6.arr.js

// 数组的常见方法
// map (some,every,filter,forEach) es5
// find findIndex es6
// reduce 收敛 叠加 
// for of();

// 1) reduce 返回的结果是叠加后的结果
// 函数的返回结果会作为下一次循环的prev
let result = [1, 2, 3, 4, 5].reduce((prev, next, currIndex, ary) => {
  console.log(prev, next, currIndex, ary)
  if (currIndex === ary.length - 1) {
    return (prev + next) / ary.length
  }
  return prev + next;
});
console.log(result);


// let total = [{ price: 10 }, { price: 20 }, { price: 30 }].reduce((prev, next, currIndex, ary) => {
//   return prev + next.price
// },0)
// console.log(total);


Array.prototype.myReduce = function (fn, prev) {
  for (let i = 0; i < this.length; i++) {
    if (typeof prev === 'undefined') {
      prev = fn(this[i], this[i + 1], i + 1, this);
      ++i; // 保证下次取值时是正确的结果
    } else {
      prev = fn(prev, this[i], i, this);
    }
  }
  return prev;
}
let total = [1, 2, 3].myReduce((prev, next, currIndex, ary) => {
  return prev + next
}, 0);
console.log(total);

let flat = [[1, 2, 3], [4, 5, 6]].reduce((prev, next, index, ary) => {
  return [...prev, ...next];
});
console.log(flat);

// 2) forEach
Array.prototype.forEach = function (fn) {
  for (let i = 0; i < this.length; i++) {
    fn(this[i], i);
  }
}
  ;[1, 2, 3].forEach((item, index) => {
    console.log(item, index);
  })

// 3.map返回值 返回值是一个新数组
Array.prototype.map = function (fn) {
  let arr = [];
  for (let i = 0; i < this.length; i++) {
    arr.push(fn(this[i], i));
  }
  return arr;
};
let arr = [1, 2, 3].map(item => {
  return item * 2;
});
console.log(arr);

// 3)filter 过滤 如果返回true表示留下 返回false表示删除
let arr = [1, 2, 3];
let filterArr = arr.filter(item => {
  return item > 2;
});
console.log(filterArr);

// 4)find 查找 他会返回查找的那一项,没有返回undefined 找到后就不会继续查找了
let f = [1, 2, 3].find(item => {
  return item === 5;
});
console.log(f);

// 5)some找到后返回true,找false可以用every 
let r = [2, 1, 3].every(item => {
  console.log(item);
  return item === 1;
});
console.log(r);

// 6) includes
console.log([1, 2, 3].includes(3));


// Array.from();


// 希望将类数组转化成数组
// 常见的类数组有 htmlCollection arguments {0:1,1:2,2:3,length:3}
function a(){
  console.log(eval(Array.from(arguments).join('+')))
}
a(1,2,3);

// of()
let ary = Array.of(3);
console.log(ary)// [3]; 
// copywithin