1. JavaScript Introduction:
JavaScript is the most popular scripting language on the Internet, and all modern HTML uses JavaScript. Since it is a scripting language, it has three characteristics:
(1) Weak type;
(2) Interpreted type Language (no compilation required);
(3) Line by line execution, if one line of code is wrong, subsequent code blocks will not continue to execute;
(4) The
console.log("hello,javascript")
(3) document.write() : Write the content directly in the HTML page, for example:
document.write("hello,javascript")
4. JavaScript variable: ## Unlike Java, ECMAScript The variables in have no specific type. When defining a variable, only use the var operator. It can be initialized to any value. The initialization format of the variable: var variable name = variable value; example:
var a = "hello";
var b = 123;
If you want to define multiple variables, you can write multiple variables on one line and separate them with commas; example:
var a = "你好", b = 123, c = "hello";
consists of letters, numbers, underscores, and $ symbols
(2) It cannot start with a number, and it is not recommended to start with an underscore;
(3 ) Strictly case-sensitive; (4) Cannot be keywords and reserved words 5. JavaScript data type:JavaScript can be divided into two types: primitive data types and reference data types:
(1) Primitive data types: Number, String, Boolean, undefined, null
Number
: Numeric type, is a number, including positive numbers, negative numbers, integers, decimals, 0, NaN, Infinity (positive infinity), -Infinity (negative infinity); Note:
NaN: abbreviation of not a number, indicating that the value is not a numerical value (also belongs to Number) String:字符串:用双引号""或单引号''包起来的0个或多个字符,如果引号中什么也没有,那么这个字符串被称为空字符串 Boolean:布尔型:包含true:表示真(成立)和false:表示假(不成立)两个值 undefined:表示变量未定义,或变量被定义出来,但是没有被赋值 null:表示一个变量没有指向任何一片存储空间,即变量存在,但是里面是空的,类似于Undefined (小提示:在Chrome浏览器控制台输出时,会发现Number类型为深蓝色,String为黑色,Boolean为浅蓝色,undefined和null都为浅灰色) (2)引用数据类型: Object(对象),Array(数组),Date(日期),RegExp(正则)。。等等 (3)如何查看一个变量的数据类型(typeof 运算符): 数值型数据:返回值为number 字符串型数据:返回值为string 布尔型数据:返回值为boolean Undefined:返回值为undefined Null:返回值为Object(历史遗留问题,说明null也是一个对象) NaN:返回值为number 6.JavaScript 数据类型的转换: (1)在使用加法(+)运算符时,任何数据与字符串类型数据相加都为字符串类型数据; 注(简单理解): 在JavaScript 中空字符串""转换为false,非空字符串转换为true(除“0”,“1”外); false转换为 0 或“0”,true转换为 1 或“1”; 做逻辑判断的时候,null,undefined,""(空字符串),0,NaN都默认为false; ==在比较的时候可以转换数据类型,===是严格比较,只要类型不匹配就返回false; 其实 (2)parseInt:将字符串转换成整数(只识别字符串中的数值): 注:会忽略字符串中的前后空格(当数值后的空格后面还有数值时,将不会再识别); 能够正确识别正负号,即保留正负号; 在转换时,遇到非数值型的字符就会停止转换; 如果字符串的第一个字符是非数值型的,那么转换的结果为NaN; (3)parseFloat:将字符串转换成小数(识别小数点,注意事项同上) (4)Number:将其它类型的数据转换成数值型,注意被转换的数据必须是纯数值构成,否则无法转换,其它注意事项同上 (5)页面中的信息框: alert(),弹出个提示框,只有确定; confirm(),弹出个确认框,有确定和取消; prompt(),弹出个输入框,可以输入内容; 本文讲解了JavaScript相关的内容讲解,更多相关之请关注php中文网。 相关推荐:console.log(typeof 123) //输出number
console.log(typeof "你好") //输出string
console.log(typeof true/false) //输出boolean
console.log(typeof undefined) //输出undefined
console.log(typeof null) //输出object
console.log(typeof NaN) //输出number
console.log("你好" + 123) //输出"你好123"
==
的比较确实是转换成字符串来比较但,但是在布尔型转换为字符串之前,要先转换成 Number
console.log("123" == true) //输出false
console.log("1" == true) //输出true
console.log("" == true) //输出false
console.log(1 == true) //输出true
console.log("" == false) //输出true
console.log('123' == false) //输出fasle
console.log('0' == false) //输出true
console.log(0 == false) //输出true
console.log('1' == 1) //输出true
console.log('0' == 0) //输出true
console.log(-true) //输出-1
console.log(parseInt("123")) //输出123
console.log(parseInt(" 1 2")) //只会输出1
console.log(parseInt(-123)) //输出-123
console.log(parseInt("hello")) //输出NaN
console.log(parseInt(true)) //输出NaN
console.log(parseInt("123hello")) //输出123,后面非数值型不会识别
console.log(parseInt(" 1 ")) //输出1,忽略空格
console.log(parseFloat("123.55")) //输出123.55
console.log(parseFloat(".1hello")) //输出0.1
console.log(Number(true)) //1
console.log(Number(false)) //0
console.log(Number(null)) //0
console.log(Number("123hello")) //NaN
console.log(Number("12.22")) //12.22
console.log(Number(undefined)) //NaN
window.alert("今天天气很好")
window.confirm("今天心情也很好")
window.prompt("password","请输入密码")
JavaScript的基础暂时先写到这里,后续都会补上。。。
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