javascript基本对象

1 String对象

创建对象

var string1 = new String("Hello");
 
var string2 = "Hello"      //也可以创建一个基本的字符串，js后台会自动把其转换为String对象

1.1 length()

长度属性

var str = "Hello,world!";
var tmp = str.length;   //12

1.2 indexOf与lastIndexOf

根据元素找索引

var myString = new String("hello world hello")
 
document.write(myString.indexOf('hello'))   //0，从前面开始匹配，匹配到后就不继续查找了
document.write(myString.indexOf('world'))   //6
document.write(myString.indexOf('123'))     //-1
 
document.write(myString.lastIndexOf('hello'))   //12，从后面开始匹配，匹配到后就不继续查找了
document.write(myString.lastIndexOf('world'))   //6
document.write(myString.lastIndexOf('123'))   //-1

练手统计字符串

<script>
    originString = 'Python is an to learn, powerful programming language. It has efficient high-level data structures and a simple but effective approach to object-oriented programming. Python’s elegant syntax and dynamic typing, together with its interpreted nature, make it an ideal language for scripting and rapid application development in many areas on most platforms.'
    var foundAtPosition = 0
    var targetCount = 0
    while (foundAtPosition != -1){
        foundAtPosition = originString.indexOf("Python",foundAtPosition)
 
        if(foundAtPosition != -1){
            targetCount++
            foundAtPosition++  //防止死循环
        }
    }
    document.write(targetCount)
</script>

1.3 substr()

复制字符串的一个子串，类似于py的切片

var str = "Hello,world!";
var tmp = str.substr(0,4);   //Hell

//获取当前网页文件名
var fileName = window.location.href;
 
fileName = fileName.substr(fileName.lastIndexOf("/") + 1);
document.write(fileName);

1.4 toLowerCase与toUpperCase

大小写转换

var str = "Hello,world!";
var tmp = str.toLowerCase();
var tmp = str.toUpperCase();

1.5 charCodeAt

字符串转为字符编码

var tmp ="A".charCodeAt(0);

1.6 fromCharCode

字符编码转为字符串

//它是一个静态方法，不需要创建对象就能使用
var tmp = String.fromCharCode(65, 66, 67);

1.7 trim

去掉两端空格

var myString = prompt("Enter some text", "Hello,world!");
var tmp = myString.trim();

2 Array对象

2.1 length

var names = [];
names[0] = "morra";
names[1] = "blue";
 
names[11] = "jack";
 
var tmp = names.length
document.write(tmp)  //12
//["morra", "blue", undefined × 9, "jack"]

2.2 push()

添加元素

使用push的时候无需指定索引

var names = ["123"];
names.push("hello1")
names.push("hello2")
 
document.write(names) //123,hello1,hello2

2.3 concat()

连接数组

var names = ["a","b","c"];
var ages = ["1","2","3"];
 
var tmp = names.concat(ages);  //["a", "b", "c", "1", "2", "3"]

2.4 slice()

切片

var names = ["a","b","c","d","e","f"];
var tmp = names.slice(1,4);
 
console.log(names)  //初始数组对象不变，["a", "b", "c", "d", "e", "f"]
console.log(tmp);  //["b", "c", "d"]

2.5 join

连接

var names = ["a", "b", "c", "d", "e", "f"];
var tmp = names.join("_")
 
console.log(tmp) //a_b_c_d_e_f

2.6 sort()

排序
排序是分大小写的
js保存的是字符串对应的unicode编码，所以排序是基于unicode编码而不是实际字母

2.7 reverse()

翻转

var names = ["a", "b", "c", "d", "e", "f"];
names.reverse();
 
console.log(names) //["f", "e", "d", "c", "b", "a"]

2.8 indexOf与lastIndexOf

根据元素找索引，没有找到目标则返回-1

var names = ["a", "b", "c", "d", "c", "f"];
var tmp1 = names.indexOf("c")
var tmp2 = names.lastIndexOf("c")
 
console.log(tmp1) //2
console.log(tmp2) //4

2.9 every()与some()

every()：判断可迭代对象中的所有元素是否全部满足目标条件
some()：判断可迭代对象中的所有为元素是否全部满足目标条件

every()与some()都返回布尔值

var numbers = [1, 2, 3, 4, 5];
 
function isLessThan(value, index, array) {
    var ret = false;
    if (value < 3) {
        ret = true;
    }
    return ret
}
 
alert(numbers.every(isLessThan));

2.10 filter()

filter()返回的是符合条件的元素组成的数组。

var numbers = [1, 2, 3, 4, 5];
 
function isLessThan(value, index, array) {
    var ret = false;
    if (value < 3) {
        ret = true;
    }
    return ret
}
 
console.log(numbers.filter(isLessThan)) //[1, 2]

2.11 forEach()和map()

都是对可迭代对象中的元素进行批量操作，但是forEach()是没有返回值的，因此只能操作但是不自动保存修改后的数据。

//forEach
var numbers = [1, 2, 3, 4, 5];
 
function double(value, index, array) {
    var ret = value * 2;
    console.log(ret)
}
numbers.forEach(double)

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

3 Math对象

Math对象提供了很多数学函数和数值操作的方法。Math对象之前不需要实例化，js会自动创建。所以在使用时直接用就好了。例如：

var myNumber = -101;
console.log(Math.abs(myNumber))

方法	注释
abs()	绝对值
min()	最小值
max()	最大值
ceil()	向上修整到最接近的最小整数，例：10.01 > 11 ， -9.99 > -9
floor()	向下修整到最接近的最小整数，例：10.01 > 10 ， -9.99 > -10
round()	四舍五入
pow()	指数
random()	返回一个0~1之间的随机浮点数

pow()
指数

console.log(Math.pow(2,3))   //2^3

random()
返回一个0~1之间的随机浮点数

console.log(Math.random())  //返回 0<=x<1 之间的随机浮点数
 
console.log(Math.random()*6)  //返回 0<=x<6 之间的随机浮点数
 
console.log(Math.floor(Math.random()*6))  //返回 0<=x<6 之间的随机整数
 
console.log(Math.floor(Math.random()*6)+1)  //返回 1<=x<7 之间的随机整数

4 Number

创建对象与String类似

var myNumber1 = new Number(123);
var myNumber2 = 123;   
 
//切记下面的这种写法js会把其识别为String对象而不是Number对象
var myNumber2 = "123";

4.1 toFixed()

四舍五入，可自定义保留几位

var myNumber1 = new Number(123.4567);
 
console.log(myNumber1.toFixed(1));  //123.5
console.log(myNumber1.toFixed(2));  //123.46
console.log(myNumber1.toFixed(3));  //123.457

5 自定义对象