add: basic function with comments

src/otp.js

@@ -4,17 +4,44 @@
  */
 
 export default class OTP  {
+    /*＊
+     * This constructor will create OTP instance.
+     *
+     * @param {secret}
+     * @type {String}
+     * @desc random base32-encoded key, it is the 
+     * key that be used to verify.
+     *
+     * @param {digits}
+     * @type {int} 
+     * @desc the length of the one-time password, default to be 6
+     *
+     * @param {digest}
+     * @type {String}
+     * @desc the key that be used to do HMAC encoding, dedault and 
+     * only to be "sha1"
+     *
+     */
     constructor(secret, digits=6, digest="sha1") {
-        /*
-         * This constructor will create OTP instance
-         */
         this.secret = secret;
         this.digits = digits;
         this.digest = digest;
-    };
+    }
 
-    generate_otp() {
+    /*＊
+     * When class HOTP or TOTP pass the input params to this
+     * function, it will generate the OTP object with params,
+     * the params may be counter or time. 
+     *
+     * @param {input}
+     * @type {int} 
+     * @desc input params to generate OTP object, maybe
+     * counter or time.
+     *
+     * @return {OTP}
+     */
+    generate_otp(input) {
         console.log("OTP.generate_otp");
-    };
+    }
  }