JS data type - basic concepts, data casts

First, the basic concept of data types

• Data type : refers to the type literal, in JS, a total of six data types
  • String String
  • Number value
  • Boolean Boolean value
  • Null null
  • Undefined Undefined
  • Object Object
• Wherein String Number Boolean Null Undefined belonging to the basic data types , belonging to the Object reference data types

1 String String

1.1 String Basics

• JS quotes required to cause the string;
• Use double or single quotes can be, but not mixed with use;
• It can not be nested quotes, double quotes not put double quotes, single quotes not put single quotes;

var str = 'hello';  // 声明一个变量str并给其赋值hello
console.log(str)  // 控制台输出str的内容

// 修改变量str的值为'我说:"今天天气真不错！"'
str = '我说:"今天天气真不错！"'; // 会修改掉原来str的值
console.log(str)

1.2 String Escape

In the string we can use \ as an escape character, when expressed some special symbols can be used \ to escape

• \ "=> Means"
• \ '=> Represents'
• \ N => represents a line feed
• \ T => tab
• \ => Representation \

str = "我说:\"今天\t天气真不错！\"";			
str = "\\\\\\";
//输出字面量 字符串str
alert("str");

1.3 String stitching

There are two forms of string concatenation: Use the + stitching; use template string

var a = "孙悟空";
var b = "我是"+a;
console.log(b);
var c = `我是${a}`;
console.log(c);

2 Number Numeric

• In JS all values are Number types , including integer and floating point (decimal)
• JS the maximum number that can be expressed in
  • Number.MAX_VALUE
  • 1.7976931348623157e + 308
• Is greater than a minimum value of 0 Number.MIN_VALUE
  • 5e -324
• If digital Number representation exceeds the maximum, it will return a
  • Infinity represents positive infinity
  • -Infinity representing negative infinity
• Use typeof check Infinity will return number
• NaN is a special number, indicates Not A Number, using typeof NaN also returns a check number

var a = 123;  //数字123
var b = "123";  //字符串123
/*
    可以使用一个运算符 typeof来检查一个变量的类型
    语法：typeof 变量	
    检查字符串时，会返回string
    检查数值时，会返回number
*/
// console.log(typeof b);
a = -Number.MAX_VALUE * Number.MAX_VALUE;
a = "abc" * "bcd";
a = NaN;
// console.log(typeof a);
a = Number.MIN_VALUE;
//console.log(a);

// 在JS中整数的运算基本可以保证精确
var c = 1865789 + 7654321;
/*
 * 如果使用JS进行浮点运算，可能得到一个不精确的结果
 * 	所以千万不要使用JS进行对精确度要求比较高的运算	
 */
var c = 0.1 + 0.2;
console.log(c);

3 Boolean Boolean value

• Boolean Boolean: Boolean only two, mainly used for determination logic
  • true - for true
  • false - false representation
• When using typeof check a Boolean value, it will return boolean

var bool = false;	
console.log(typeof bool);
console.log(bool);

4 Null null and undefined Undefined

• The null value is used to represent a special null object
  • When using typeof check for a null value, returns object
• Value Undefined (undefined) only one type, it undefind
  • When you declare a variable, but not when assigned to a variable, its value is undefined, also returns undefined undefined when using a check typeof

var a = null;
var b = undefined;
console.log(typeof b);

Second, the data type conversion

Cast

• It refers to a data type cast to other data types
• Mainly refers to the type of conversion, the other types of data, converted to a String, Number, Boolean

1 other data type to String

• Convert other data types to String
• method one:
  • Calls are converted to the data type toString () method
  • This method does not affect the original variables, the results of which will be converted Return
  • But note: null and undefined these two values ​​do not toString () method, if you call their approach will complain
• Second way:
  • Call String () function, and the converted data is passed to the function as a parameter
  • Use String () function to do when cast for Number and Boolean actually call toString () method
• But for null and undefined , it will not call the toString () method, it will directly convert null "null", will directly convert undefined "undefined".

//调用toString()方法，来将a转换为字符串
var a = 123; // 数值转String
a = a.toString();
a = true; // 布尔转String
a = a.toString();
a = null; // null转String
a = a.toString(); //报错
a = undefined;  // undefined转String
a = a.toString(); //报错

//调用String()函数，来将a转换为字符串
a = 123;
a = String(a);
a = null;
a = String(a);
a = undefined;
a = String(a);
console.log(typeof a);
console.log(a);

2 is converted to a different data type Number

2.1 Data Conversion

• Other data types converted to Number

• A conversion mode:

• Use Number () function

• String -> Digital

  • 1. If the string is purely digital, it is directly converted into digital
  • 2. If the content of non-numeric string, the conversion is NaN
  • 3. If the string is a null string is a space or a full string, then converted to 0

• Istanbul -> Digital

  • converted into a true
  • false turn into 0

• null -> Digital 0

• undefined -> Digital NaN

• Conversion way:

  • This way designed to deal with string
  • the parseInt () to convert a string to an integer
  • parseFloat () to convert a string into a float

//调用Number()函数来将a转换为Number类型
var a = "123";
a = Number(a);   // 123
a = false;
a = Number(a);  // 0
a = null;
a = Number(a);  // 0
a = undefined;
a = Number(a);  // Nan
//调用parseInt()函数将a转换为Number
a = "123567a567px";
// parseInt()可以将一个字符串中的有效的整数内容去出来，然后转换为Number
a = parseInt(a);
// parseFloat()作用和parseInt()类似，不同的是它可以获得有效的小数
a = "123.456.789px";
a = parseFloat(a);
// 如果对非String使用parseInt()或parseFloat(),它会先将其转换为String然后在操作
a = true;
a = parseInt(a);
a = 198.23;
a = parseInt(a);
console.log(typeof a);
console.log(a);

2.2 hex value

• In js, if you need to represent hexadecimal numbers, you need to begin with 0x
• If you need to represent octal numbers, you need to start with 0
• If you want to represent binary numbers, you need to begin with 0b
• But not all browsers support

//十六进制
a = 0x10;
a = 0xff;
a = 0xCafe;
//八进制数字
a = 070; //像"070"这种字符串，有些浏览器会当成8进制解析，有些会当成10进制解析
//二进制数字
a = 0b10;

a = "070";
//可以在parseInt()中传递一个第二个参数，来指定数字的进制
a = parseInt(a,10);
console.log(typeof a);
console.log(a);

3 is converted to a different data type Boolean

• Converting the other data type Boolean
• Using Boolean () function
• Digital -> Boolean
  • In addition to 0 and NaN, the rest are true
• String -> Boolean
  • In addition to the empty string, the rest are true
• null and undefined are converted to false
• Objects can also be converted to true

// 数值转布尔值
var a = 123; //true
a = -123; //true
a = Infinity; //true
a = NaN; //false
a = 0; //false
// 调用Boolean()函数来将a转换为布尔值
a = " "; // ture
a = null; //false
a = undefined; //false
a = Boolean(a);
console.log(typeof a);
console.log(a);

// 数值转布尔值
var a = 123; //true
a = -123; //true
a = Infinity; //true
a = NaN; //false
a = 0; //false
// 调用Boolean()函数来将a转换为布尔值
a = " "; // ture
a = null; //false
a = undefined; //false
a = Boolean(a);
console.log(typeof a);
console.log(a);