Detailed explanation of variables and data types in js-JS Tutorial-php.cn

1. Variables

Variables in js are an abstract concept. Variables are used to store values and represent values. Defining a variable in js is very simple: var variable name = variable value

= is an assignment operation, with the variable name on the left and the stored value on the right

Variables in js are loosely typed : Any data type can be stored through a var variable name

For example, var name = '李思'

2. Data type

　1. Basic data types: composed of simple structures

Number (number), string (string), Boolean (boolean), null, undefined

　2. Reference data type: The structure is relatively complex

Object data type (object)

　　Function data type (function)

3. Detailed explanation of data types

1. Number: positive number, negative number, 0, decimal NaN (not a valid number, but belongs to the number data type )

##　　　NaN==NaN //false is not equal

　var num = 12 //= is assignment == is to determine whether the values on the left and right sides are Equality

isNaN(); tests whether a proposition whose value is not a valid number is true. If it is a valid number, it returns false. If it is not a valid number, it returns true

If the value detected is not of type number , the browser will convert it into number by default

Number(): Forces other data types to be converted into number types. It is required that if it is a string, all the strings must be numbers before it can be converted

For example: Number('12')==>12 Number('12px')==>NaN

Non-mandatory data type conversion parseInt/parseFloat:

parseInt: Search characters one by one from left to right, converting numbers into valid numbers. If you encounter a non-valid number in the middle, you will not continue searching

ParseFloat: Same as the above one, you can Identify one more decimal point

Interview questions: 　

var val = Number('12px');if(val==12){ console.log(12) }else if(val==NaN){ console.log(NaN) }else{ console.log('以上都不成立')//输出这个 }

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2. boolean: true false

! : An exclamation point is negated. First convert the value into a Boolean type, and then negate it

　　 console.log(!3)//Convert 3 to boolean first, and then negate it

　　　! ! : Convert other data types into boolean types, equivalent to Boolean()

Rules for data type conversion:

1) If there is only one value, determine whether this value is true Or false, follow: only 0 NaN " " null undefined These five are false and the rest are true

2) If two values are compared to see if they are equal

　　　　　val1==val2 The two values may not be of the same data type. If compared with ==, the default data type conversion will be performed

　　　①Object == object, never equal

　　　②、Object == string, first convert the object into a string (call the toString method), and then compare it

　　　　　　[]Convert into a string ""

　　　　　{}Convert to string "[object,object]"

　　　　③、Object == Boolean type, the object is first converted into a string (toString), and then converted into a number (Number "" becomes 0), the Boolean type is also converted to a number (true is 1, false is 0), the last two numbers are compared

　　　　④. Object == number, the object is first converted into a string (toString), and then converted is a number (Number "" becomes 0), the last two numbers are compared

　　　　⑤、Number == Boolean, Boolean is converted into a number,

　　　　⑥、 Number == string, string Capture it into a number

　　　　　⑦、String == Boolean are all converted into numbers

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　ulous in in Boolean, all are converted intoÃ¢â‚¬Å¦ The data types are not equal in comparison

3), except == is a comparison, === is also a comparison (absolute comparison)

　　　val1===val2 If the data The types are not equal, definitely not equal

3. Object data type (composed of multiple groups [attribute name and attribute value], multiple groups of key-value pairs, multiple key:value. Attribute name and attribute The value is used to describe the characteristics of this object)

For example: literal creation method instance creation method

personInfo ="小李"28"60kg"

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　　　　对象数据类型中，还可以具体的细分:对象类（Object），数组类（Array），正则类（RegExp），时间类(Date)，字符串类（String），布尔类（Boolean），Math数学函数...等对应的实例:对象、数组、正则、时间...
js中对象、类、实例的区别:对象是泛指，js中万物皆对象，类是对对象的具体的细分，实例是类中的一个具体的事物
　　　　举例:自然界中万物皆对象，所有的东西可以分为人类、植物类、动物类、物体类，每一个人都是人类中的一个具体的实例
4、基本数据类型和引用数据类型的区别
　　　　　面试题:

var num1 = 12;var num2 = num1;//把num1代表的值给了num2变量num2++//等于num2 = num2+1 也可以写成num2+=1console.log(num2);//13console.log(num1);//12var obj1 = {name:"小李"};var obj2 = obj1; obj2.name = '小李小李'; console.log(obj1.name)//小李小李console.log(obj2.name)//小李小李

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　　　　　总结:基本数据类型没有跟着改变，引用数据类型跟着改变了
　　　　　可以看一下下面的图：
基本数据类型和引用数据类型的本质区别:基本数据类型操作的是值，引用数据类型操作得是对新空间的引用地址
　　　　　　基本数据类型是把值直接的给变量，接下来在操作的过程中，直接拿这个值操作的，可能两个变量存储一样的值，但是互不干扰，其中一个改变，另一个没有任何的影响。
　　　　　　引用数据类型:
　　　　　　　　1)、定义一个变量
　　　　　　　　2）、开辟一个新的空间，然后把属性名和属性值保存在这个空间中，并且有一个空间地址
　　　　　　　　3）、把空间的地址给了这个变量，变量并没有存储这个数值，而是存储的是对这个空间的引用
　　　　　　　　4）、接下来把这个地址，又告诉给了另外一个变量，另外一个变量存储的也是这个地址，此时两个变量操作的是同一个空间
　　　　　　　　5）、其中一个改变了空间的内容，另外一个也跟着改变了
　5、检测数据类型的方式:typeof instanceof constructor Object.prototype.toString.call()
　　　　　　typeof用来检测数据类型的：typeof 要检查的值
　　　　　　返回值:是一个字符串，包含了数据类型字符"number"、"string"、“boolean”、“undefined”、“object”、"function"
　　　　　　typeof null 的结果是“object”
　　　　　　typeof的局限性:不能具体检查object下细分的类型，检查这些返回的都是“object”
　　　　　　面试题:
　　　　　　　　console.log(typeof typeof typeof [])// "string" 出现两个以上的typeof 最终结果都是“string”

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