js数组方法的总结

摘要：例如会删除当前数组位置的项，然后再从位置开始插入字符串。对数组中的每一项运行给定函数，返回该函数会返回的项组成的数组。则从数组的最后一项开始，向前遍历到第一项。

1、队尾插入push

   var colors = ["red","green"];
   colors.push("black"):
   console.log(colors); //["red","green","black"]

2、队尾删除并返回删除的最后一项pop

   var colors = ["red","green","black"];
   var item = colors.pop();
   console.log(item); //"black"

3、队首插入unshift()

   var colors = ["red","green"];
   colors.unshift("black");
   var item = colors.pop();
   console.log(item); //"black"

4、队首删除shift()

   var colors = ["red","green","black"];
   colors.shift();
   console.log(colors); //["green","black"]

5、数组一添加数组二concat()

   var colors = ["red","green","black"];
   var colors2 = colors.concat("yellow",["blue","brown"]);
   console.log(colors); //["red","green","black"]
   console.log(colors2); //["red","green","black","yellow","blue","brown"]

6、数组的截取slice()
只传一个参数：从数组这个参数的下标开始截取一直到数组结束。

   var colors = ["red","green","black"];
   colors.slice(1); //["green","black"]
   console.log(colors); //["red","green","black"]

传两个参数：第一个是截取开始的位置，第二个是截取结束的位置

   var colors = ["red","green","black","yellow","blue","brown"];
   colors.slice(1,3)//从位置1开始，到位置2结束["green","black"];

7、数组的splice()方法

   有三种用法：

删除：可以删除任意数量的项，只需指定两个参数：第一个参数为删除开始的位置，第二个参数为删除项数。

插入：可以向指定位置插入任意数量的项，只需提供3个参数：起始位置、0（要删除的项数）和要插入的项。例如：splice（2,0,"red","green")，会从当前数组的位置2开始插入字符串"red"和"green"。

替换: 可以向指定位置插入任意数量的项，且同事删除任意数量的项，只需提供3个参数：起始位置、要删除的项数和要插入的任意数量的项。插入的项数不必与删除项数相等。例如：splice (2,1,"red","green") 会删除当前数组位置2的项，然后再从位置2开始插入字符串。

   var colors = ["red","green","black"];
   var removed = colors.splice(0,1);
   console.log(colors); //["green","black"]
   console.log(removed); //["red"]
    
   removed = colors.splice(1,0,"yellow","orange");
   console.log(colors); //["green","yellow","orange","black"]
   console.log(removed); //[]

   removed = colors.splice(1,1,"red","purple");
   console.log(colors); //["green","red","purple","orange","black"]
   console.log(removed); //["yellow"]

8、位置方法indexOf()和lastIndexOf()

indexOf()和lastIndexOf()都接收两个参数，第一个参数是要查找的项，第二个（可选）查找开始的位置，indexOf()是从数组头开始查，lastIndexOf()是从数组尾开始查找。

   var numbers = [1,2,3,4,5,4,3,2,1];

   console.log(numbers.indexOf(4)); //3
   console.log(numbers.lastIndexOf(4)); //5
   
   console.log(numbers.indexOf(4,4)); //5
   
   var person = {name: "vivi"};
   var people = [{name: "vivi"}];
   
   var morePeople = [person];
   console.log(people.indexOf(person)); //-1
   console.log(morePeople.indexOf(person)); //0

9、查找find()方法
查找符合条件的第一项

   var inventory = [
    {name: "apples", quantity: 2},
    {name: "bananas", quantity: 0},
    {name: "cherries", quantity: 5}
   ];
   const inventorItem = inventory.find((item) => item.name === "apples");
   console.log(inventorItem); //{name: "apples", quantity: 2}

10、迭代方法
传入这些方法中的函数会接收三个参数：数组项的值、该项在数组中的位置和数组对象本身。

every(): 对数组中的每一项运行给定函数，如果该函数对每一项都返回true，则返回true。

filter(): 对数组中的每一项运行给定函数，返回该函数会返回true的项组成的数组。

forEach(): 对数组中的每一项运行给定函数，这个方法没有返回值。

map(): 对数组中的每一项运行给定函数，返回每次函数调用的结果组成的数组。

some(): 对数组中的每一项运行给定函数，如果该函数对任一项返回true，则返回true。

   var numbers = [1,2,3,4,5,4,3,2,1];
   var everyResult = numbers.every((item, index, array) => {
        return item > 2;
   });
   console.log(everyResult); //false
   
   var someResult = numbers.some((item, index, array) => {
       return  item > 2;
   });
   console.log(someResult); //true

  var numbers = [1,2,3,4,5,4,3,2,1];
  var filterResult = numbers.filter((item, index, array) => {
    return item > 2;
  });
  console.log(filterResult); //[3,4,5,4,3]

   var numbers = [1,2,3,4,5,4,3,2,1];
   var mapResult = numbers.map((item, index, array) => {
    return item * 2;
  });
  console.log(mapResult); //[2,4,6,8,10,8,6,4,2]

   var numbers = [1,2,3,4,5,4,3,2,1];
   numbers.forEach((item, index, array) => {
       //执行某些操作
   });

11、归并方法reduce()和reduceRight()

这两个方法都会迭代数组的所有项，然后构建一个最终返回的值。
reduce（）方法从数组的第一项开始，逐个遍历到最后。                                  
reduceRight（）则从数组的最后一项开始，向前遍历到第一项。

   var values = [1,2,3,4,5];
   var sum = values.reduce((prev, cur, index, array) => {
       return prev + cur;
   });
   console.log(sum); //15

   var values = [1,2,3,4,5];
   var sum = values.reduceRight((prev, cur, index, array) => 
   {
       return prev + cur;
   });
   console.log(sum); //15