JS学习-JavaScript 基本语法

Posted 2022-10-22 znapast

1

关键字var用于定义局部变量，去掉关键字var则可以定义全局变量.

//局部变量1
var message = “hi”,
    found = false,
    age = 29;

//局部变量2
function test()
    var message = “hi”; //local variable

test();
alert(message); //error!

//全局变量
function test()
    message = “hi”; //global variable

test();
alert(message); //”hi”

2

JS有5种基本数据类型和一种复杂数据类型： Undefined, Null,Boolean, Number, 和 String，以及Object。使用typeof操作符可以得到变量的数据类型：

• “undefined” if the value is undefined
• “boolean” if the value is a Boolean
• “string” if the value is a string
• “number” if the value is a number
• “object” if the value is an object (other than a function) or null
• “function” if the value is a function

var message = “some string”;
alert(typeof message); //”string”
alert(typeof(message)); //”string”
alert(typeof 95); //”number”

var message;
alert(message == undefined); //true

var car = null;
alert(typeof car); //”object”

//undefined 类型继承自 null类型
alert(null == undefined); //true

var message = “Hello world!”;
var messageAsBoolean = Boolean(message);//true
var b1 = Boolean("")//false
var b2 = Boolean(0)//false
var b3 = Boolean(NaN)//false
var b4 = Boolean(1)//true
var b5 = Boolean(-1)//true
var b6 = Boolean(null)//false
var b7 = Boolean(undefined)//false
var b8 = Boolean(n/a);//true
//NaN不等于任何对象，包括NaN
alert(NaN == NaN); //false

var num1 = Number(“Hello world!”); //NaN
var num2 = Number(“”); //0
var num3 = Number(“000011”); //11
var num4 = Number(true); //1
var num1 = parseInt(”AF”, 16); //175
var num2 = parseInt(”AF”); //NaN

var num1 = parseFloat(“1234blue”); //1234 - integer
//十六进制数都会变为0
var num2 = parseFloat(“0xA”); //0
var num3 = parseFloat(“22.5”); //22.5
var num4 = parseFloat(“22.34.5”); //22.34
var num5 = parseFloat(“0908.5”); //908.5
var num6 = parseFloat(“3.125e7”); //31250000

var num = 10;
alert(num.toString()); //”10”
alert(num.toString(2)); //”1010”
alert(num.toString(8)); //”12”
alert(num.toString(10)); //”10”
alert(num.toString(16)); //”a”

3

Object对象实例有以下属性和方法：

• constructor ：构造方法
• hasOwnProperty(propertyName) ： 使用有指定的属性
• isProtypeOf(object)： 是否是某个实例对象的原型
• propertyIsEnumerable(propertyName)： 属性是否可枚举
• toLocaleString()： 转换为表示该对象的字符串
• valueOf()： 得到代表该对象的字符串或者数字或者布尔值

对象比较”==” 和“===”

var result1 = (“55” == 55); //true - equal because of conversion
var result2 = (“55” === 55); //false - not equal because different data types

var result1 = (“55” != 55); //false - equal because of conversion
var result2 = (“55” !== 55); //true - not equal because different data types

逗号操作符

var num1=1, num2=2, num3=3;
var num = (5, 1, 4, 8, 0); //num becomes 0

Javascript中没有块作用域的概念，在块内定义的变量在块之外同样有效，例如：

var count = 10;
for (var i=0; i < count; i++)
alert(i);

//i同样的有效
alert(i); //10

for-in 表达式用于遍历对象中的属性。如下实例遍历window 对象中的所有属性

for (var propName in window) 
    document.write(propName);