JS second week

Looping Statements

**1.while**

while (condition) {
Code executed when the condition is met
}
**2.do...while**

do{
Code to execute
}while (condition)
**3.for**

3.1 for (initialization expression; conditional-judgment expression; change loop-condition expression) {
Code to execute
}
3.2 Iterating through an array
Example: Let arr=[' 1 ', ' 2 ', ' 3 ']
for (let i=0;i<arr.length;i++) {
Console.log[i];
}
3.3 Interrupt Loop
        break
         Example: for (let i=0;i<10;i++) {
Span style= "COLOR: #008000" >                                 if (i==5) {
                                          break; /continue;
         break is a direct end to the entire loop, and only interrupts the nearest loop from his
         continue end of current round loop

JS. First Class
**1. Notes * *
A. Single-line comment: Double slash/middle is comment content/, only in current line
B. Multi-line Comment:/* The middle is the content of the comment */, can be many lines
**2. Identifiers * *
A. Rigid requirements
-can consist of letters, numbers, underscores, and dollar signs, no other special symbols are allowed
-Cannot start with a number
-Prohibit use of keywords and reserved words in JavaScript
-Strictly case-sensitive
B. Soft requirements
-Wang Wen Zhi-yi
-Using the Hungarian nomenclature, the Hump nomenclature, the Snake name method
**3. Data type Introduction * *
ES5
A. Strings string
B. Number
C. Boolean Boole results only true and false
D. Undefined not defined
E. NULL null
ES6 New
F. Unique symbol
G. Object
A-F: Basic data (simple data)
G: Reference data (complex data)
**4. viewing Data types * *
Example: Console.log (typeof data);

**5. Variables * *

Meaning: A container for storing data that can be changed in the presence of variables

A. Declaration of a variable (created)

ES5:

Example: Var A;

ES6:

Example: let A;
B. Variable initialization

Example: let c=123; (give a value at the beginning)
C. Issues with variable declarations

-Duplicate Declaration
-Omission Statement
-Continuous Declaration

**6. Scope of Variables * *

is the scope of the variable.
Divided into global scope, local scope
Local scope of ES5: partitioning by functions
Local scope of ES6: by curly Braces
Free variable: A variable declared outside the current scope, which is called a free variable for the current scope

**7. Variable Lift * *

Before all code runs once, there is a process to parse the code (browse but not run). In this process, all variables declared through Var are found, and the declaration of the variable is promoted to the head of the code (the assignment of the variable remains in the original position)

**8. Constants * *

Example: const a=1;
Constant declaration must be initialized at the same time
Modifying values is not allowed
Duplicate declarations are not allowed

The difference between **9.var and let * *

A.var allow duplicate declarations, let do not allow
B.var scopes are distinguished by functions, let is distinguished by braces
C.var allows variable elevation, let does not allow

JS second week