1. 关于数组的一些认识

  1. 使用构造器函数创建数组时:
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    let ary = new Array();
  • 若参数只有一个，且为数字，let ary1 = new Array(3)，这是其实我们是在指定数组的长度的，即 ary1.length === 3。ary1[0]则为undefined;
  • 当定义let ary2 = new Array(1, 2, 3)时，此时ary2 === [1, 2, 3];
  1. JS中的数组定义非常的自由：
    看下面的例子：
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    let ary = [1, 2, 3];
    ary[5] = 5;
    console.log(ary);          // [1,2,3,undefined,undefined,5]
    console.log(ary.length);   // 6
    再看：
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    let ary2 = [1, 2, 3, 4, 5];
    ary2.length = 3;
    console.log(ary2);      // [1, 2, 3]

    2. 常用的数组API

  2. 万能方法 splice()
  • splice()方法允许我们对数组进行插入、替换和删除的功能。splice方法返回一个有删除元素组成的新数组，没有删除时则返回一个空数组，简直完美呀！
  • splice()方法接受三个参数，第一个为开始索引，第二个为删除元素的位置，第三个为插入的元素，可以为第二个，当然也可省略（表示删除元素）。
  • splice()方法会修改原数组！
  • 通过以下三个demo了解一下具体的用法吧：
    插入: 
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    let ary1 = ["first", "second", "third", "forth", "fifth"];
    let ary2 = ary1.splice(1,0,"add1");
    console.log(ary1);   // [ 'first', 'add1', 'second', 'third', 'forth', 'fifth' ]
    console.log(ary2);   // [] 没有删除则放回一个空数组
    替换： 
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    let ary1 = ["first", "second", "third", "forth", "fifth"];
    let ary2 = ary1.splice(1,1,"replace");
    console.log(ary1);   // [ 'first', "replace, 'third', 'forth', 'fifth' ]
    console.log(ary2);   // ["second"] 返回被删除(即替换)的数组
    删除： 
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    let ary1 = ["first", "second", "third", "forth", "fifth"];
    let ary2 = ary1.splice(1,3);
    console.log(ary1);   // ["second", "third", "forth"]
    console.log(ary2);   // ["first", "fifth"]
  1. slice() 方法
    • 可以接受两个参数(start, end)；
    • 可从已有的数组中返回选定的元素；
    • 可提取字符串的某个部分，并以新的字符串返回被提取的部分；
    • 不会改变原始数组，而是返回一个新数组。
      demo1:
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      let ary = ["first", "second", "third", "forth", "fifth"];
      console.log (ary.slice(1,2) );   // ["second"]
      let ary1 = ary.slice(1,2);       // ["second"]
      let ary2 = ary.slice(1,3);       // ["second", "third"]
      console.log(ary);                // ["first", "second", "third", "forth", "fifth"];
      demo2:
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      let ary = ["first", "second", "third", "forth", "fifth"];
      console.log(ary.slice(1,2));  // ["second"]
      console.log(ary.slice(1,3));  // ["second", "third"]
      console.log(ary));            // ["first", "second", "third", "forth", "fifth"];
      demo3:
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      let ary = ["first", "second", "third", "forth", "fifth"];
      let ary1 = ary.slice();    // ["first", "second", "third", "forth", "fifth"];
      let ary2 = ary.slice(0);   // ["first", "second", "third", "forth", "fifth"];

      ary === ary1;       // false
      ary === ary2;       // false
      ary1 === ary2;      // false
      看出来啥猫腻没，这不是深复制一个数据吗！