• 字符串 ,数字 , 对象 , 数组的扩展


     字符串扩展
      
      1.是否有某个字符
    let str = 'http://127.0.0.1:3000/index.html';
    console.log(str.includes('index'));//true
    console.log(str.includes('logo'));//false 
      2.是否以某个字符开头
    let str = 'http://127.0.0.1:3000/index.html';
    console.log(str.startsWith('http://'));//true
    console.log(str.startsWith('login'));//false
    console.log(str.startsWith('http'));//true
    console.log(str.startsWith('h'));//true
      3.是否以某个字符结尾
    let str = 'http://127.0.0.1:3000/index.html';

    console.log(str.endsWith('.html'));//true
    console.log(str.endsWith('index.html'));//true 
      4.重复字符串
    console.log('#'.repeat(30));
    数值类型的扩展
     
      1.判断数字是否是有限大
      
    console.log(Number.isFinite(900000*90000000));//true
    console.log(Number.isFinite(1.7976931348623157e+309));//false
    console.log(Number.isFinite(Infinity));//false

      

      2.数字的最大值
     console.log(Number.MAX_VALUE);

      

      3.判断是否为整数
    console.log(Number.isInteger(0));//true
    console.log(Number.isInteger(0.2));//false

      

      4. 将字符串转换为整型
    console.log(Number.parseInt('123'))   //123
    console.log(Number.parseInt('123q'))  //123
    console.log(Number.parseInt('12a3'))  //12
     // 向下取整
    console.log(Math.trunc(12.8));//12
     
    对象的扩展
     
      
     let arr1 = [1,2,3];
     let arr2 = [1,2,3];
     console.log(Object.is(arr1,arr2));//false
     let obj1 = {name : 'E'};
     let obj2 = {name : 'E'};
     console.log(Object.is(obj1,obj2));//false
     let obj3 = obj1;
     console.log(Object.is(obj3,obj1));//true
     let str1 = 'asd';
     let str2 = 'asd';
     console.log(Object.is(str1,str2));//true
     console.log(Object.is(NaN,NaN));//true

      

     2.将一个对象的属性或者方法分配给另一个对象Object.Assign(newObj,oldObj)
          置两个参数  第一个参数是被分配的对象   第二个参数是原对象
          注意  如果说  被分配的对象有同名的key那么原来的数据会被覆盖 
          assign分配过去对象之后  新对象和原对象没有任何关系了
    let newObj = {};
    let oldObj = {};
    newObj.name = 'li';
    oldObj.name = 'zhang';
    Object.assign(newObj,oldObj);
    console.log(newObj);//{ name: 'zhang' }
    数组的扩展
      
      1.Arrr.from  将伪数组对像或者可遍历的对象转换为真数组
    let btns = document.getElementsByTagName('button');
    let qbtns=document.querySelectorAll('button');
    console.log(Object.prototype.toString.call(btns));//[object NodeList]
    console.log(Object.prototype.toString.call(qbtns));//[object NodeList]
    let arr = Array.from(btns);
    console.log(Object.prototype.toString.call(arr)); //[object Array]

      

      2.Array.of    将一系列的值转换为数组
    let arr = Array.of(1,3,5,7,9);
    console.log(arr);//[ 1, 3, 5, 7, 9 ]
    console.log(Object.prototype.toString.call(arr));//[object Array]

      

      3.find    找出第一个满足条件的元素
           find方法   查找数组中第一个为真的数据  filter
           filter是返回所有为真的元素  返回值是一个数组  哪怕是一个元素  他也是数组
    //let arr = ['Eric','Mary','Tom','Jerry','Jack'];
    //let result = arr.filter(element=>element.startsWith('M'));
    //let result1 = arr.filter(element=>element.endsWith('k'));
    //console.log(result);//[ 'Mary' ]
    //console.log(result1);//[ 'Jack' ]
    
    let result = arr.find(element=>element.startsWith('M'));
    let result1 = arr.find(element=>element.endsWith('k'));
    console.log(result);//Mary
    console.log(result1);//Jack
      4.findIndex    找出第一个满足条件元素的索引
    let arr = ['Eric','Mary','Tom','Jerry','Jack'];
    let result = arr.findIndex(element=>element.startsWith('J'));
    let result1 = arr.findIndex(element=>element.endsWith('y')); 
    console.log(result);//3
    console.log(result1);//1
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  • 原文地址:https://www.cnblogs.com/ximenchuifa/p/13629552.html
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