const originalData = [
  { name: 'jch', age: 30, score: 90, sex: 1, lesson: 'math' },
  { name: 'oh', age: 31, score: 80, sex: 1, lesson: 'math' }, 
  { name: 'jia', age: 27, score: 70, sex: 0, lesson: 'math' }, 
  { name: 'jch', age: 30, score: 80, sex: 1, lesson: 'english' }
];

function findAgeGreaterThan(age) {
  return originalData.filter((obj) => obj['age'] > age);
}

// Answer for #1.1.1
findAgeGreaterThan(28);

function namesAndLessons() {
  let data = findAgeGreaterThan(28);
  let result = {};
  result['name'] = toDistinctArray(data.map((obj) => obj['name']));
  result['lesson'] = toDistinctArray(data.map((obj) => obj['lesson']));
  return result;
}

// Answer for #1.1.2
namesAndLessons()

function toDistinctArray(arr){
  return Array.from(new Set(arr));
}

function groupBy(data, reduceFunc, groupByProperties){
  let groupByResult = {};
  groupByProperties = Array.from(groupByProperties);
  let property = groupByProperties.shift();
  if(property != undefined){
    data.forEach(function(item){
      if(item.hasOwnProperty(property)){
        let propertyValue = item[property];
        if(!groupByResult.hasOwnProperty(propertyValue)){
          groupByResult[propertyValue] = [];
        }
        groupByResult[propertyValue].push(item);
      }
    });
    
    for(let propertyValue in groupByResult){
      var data = groupByResult[propertyValue];
      groupByResult[propertyValue] = groupBy(data, reduceFunc, groupByProperties);
    }
    return groupByResult;
  } else {
    return data.reduce(reduceFunc, 0);
  }
}

let averageReduceFunc = function(previousVal, currentVal, index, array) {
  if(index == array.length - 1) {
    return (previousVal + currentVal.score) / array.length;
  } else {
    return previousVal + currentVal.score;
  }
} 

// Answer for #1.1.3
groupBy(originalData, averageReduceFunc, ['lesson', 'sex'])

// Reference #1: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce