Branch statement

if branch statement

1. Decide whether to execute a piece of code according to a concept or a condition

2. Grammatical form

(1) if statement
if (conditional) {snippet}

Meaning: when the condition is true, the code segment in {} will be executed, otherwise it will not be executed

var age=16
if (age>=18){
    console.log("Go to the Internet cafe")
}

(2) if else statement

1) if (condition) {snippet} else {snippet}

2) Meaning: if the condition is true, execute the code in {} after if, otherwise execute the code in {} after else

3) Note: code within two {} can and can only execute one

var age = 16
if (age >= 18) {
  console.log('Go to the Internet cafe')
} else {
  console.log('Borrow an ID card')
}

(3) If else... Statement

1) If (condition) {snippet} else if (condition) {snippet}

2) Meaning: start from the first if and judge each condition in turn. Which condition is true and execute the code in {} after which condition. As long as the previous if condition is true, the latter will be ignored; If none of the conditions holds, then none will be implemented

3) Note: only one code in multiple {} can be executed at most

var age = 5
if (age >= 18) {
  console.log('go to university')
} else if (age >= 12) {
  console.log('middle school')
} else if (age >= 6) {
  console.log('primary school')
}

(4) If else statement

1) If (condition) {snippet} else if (condition) {snippet} Else {snippet}

2) Meaning: start from the first if, judge each condition in turn, which condition holds, and execute the code in {} after which condition;
As long as the previous if condition is true, the latter will be ignored; If none of the conditions holds, execute the code in {} after else

3) Note: Code in multiple {} can and can only execute one

var age = 5
if (age >= 18) {
   console.log('go to university')
} else if (age >= 12) {
   console.log('middle school')
} else if (age >= 6) {
   console.log('primary school')
} else {
   console.log('Play at home')
}

**Judge whether a number is even or odd**
var n = 13
if (n % 2 === 0) {
  console.log('even numbers')
} else {
  console.log('Odd number')
}
-----------------------------------------
leap year: year Is a multiple of 4, and year Not a multiple of 100
 leap year: year Is a multiple of 400
     Namely
 leap year: year % 4 === 0 && year % 100 !== 0
 leap year: year % 400 === 0
var year = 1900
if (year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
  console.log(year + ' It's a leap year')
} else {
  console.log(year + ' It's a normal year')
}

switch branch statement

1. Basic syntax:

switch (data){

case 1:

Code executed when condition 1 is met

​ break

case 2:

Code executed when condition 2 is met

​ break

​ default:

Code executed when all conditions are not met

​ }

Note 1: the contents in: () will not be converted to Boolean, but will get a final result and enter the switch to judge

Note 2: the data to be judged and each case must have the same value and data type (= = =) to meet the condition

Note 3: try not to write an expression in the case position, because conditional judgment will be made according to the result of the expression

Note 4: switch is not responsible for making scope judgment

Note 5: default can not be written. When it is not written, there will be no code execution if all case s are not satisfied

Note 6: if the break is not written, the break penetration effect will occur

• Penetration effect: whether the next case is satisfied or not, the code will be executed directly until the next break is encountered or the whole code is executed

According to 1 ~ 12 How many days are these numbers output in the current month(2 The month is temporarily calculated as 28 days)
var month = 9
switch (month) {
  case 1:
  case 3:
  case 5:
  case 7:
  case 8:
  case 10:
  case 12:
         console.log('31')
         break
  case 4:
  case 6:
  case 9:
  case 11:
         console.log('30')
         break
  case 2:
        console.log('28')
        break
    }
-----------------------------------------------
The judgment is the day of the year
 var year = 2020
 var month = 12
 var date = 31
 var total = date
 switch (month) {
     case 12: total += 30
     case 11: total += 31
     case 10: total += 30
     case 9: total += 31
     case 8: total += 31
     case 7: total += 30
     case 6: total += 31
     case 5: total += 30
     case 4: total += 31 // Superimposed March
     case 3:
 total += year % 4 === 0 && year % 100 !== 0 || year % 400 === 0 ? 29 : 28
 case 2: total += 31 // Superimposed January
 }
console.log(total)
-----------------------------------------------
var year = 2020
var month = 12
var date = 31
var total = date
if (month > 1) total += 31
if (month > 2) total += year % 4 === 0 && year % 100 !== 0 || year % 400 === 0 ? 29 : 28
if (month > 3) total += 31
if (month > 4) total += 30
if (month > 5) total += 31
if (month > 6) total += 30
if (month > 7) total += 31
if (month > 8) total += 31
if (month > 9) total += 30
if (month > 10) total += 31
if (month > 11) total += 30
console.log(total)