JavaScript

1、js It's an explanatory statement , There is no difference between compiling and running , Only runtime errors .

2、①alert(message)  Popup （ news ）        ②prompt(message)  Pop up the input box , And get the value entered by the user

③console.log(message)    Print a log on the console        

④console.error(message)  Print an error log on the console

3、JS The data type of the variable in

1） data type ：number、boolean、string、undefined、null、function、object、array

2) Variable ： There is only one （ Reference type ）;

Data types , Variables have no type ;

3）js The variables in the , Must be defined first , And then you can use it . A variable is a logical abstraction of a space in memory .

var a;// Definition      a = 10; // Use       b = a; // Use

4） Assignment operation ： Put the value to the right of the equal sign , Load the memory space to the left of the equal sign .

5）number type : ① -Infinity: Negative infinity         ②Infinity: It's just infinite         ③NaN : not a number It's not a number

6）string type ： Single or double quotation marks ; Template Strings use backquotes

eg:  `hello${a}`        Try to put world Variable expansion

7）undefines/null : It's all about an abstraction that doesn't exist , But it's divided into two undefined( Undefined )、null( empty )

4、

1）== and ===     

  a==b Type consistency is not required ,a===b Strict equality judgment , Must ask for a and b It's the same type , also a The value is equal to the b Value

2）!= and !==

In the same way

5、if( Conditions ){...}  , A condition is an expression , An expression is defined by its type , According to its type + Its value is determined true / false

What conditions true？

①boolean type ： eligible true

②number type ： No 0

③string type ： Not an empty string ""

④ Other types ： No undefined as well as null

6、 Array （array)： More like a sequence table , Can grow dynamically

①a.push() Tail insertion         ②a.unshift() Head insertion          ③a.pop() Deletion at the end        ④a.shift() Head deletion         ⑤a.sort() Sort

⑥a[index] similar a.get()        ⑦a[index] = ...    similar a.set() 

similar js Variables in are typeless ,js The elements of the array are also typeless （ The types of elements can be different ）;

7、 function （function） There is no clear way

eg:function add(x,y){return x+y;}

function keyword ; No return value type （ because js Variable in has no type ）; The parameter in the parameter list has no type , Don't need to, var. Function is also a data type .