JavaScript简单语法

最新推荐文章于 2025-06-08 17:32:39 发布

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#javascript #string #object #delete

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本文深入探讨了JavaScript的基础语法，包括类型判断、函数默认返回值、数值精度表示、字符串对象的方法、常用方法应用、字符串操作、位运算、逻辑运算、全等与非全等比较等关键知识点。

JavaScript的简单语法知识：

//typeof：返回变量的类型
alert(typeof 45);
var undefine；
alert(typeof undefine);
//函数无明确返回值时，返回的为undefined
function testFunc(){}
alert(testFunc()==undefined);
//用指定的小数位数表示
var oNumberObject = new Number(68);
alert(oNumberObject.toFixed(2));
//String对象的valueOf方法和toString方法
var oStringObject = new String("hello");
alert(oStringObject.valueOf()==oStringObject.toString());
alert(oStringObject.length);
//String对象的一些常用方法
var oStr = new String("myString");
var Str = oStr.concat("hello");
alert(oStr.charAt(2)+" "+oStr.charCodeAt(2));
alert(Str);
alert(Str.indexOf("S")+" "+Str.lastIndexOf("S"));
//slice()和substring()方法
var str = new String("hello world");
alert(str.slice("3")+"<>"+str.substring("3")+"<>"+str.slice("3,7")+"<>"+str.substring("3,7"));
var str = new String("hello world");
alert(str.slice("-3")+"<>"+str.substring("-3"));
//instanceof运算符
var str = new String("hello world");
alert(str instanceof String);
//delete运算符
var o = new Object;
o.name ="hello";
alert(o.name);
delete o.name;
alert(o.name);
//位运算
//1,NOT
var iNum1 = 25;
var iNum2 = ~iNum1;
var num1 = 25;
var num2 = -num1-1;
alert(iNum2+" "+num2);
//2,AND
var num = 25 & 3;
alert(num);
//3,OR
var num = 25 | 3;
alert(num);
//4,XOR
var num = 25^3;
alert(num);
//5，左移<<
var num = 2;
var inum =num<<3;
alert(inum);
//6,有符号右移>>
var num = 64;
var inum = num>>4;
alert(inum);
//7，无符号右移>>>
var num = 64;
var inum = num >>>3;
alert(inum);
//逻辑运算
//1,NOT !
var ss = false;
var num = 0;
var object = new Object;
alert(!num+" "+!object);
//2,AND &&
var aa = false;
var bb = (aa && mm);
alert(bb);
//3,OR ||
var aa = true;
var bb = false;
alert(aa||bb);

//全等和非全等号
var aa = 55;
var bb = "55";
alert(aa==bb);
alert(aa===bb);

alert(typeof 45);
var undefine；
alert(typeof undefine);
//函数无明确返回值时，返回的为undefined
function testFunc(){}
alert(testFunc()==undefined);
//用指定的小数位数表示
var oNumberObject = new Number(68);
alert(oNumberObject.toFixed(2));
//String对象的valueOf方法和toString方法
var oStringObject = new String("hello");
alert(oStringObject.valueOf()==oStringObject.toString());
alert(oStringObject.length);
//String对象的一些常用方法
var oStr = new String("myString");
var Str = oStr.concat("hello");
alert(oStr.charAt(2)+" "+oStr.charCodeAt(2));
alert(Str);
alert(Str.indexOf("S")+" "+Str.lastIndexOf("S"));
//slice()和substring()方法
var str = new String("hello world");
alert(str.slice("3")+"<>"+str.substring("3")+"<>"+str.slice("3,7")+"<>"+str.substring("3,7"));
var str = new String("hello world");
alert(str.slice("-3")+"<>"+str.substring("-3"));
//instanceof运算符
var str = new String("hello world");
alert(str instanceof String);
//delete运算符
var o = new Object;
o.name ="hello";
alert(o.name);
delete o.name;
alert(o.name);
//位运算
//1,NOT
var iNum1 = 25;
var iNum2 = ~iNum1;
var num1 = 25;
var num2 = -num1-1;
alert(iNum2+" "+num2);
//2,AND
var num = 25 & 3;
alert(num);
//3,OR
var num = 25 | 3;
alert(num);
//4,XOR
var num = 25^3;
alert(num);
//5，左移<<
var num = 2;
var inum =num<<3;
alert(inum);
//6,有符号右移>>
var num = 64;
var inum = num>>4;
alert(inum);
//7，无符号右移>>>
var num = 64;
var inum = num >>>3;
alert(inum);
//逻辑运算
//1,NOT !
var ss = false;
var num = 0;
var object = new Object;
alert(!num+" "+!object);
//2,AND &&
var aa = false;
var bb = (aa && mm);
alert(bb);
//3,OR ||
var aa = true;
var bb = false;
alert(aa||bb);

//全等和非全等号
var aa = 55;
var bb = "55";
alert(aa==bb);
alert(aa===bb);