diff --git a/README.md b/README.md
index 0ed7e82..57f3c95 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,10 @@
 
 `$ npm install wechatpay-axios-plugin`
 
+## Requirements
+
+`Rsa.encrypt` and `Rsa.decrypt` with `oaepHash` was added on Node v12.9.0. So that the Node minimum version should be 12.9.0(I'm not very sure).
+
 ## Examples
 
 ### Initialization
@@ -54,6 +58,17 @@ client.post('/v3/combine-transactions/jsapi', {}).then(response => {
 })
 ```
 
+#### POST `/v3/smartguide/guides/{guide_id}/assign` mixed `RESTful` parameter with `JSON` body payload, response as `204` status code.
+
+```js
+client.post(`/v3/smartguide/guides/${guide_id}/assign`, {
+  sub_mchid,
+  out_trade_no,
+}).catch(error => {
+  console.error(error)
+})
+```
+
 ### Async/Await style
 
 #### GET `/v3/merchant-service/complaints` with `query` parameters
@@ -71,7 +86,7 @@ client.post('/v3/combine-transactions/jsapi', {}).then(response => {
     })
     console.info(res.data)
   } catch (error) {
-    console.info(error)
+    console.error(error)
   }
 })()
 ```
@@ -96,7 +111,7 @@ client.post('/v3/combine-transactions/jsapi', {}).then(response => {
     })
     console.info(res.data.code_url)
   } catch (error) {
-    console.info(error)
+    console.error(error)
   }
 })()
 ```
@@ -139,7 +154,7 @@ client.post('/v3/combine-transactions/jsapi', {}).then(response => {
     })
     console.info(res.data)
   } catch (error) {
-    console.info(error)
+    console.error(error)
   }
 })()
 ```
diff --git a/lib/aes.js b/lib/aes.js
index abc4b5c..6b805e7 100644
--- a/lib/aes.js
+++ b/lib/aes.js
@@ -7,8 +7,21 @@ const base64 = `base64`
 const BLOCK_SIZES = 16
 const algorithm   = `aes-256-gcm`
 
+/**
+ * Aes encrypt/decrypt using `aes-256-gcm` algorithm with `AAD`.
+ */
 class Aes {
 
+  /**
+   * Encrypts plaintext.
+   *
+   * @param {string} iv - The initialization vector, 16 bytes string.
+   * @param {string} key - The secret key, 32 bytes string.
+   * @param {string} plaintext - Text to encode.
+   * @param {string} aad - The additional authenticated data, maybe empty string.
+   *
+   * @returns {object}
+   */
   static encrypt(iv, key, plaintext, aad = '') {
     const cipher = crypto.createCipheriv(
       algorithm, key, iv
@@ -21,6 +34,16 @@ class Aes {
     ]).toString(base64)
   }
 
+  /**
+   * Decrypts ciphertext.
+   *
+   * @param {string} iv - The initialization vector, 16 bytes string.
+   * @param {string} key - The secret key, 32 bytes string.
+   * @param {string} ciphertext - Base64-encoded ciphertext.
+   * @param {string} aad - The additional authenticated data, maybe empty string.
+   *
+   * @returns {string}
+   */
   static decrypt(iv, key, ciphertext, aad = '') {
     const buf     = Buffer.from(ciphertext, base64)
     const tag     = buf.slice(-BLOCK_SIZES)
diff --git a/lib/formatter.js b/lib/formatter.js
index 270e7d8..2e9a50f 100644
--- a/lib/formatter.js
+++ b/lib/formatter.js
@@ -1,24 +1,69 @@
-
+/**
+ * Provides easy used methods using in this project.
+ */
 class Formatter {
 
+  /**
+   * Generate a random string aka `nonce`, similar as `crypto.randomBytes`.
+   *
+   * @param {number} size - Nonce string length, default is 32 bytes.
+   *
+   * @returns {string}
+   */
   static nonce(size = 32) {
     const chars = `0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ`
 
     return `0`.repeat(size).replace(/0/g, _ => chars[Math.random()*62|0])
   }
 
+  /**
+   * Retrieve the current `Unix` timestamp.
+   *
+   * @returns {number}
+   */
   static timestamp() {
     return +(new Date)/1000|0
   }
 
+  /**
+   * Formatting for the heading `Authorization` value.
+   *
+   * @param {string} mchid - The merchant ID.
+   * @param {string} nonce_str - The Nonce string.
+   * @param {string} signature - The base64-encoded `Rsa.sign` ciphertext.
+   * @param {string} timestamp - The `Unix` timestamp.
+   * @param {string} serial_no - The serial number of the merchant public certification.
+   *
+   * @returns {string}
+   */
   static authorization(mchid, nonce_str, signature, timestamp, serial_no) {
     return `WECHATPAY2-SHA256-RSA2048 mchid="${mchid}",nonce_str="${nonce_str}",signature="${signature}",timestamp="${timestamp}",serial_no="${serial_no}"`
   }
 
+  /**
+   * Formatting this `HTTP.request` for `Rsa.sign` input.
+   *
+   * @param {string} method - The merchant ID.
+   * @param {string} uri - Combined string with `URL.pathname` and `URL.search`.
+   * @param {string} timestamp - The `Unix` timestamp, should be the one used in `authorization`.
+   * @param {string} nonce - The `Nonce` string, should be the one used in `authorization`.
+   * @param {string} body - The playload string, HTTP `GET` should be an empty string.
+   *
+   * @returns {string}
+   */
   static request(method, uri, timestamp, nonce, body = '') {
     return `${method}\n${uri}\n${timestamp}\n${nonce}\n${body}\n`
   }
 
+  /**
+   * Formatting this `HTTP.response` for `Rsa.verify` input.
+   *
+   * @param {string} timestamp - The `Unix` timestamp, should be the one from `response.headers[wechatpay-timestamp]`.
+   * @param {string} nonce - The `Nonce` string, should be the one from `response.headers[wechatpay-nonce]`.
+   * @param {string} body - The response payload string, HTTP status(`204`) should be an empty string.
+   *
+   * @returns {string}
+   */
   static response(timestamp, nonce, body = '') {
     return `${timestamp}\n${nonce}\n${body}\n`
   }
diff --git a/lib/rsa.js b/lib/rsa.js
index c28b0de..840321b 100644
--- a/lib/rsa.js
+++ b/lib/rsa.js
@@ -7,8 +7,20 @@ const base64 = `base64`
 const sha256WithRSAEncryption = `sha256WithRSAEncryption`
 const RSA_PKCS1_OAEP_PADDING  = crypto.constants.RSA_PKCS1_OAEP_PADDING
 
+/**
+ * Provides some methods for the RSA `sha256WithRSAEncryption` with `RSA_PKCS1_OAEP_PADDING`.
+ */
 class Rsa {
 
+  /**
+   * Encrypts text with sha256WithRSAEncryption/RSA_PKCS1_OAEP_PADDING.
+   * Node Limits >= 12.9.0 (`oaepHash` was added)
+   *
+   * @param {string} plaintext - Cleartext to encode.
+   * @param {string|Buffer} publicCertificate - A PEM encoded public certification.
+   *
+   * @returns {string}
+   */
   static encrypt(plaintext, publicCertificate) {
     return crypto.publicEncrypt({
       oaepHash : sha1,
@@ -17,6 +29,15 @@ class Rsa {
     }, Buffer.from(plaintext, utf8)).toString(base64)
   }
 
+  /**
+   * Decrypts base64 encoded string with `privateKeyCertificate`.
+   * Node Limits >= 12.9.0 (`oaepHash` was added)
+   *
+   * @param {string} ciphertext - Was previously encrypted string using the corresponding public certication.
+   * @param {string|Buffer} privateKeyCertificate - A PEM encoded private key certification.
+   *
+   * @returns {string}
+   */
   static decrypt(ciphertext, privateKeyCertificate) {
     return crypto.privateDecrypt({
       oaepHash : sha1,
@@ -25,6 +46,14 @@ class Rsa {
     }, Buffer.from(ciphertext, base64)).toString(utf8)
   }
 
+  /**
+   * Creates and returns a `Sign` string that uses `sha256WithRSAEncryption`.
+   *
+   * @param {string} message - Content will be `crypto.Sign`.
+   * @param {string|Buffer} privateKeyCertificate - A PEM encoded private key certification.
+   *
+   * @returns {string}
+   */
   static sign(message, privateKeyCertificate) {
     return crypto.createSign(
       sha256WithRSAEncryption
@@ -34,6 +63,15 @@ class Rsa {
     )
   }
 
+  /**
+   * Verifying the `message` with given `signature` string that uses `sha256WithRSAEncryption`.
+   *
+   * @param {string} message - Content will be `crypto.Verify`.
+   * @param {string} signature - The base64-encoded ciphertext.
+   * @param {string|Buffer} publicCertificate - A PEM encoded public certification.
+   *
+   * @returns {string}
+   */
   static verify(message, signature, publicCertificate) {
     return crypto.createVerify(
       sha256WithRSAEncryption