Add docs for methods

Author: 100gle

static/js/passed-state.js

@@ -5,6 +5,13 @@ class PassedState {
     _key = 'python-type-challenges';
     _state = null;
 
+    /**
+     * Initializing when there is no state in the local storage. If there is no state in the local storage, the initial state is required.
+     * this function will check the new state and the old state whether is undefined or not and updated the old state based on new state.
+     * 
+     * @param {object} initialState - the initial state of the challenges which grouped by the level.
+     * @returns void
+     */
     init(initialState) {
         const currentState = localStorage.getItem(this._key);
         // initialize the state when there is no state in the local storage.
@@ -19,10 +26,10 @@ class PassedState {
         const state = this._checkAndMerge(JSON.parse(currentState), rawState);
         this._save(state);
         this._state = state;
-        return
     }
     /**
      * prepare the state for initialization.
+     * 
      * @param {object} rawState 
      * @returns state - the state contains the challenge name and whether the challenge is passed.
      */
@@ -54,12 +61,17 @@ class PassedState {
         return this._state;
     }
 
+    /**
+     * Save the state to the local storage with JSON format.
+     * @param {object} state - the state contains the challenge name and whether the challenge is passed.
+     */
     _save(state) {
         localStorage.setItem(this._key, JSON.stringify(state));
     }
 
     /**
-     * Set the challenge as passed in the state.
+     * Set the target challenge as passed in the state.
+     * 
      * @param {'basic' | 'intermediate' | 'advanced' | 'extreme'} level - the level of the challenge.
      * @param {string} challengeName - the name of the challenge.
      * @returns void
@@ -81,10 +93,14 @@ class PassedState {
     }
 
     /**
-     * Merge the new state to the current state. this function will compare the new state with the current state and finally overwrite the current state based on the new state: 
-     * - If the old key in the current state is not in the new state, the old key will be removed from the current state. 
-     * - If the new key in the new state is not in the current state, the new key will be added to the current state.
-     * @param {object} newState 
+     * Merge the new state and the current state. 
+     * this function will compare the new state with the current state and finally overwrite the current state based on the new state: 
+     * - If the old key in the current state isn't in the new state, the old key will be removed from the current state. 
+     * - If the new key in the new state isn't in the current state, the new key will be added to the current state.
+     * 
+     * @param {object} oldState - the current state stored in the local storage.
+     * @param {object} newState - the latest state from the server.
+     * @returns mergedState - the merged state.
      */
     _checkAndMerge(oldState, newState) {
         if (!newState && !oldState) {