The following is an example of a FakeDataContext in JavaScript.

export class FakeDataContext {
   _identityCounter = 1;
   _dataSet = [];
 
   get DataSet() {
     return this._dataSet;
   }
 
   set DataSet(value) {
     this._dataSet = value;
   }
 
   get(predicate) {
     if (typeof(predicate) !== 'function') {
       throw new Error('Predicate must be a function');
     }
 
     const resultSet = this_dataSet.filter(predicate);
 
     return resultSet.length >= 1 ? {...resultSet[0]} : null;
   }
 
   getAll() {
     return this._dataSet.map((x) => {
       return {...x};
     });
   }
 
   save(item) {
     return item.id ? this.update(item) : this.create(item);
   }
 
   update(item) {
     if (!this._dataSet.some(x => x.id === item.id)) {
       this._dataSet.push({...item});
     } else {
       let itemIndex = this._dataSet.findIndex(x => x.id === item.id);
       this._dataSet[itemIndex] = {...item};
     }
 
     return {...item};
   }
 
   create(item) {
     let newItem = {...item};
     newItem.id = this._identityCounter++;
     this._dataSet.push({...newItem});
 
     return {...newItem};
   }
 }