Take you to learn more about JS basic grammar (recommended Collection)

Literal
Concept ： Some unalterable values , Can be used directly , But generally, it will not directly make use .
for example ：1 3 5 7;

Variable
Concept ： Used to store literal values , And the value can be changed arbitrarily .
Statement ： By keyword var Declare variables .
for example ：var btn=5;

identifier
Concept ：js All that can be independently named by us can become identifiers .
for example ： Variable name Function name Attribute names are identifiers
Naming rules ：1. It can contain letters 、 Numbers 、 Underline _、$
2. Cannot start with a number
3. Out of commission ES Keywords and reserved words in
4. It can be in Chinese
5. Hump writing is generally used
for example ： Little hump helloWorld Hump HelloWorld

data type
Concept ： Data types are literal types
Check data type Syntax ：typeof Variable
example ：var btn=”123”; console.log(typeof btn);

share Six kinds data type ：

Basic types ：

1.string character string
example ：var str=”hello world”;

2.number The number
Concept ：js All the values in are number type , Including whole Count 、 Floating point numbers （ decimal ）, If the maximum value is exceeded, a Infinity It means positive infinity , Plus a minus sign is negative infinity
example ：var a=123;

3.Null Null value
Concept ： Specifically used to represent an empty object
Use typeof When checking, it will return object

4.undifined Undefined
Concept ： When a variable is declared but not assigned a value , Its value is undefined
example ：var c;

5.Boolean Boolean value true/false
Concept ： Mainly used to make logical judgments -true It's true -false For false
example ：var bl=false;

6. Reference type ：
object object
Type conversion

Cast ：
Concept ： Cast one data type to another
Type conversion mainly refers to converting other data types to
String number Boolean

1. Other data types are converted to string type

Law 1 ：· call toString() Method , This method does not affect the original variable , Only the converted results will be returned
null and undefined These two are not worth toString() Method

Law two ： call String() function , The transformed data is passed to the function as parameters

example ：
var a=123;// value type -》 String type
var b=a.toString();
console.log(typeof b);
var bl=false;// Boolean type -》 String type
var b=a.toString();
console.log(typeof b);
var c=123;
var d=string;
console.log(typeof c);//number
console.log(typeof d);//string

2. Other data types are converted to number type
Concept ：
Law 1 ：
character string –> Numbers
1. A pure numeric string is directly converted to a number
2. If there is non numeric content in the string, it is converted to NaN
3. If the string is empty or full space, it is converted to 0

Boolean value –> Numbers true Convert to 1,false Convert to 0
null–> The number is 0
undefined–> The number is NaN

example ：var num=”123”/”123sdf1”/” ”/false/null/undefined;
var a=Number(num);
console.log(a);/123/NaN/0/0/0/NaN

Law two ： Specifically used to convert string types
parseInt() You can convert strings to integers , Take out the valid integers in the string , And then convert to Number
parseFloat() You can convert a string to a floating point ( decimal ), Gets the valid decimal number in the string
example ：var num=”123.23”
num=parseFloat(num);/parseInt(num);
Console.log(num);/123.23/123

3. Other data types are converted to Boolean type

Method ： Use Boolean() function

1. Numbers –> Boolean except 0 and NaN yes false, Others are all true
2. character string –> Boolean Except the empty string is false, Others are all true
3.null and undefined Will be converted to false
4.object object Convert to true

example ：var a=123.12;/0/NaN
a=Boolean(a);
console.log(a);/true/false/false

var str=”234fg”;/””/undefined
s=Boolean(str);
console.log(s);/true/false/false