Edit models/challenge.js Typo

 const mongoose = require("mongoose");
 
-const { Schema }=mongoose;
+const { Schema } = mongoose;
 
-const GroupSchema=new Schema({
-    members:{type:[String]},
+const GroupSchema = new Schema({
+  members: { type: [String] },
 });
 
-const ChallengeSchema=new Schema({
-    challengeName: {type: String, required: true},
-    startDate: {type: Object, required: true},
-    endDate: {type: Object, required: true},
-    durationPerSession: {type: String, required: true}, // '1d' means one day per session, '2w' means 2 weeks per session, '3m' means 3 months per session.
-    goalPerSession: {type: Number, required:true}, // number of problems for one session
-    groups:{type: [GroupSchema], required:true}, // groups attending challenge, group of only one member supposed to be single
+const ChallengeSchema = new Schema({
+  challengeName: { type: String, required: true },
+  startDate: { type: Object, required: true },
+  endDate: { type: Object, required: true },
+  durationPerSession: { type: String, required: true }, // '1d' means one day per session, '2w' means 2 weeks per session, '3m' means 3 months per session.
+  goalPerSession: { type: Number, required: true }, // number of problems for one session
+  groups: { type: [GroupSchema], required: true }, // groups attending challenge, group of only one member supposed to be single
 });
 
-ChallengeSchema.statics.findByChallengeName=function(challengeName){
-    return this.findOne({challengeName:challengeName});
-}
+ChallengeSchema.statics.findByChallengeName = function (challengeName) {
+  return this.findOne({ challengeName: challengeName });
+};
 
-ChallengeSchema.methods.addNewGroup=function(group){
-    this.groups.push(group);
-    return this.save();
-}
+ChallengeSchema.methods.addNewGroup = function (group) {
+  this.groups.push(group);
+  return this.save();
+};
 
-ChallengeSchema.methods.removeGroup=function(group_id){
-    const idx=this.groups.findIndex((item)=>item._id===group_id);
-    this.groups.splice(idx,1);
-    return this.save();
-}
+ChallengeSchema.methods.removeGroup = function (group_id) {
+  const idx = this.groups.findIndex((item) => item._id === group_id);
+  this.groups.splice(idx, 1);
+  return this.save();
+};
 
-ChallengeSchema.methods.getChallengeName=function(){
-    return this.challengeName;
-}
+ChallengeSchema.methods.getChallengeName = function () {
+  return this.challengeName;
+};
 
-ChallengeSchema.methods.getStartDate=function(){
-    return this.startDate;
-}
+ChallengeSchema.methods.getStartDate = function () {
+  return this.startDate;
+};
 
-ChallengeSchema.methods.getEndDate=function(){
-    return this.endDate;
-}
+ChallengeSchema.methods.getEndDate = function () {
+  return this.endDate;
+};
 
-ChallengeSchema.method.getDurationPerSession=function(){
-    return this.durationPerSession;
-}
+ChallengeSchema.methods.getDurationPerSession = function () {
+  return this.durationPerSession;
+};
 
-ChallengeSchema.methods.getGoalPerSession=function(){
-    return this.goalPerSession;
-}
+ChallengeSchema.methods.getGoalPerSession = function () {
+  return this.goalPerSession;
+};
 
-ChallengeSchema.methods.getGroups=function(){
-    return this.groups;
-}
+ChallengeSchema.methods.getGroups = function () {
+  return this.groups;
+};
 
-ChallengeSchema.methods.serialize=function(){
-    return this.toJSON();
-}
+ChallengeSchema.methods.serialize = function () {
+  return this.toJSON();
+};
 
-const Challenge=mongoose.model('Challenge',ChallengeSchema);
-module.exports=Challenge;
\ No newline at end of file
+const Challenge = mongoose.model("Challenge", ChallengeSchema);
+module.exports = Challenge;