js数组篇2----ECMASCript5为数组定义了5个迭代方法和归并方法reduce()和reduceRght()

let sum = [0, 1, 2, 3].reduce(function (accumulator, currentValue) {
  return accumulator + currentValue;
}, 0); // sum 6

//箭头函数的形式：
let total = [ 0, 1, 2, 3 ].reduce(
  ( acc, cur ) => acc + cur,
  0
);

let  initialValue = 0;
let sum = [{x: 1}, {x:2}, {x:3}].reduce(function (accumulator, currentValue) {
    return accumulator + currentValue.x;
},initialValue)

console.log(sum) // logs 6

//你也可以写成箭头函数的形式：
let initialValue = 0;
let sum = [{x: 1}, {x:2}, {x:3}].reduce(
    (accumulator, currentValue) => accumulator + currentValue.x
    ,initialValue
);

console.log(sum) // logs 6

let flattened = [[0, 1], [2, 3], [4, 5]].reduce(
  function(a, b) {
    return a.concat(b);
  },
  []
);
// flattened is [0, 1, 2, 3, 4, 5]
//你也可以写成箭头函数的形式：
let flattened = [[0, 1], [2, 3], [4, 5]].reduce(
 ( acc, cur ) => acc.concat(cur),
 []
);

//计算数组中每个元素出现的次数
let names = [‘Alice‘, ‘Bob‘, ‘Tiff‘, ‘Bruce‘, ‘Alice‘];
let countedNames = names.reduce(function (allNames, name) { 
  if (name in allNames) {
    allNames[name]++;
  }
  else {
    allNames[name] = 1;
  }
  return allNames;
}, {});
//countedNames is:
// { ‘Alice‘: 2, ‘Bob‘: 1, ‘Tiff‘: 1, ‘Bruce‘: 1 }

let people = [
  { name: ‘Alice‘, age: 21 },
  { name: ‘Max‘, age: 20 },
  { name: ‘Jane‘, age: 20 }
];

function groupBy(objectArray, property) {
  return objectArray.reduce(function (acc, obj) {
      console.log(acc, obj)
    var key = obj[property];
    if (!acc[key]) {
      acc[key] = [];
    }
    acc[key].push(obj);
    return acc;
  }, {});
}

var groupedPeople = groupBy(people, ‘age‘);

------->result::
groupedPeople is:
{ 
  20: [
    { name: ‘Max‘, age: 20 }, 
    { name: ‘Jane‘, age: 20 }
  ], 
  21: [{ name: ‘Alice‘, age: 21 }] 
}