item[1]));
+    } else {
+      // it doesn't exist, just put the found value in the data object
+      data[item[0]] = decodeURIComponent(item[1]);
     }
+  }
 
-    return data;
+  return data;
 };
 
 /**
@@ -236,13 +273,12 @@ OAuth.prototype.deParam = function(string) {
  * @param  {String} url
  * @return {Object}
  */
-OAuth.prototype.deParamUrl = function(url) {
-    var tmp = url.split('?');
+OAuth.prototype.deParamUrl = function (url) {
+  var tmp = url.split('?');
 
-    if (tmp.length === 1)
-        return {};
+  if (tmp.length === 1) return {};
 
-    return this.deParam(tmp[1]);
+  return this.deParam(tmp[1]);
 };
 
 /**
@@ -250,13 +286,13 @@ OAuth.prototype.deParamUrl = function(url) {
  * @param  {String} str
  * @return {String} percent encoded string
  */
-OAuth.prototype.percentEncode = function(str) {
-    return encodeURIComponent(str)
-        .replace(/\!/g, "%21")
-        .replace(/\*/g, "%2A")
-        .replace(/\'/g, "%27")
-        .replace(/\(/g, "%28")
-        .replace(/\)/g, "%29");
+OAuth.prototype.percentEncode = function (str) {
+  return encodeURIComponent(str)
+    .replace(/\!/g, '%21')
+    .replace(/\*/g, '%2A')
+    .replace(/\'/g, '%27')
+    .replace(/\(/g, '%28')
+    .replace(/\)/g, '%29');
 };
 
 /**
@@ -264,26 +300,28 @@ OAuth.prototype.percentEncode = function(str) {
  * @param  {Object} data
  * @return {Object} percent encoded data
  */
-OAuth.prototype.percentEncodeData = function(data) {
-    var result = {};
-
-    for(var key in data) {
-        var value = data[key];
-        // check if the value is an array
-        if (value && Array.isArray(value)){
-          var newValue = [];
-          // percentEncode every value
-          value.forEach((function(val){
-            newValue.push(this.percentEncode(val));
-          }).bind(this));
-          value = newValue;
-        } else {
-          value = this.percentEncode(value);
-        }
-        result[this.percentEncode(key)] = value;
+OAuth.prototype.percentEncodeData = function (data) {
+  var result = {};
+
+  for (var key in data) {
+    var value = data[key];
+    // check if the value is an array
+    if (value && Array.isArray(value)) {
+      var newValue = [];
+      // percentEncode every value
+      value.forEach(
+        function (val) {
+          newValue.push(this.percentEncode(val));
+        }.bind(this)
+      );
+      value = newValue;
+    } else {
+      value = this.percentEncode(value);
     }
+    result[this.percentEncode(key)] = value;
+  }
 
-    return result;
+  return result;
 };
 
 /**
@@ -291,48 +329,57 @@ OAuth.prototype.percentEncodeData = function(data) {
  * @param  {Object} oauth_data
  * @return {String} Header data key - value
  */
-OAuth.prototype.toHeader = function(oauth_data) {
-    var sorted = this.sortObject(oauth_data);
+OAuth.prototype.toHeader = function (oauth_data) {
+  var sorted = this.sortObject(oauth_data);
 
-    var header_value = 'OAuth ';
+  var header_value = 'OAuth ';
 
-    if (this.realm) {
-        header_value += 'realm="' + this.realm + '"' + this.parameter_seperator;
-    }
+  if (this.realm) {
+    header_value += 'realm="' + this.realm + '"' + this.parameter_seperator;
+  }
 
-    for(var i = 0; i < sorted.length; i++) {
-        if (sorted[i].key.indexOf('oauth_') !== 0)
-            continue;
+  for (var i = 0; i < sorted.length; i++) {
+    if (sorted[i].key.indexOf('oauth_') !== 0) continue;
 
-        header_value += this.percentEncode(sorted[i].key) + '="' + this.percentEncode(sorted[i].value) + '"' + this.parameter_seperator;
-    }
+    header_value +=
+      this.percentEncode(sorted[i].key) +
+      '="' +
+      this.percentEncode(sorted[i].value) +
+      '"' +
+      this.parameter_seperator;
+  }
 
-    return {
-        Authorization: header_value.substr(0, header_value.length - this.parameter_seperator.length) //cut the last chars
-    };
+  return {
+    Authorization: header_value.substr(
+      0,
+      header_value.length - this.parameter_seperator.length
+    ) //cut the last chars
+  };
 };
 
 /**
  * Create a random word characters string with input length
  * @return {String} a random word characters string
  */
-OAuth.prototype.getNonce = function() {
-    var word_characters = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
-    var result = '';
-
-    for(var i = 0; i < this.nonce_length; i++) {
-        result += word_characters[parseInt(Math.random() * word_characters.length, 10)];
-    }
+OAuth.prototype.getNonce = function () {
+  var word_characters =
+    'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
+  var result = '';
+
+  for (var i = 0; i < this.nonce_length; i++) {
+    result +=
+      word_characters[parseInt(Math.random() * word_characters.length, 10)];
+  }
 
-    return result;
+  return result;
 };
 
 /**
  * Get Current Unix TimeStamp
  * @return {Int} current unix timestamp
  */
-OAuth.prototype.getTimeStamp = function() {
-    return parseInt(new Date().getTime()/1000, 10);
+OAuth.prototype.getTimeStamp = function () {
+  return parseInt(new Date().getTime() / 1000, 10);
 };
 
 ////////////////////// HELPER FUNCTIONS //////////////////////
@@ -343,15 +390,15 @@ OAuth.prototype.getTimeStamp = function() {
  * @param  {Object} obj2
  * @return {Object}
  */
-OAuth.prototype.mergeObject = function(obj1, obj2) {
-    obj1 = obj1 || {};
-    obj2 = obj2 || {};
+OAuth.prototype.mergeObject = function (obj1, obj2) {
+  obj1 = obj1 || {};
+  obj2 = obj2 || {};
 
-    var merged_obj = obj1;
-    for(var key in obj2) {
-        merged_obj[key] = obj2[key];
-    }
-    return merged_obj;
+  var merged_obj = obj1;
+  for (var key in obj2) {
+    merged_obj[key] = obj2[key];
+  }
+  return merged_obj;
 };
 
 /**
@@ -359,19 +406,19 @@ OAuth.prototype.mergeObject = function(obj1, obj2) {
  * @param  {Object} data
  * @return {Array} sorted array
  */
-OAuth.prototype.sortObject = function(data) {
-    var keys = Object.keys(data);
-    var result = [];
-
-    keys.sort();
-
-    for(var i = 0; i < keys.length; i++) {
-        var key = keys[i];
-        result.push({
-            key: key,
-            value: data[key],
-        });
-    }
+OAuth.prototype.sortObject = function (data) {
+  var keys = Object.keys(data);
+  var result = [];
+
+  keys.sort();
+
+  for (var i = 0; i < keys.length; i++) {
+    var key = keys[i];
+    result.push({
+      key: key,
+      value: data[key]
+    });
+  }
 
-    return result;
+  return result;
 };