Branch structure statement in JS
JS code should be executed from top to bottom
Branch structure statement: when a part is executed, select whether to execute a section of code according to the condition
Conditional branch statement in JS - if
There are four ways to write
1, if branch
1. if statement
if (condition) {code} - single branch
Syntax: if (condition) {code}
Execute the code in {} as long as the condition is satisfied, and execute the code in {} as long as the condition is not satisfied
Satisfied: the condition is true
Not satisfied: the condition is false
var age = 16
// Because the result of age > = 18 is false
// So the code in {} will not execute
if (age >= 18) {
console.log('Go online in the Internet bar')
}
2,if … else statement
If (condition) {execute if condition is established} else {execute if condition is not established} - double branch
Syntax: if (condition) {execute if condition is established} else {execute if condition is not established}
If the condition is satisfied, execute the code in {} after if. If the condition is not satisfied, execute the code in {} after else
Two conditions are bound to carry out one
var num = 203
if (10 < num && num < 100) {
console.log(num + ' It's at 10 ~ 100 between')
} else {
console.log(num + ' Not at 10 ~ 100 between')
}
var age = 16
if (age >= 18) {
console.log('Go to the Internet bar')
} else {
console.log('Online at home')
}
/*
leap year:
The number of years in the Gregorian calendar is a multiple of 4, not a multiple of 100, which is a common leap year
That is: year% 4 = = = 0 & & year / 100! = = 0
The number of years in the Gregorian calendar is a multiple of 100 and 400, which is a leap year of the century
That is: year% 400 = = = 0
*/
var year = prompt('Please enter the year')
if (year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
alert(year + ' It's a leap year')
} else {
alert(year + ' It's the year of the year')
}
Extension: ternary operator
For if The abbreviation of else statement, only if Else statement
Syntax:
Condition judgment? Execute when the condition is established: execute when the condition is not established
Strict Grammar: two symbols, divided into three sections
The sign rules of ternary operators are? And:
var age = 15
age >= 18 ? console.log('adult') : console.log('under age') // under age
3,if … else if statement
If (condition 1) {if condition 1 is satisfied, execute} else if (condition 2) {if condition 2 is satisfied, execute} - multi branch
Syntax: if (condition 1) {if condition 1 is satisfied, execute} else if (condition 2) {if condition 2 is satisfied, execute}
From the first condition, if any condition is satisfied, execute the code in {} after the condition
If the previous conditions are met, all the later conditions will be ignored. Skip it directly
var age = 2
if (age >= 23) {
console.log('I can get married')
} else if (age >= 14) {
console.log('I can fall in love')
} else if (age >= 6) {
console.log('Can sit at the same table with female students')
}
4,if … else if … Else statement
If (condition) {execute on condition 2} else if (condition 2) {execute on condition 2} Else {} - multi branch
Syntax: if (condition) {execute when established} else if (condition 2) {execute when condition 2 is established} else {}
From the first condition, if any condition is satisfied, execute the code in {} after the condition
When all the conditions are not satisfied, execute the code in {} after else
If the previous conditions are met, all the later conditions will be ignored. Skip directly
var age = 5
if (age >= 23) {
console.log('I can get married')
} else if (age >= 14) {
console.log('I can fall in love')
} else if (age >= 6) {
console.log('Can sit at the same table with female students')
} else {
console.log('I can only play with my aunt at home')
}
5. Expansion: a small interaction with the page
Syntax: prompt('text content of prompt ')
Function: pop up an input box in the browser, and write a prompt text content in parentheses
Return value: the content entered by the user in the text box
Note: the content obtained from the page is in a string format
If you want to add, remember to change the value type first
// prompt will pop up an input box for the user to input
// What result receives is what the user enters
var result = prompt('Please enter your bank deposit')
// Judge if your bank deposit is more than 100000, we can talk for a while
// Judge if your bank deposit is less than 100000, then I'm sorry
if (result >= 100000) {
alert('hello nice to meet you!')
} else {
alert('I have something else to do, Let's go, sorry')
}
vscode
1, vscode plug in
1. chinese plug in
2. Color difference in Bracket Pair Colorizer brackets
3. comment translate translation plug-in
4. one dark pro theme plug-in
5. open in browser
6. Vscode Icon Editor Icon plug in
2, Shortcut key
Select the same content ctrl + d
Move up the whole line alt + up arrow
Move down one line as a whole alt + down arrow
3, Code snippet
Lower left gear - > user code snippet - > new global code snippet - > write a name by yourself - > Enter
"Print to console": {
"scope": "javascript,typescript", // Indicates that it takes effect in js file and ts file
"prefix": "abc", // Your shortcut text
"body": [ // That's what happens when you press the tab key
"console.log('$1');",
"$2"
],
"description": "Log output to console"
}
Conditional branch statement in JS - switch
The switch statement is also one of our conditional branch statements
switch Statements
1, Syntax:
switch (variable to judge){
case 1:
Code executed when case 1 is satisfied
break
case 2:
Code executed when case 2 is satisfied
break
default :
Code executed when all conditions are not satisfied
}
// Prepare a variable
var num = 1
// switch syntax
switch (num) {
case 1:
console.log('You entered the number 1')
break
case 2:
console.log('You entered the number 2')
break
case 3:
console.log('You entered the number three')
break
default:
console.log('All conditions are not met')
}
console.log('Follow up code continues')
2, Explain words
Switch switch
case scenario
Default default
Break break
3, Attention
1. It is OK to write the variables to be judged directly in parentheses, without symbols
2. The case after case can't write the comparison of range, only the full (=====)
3. The break after each case should be written. If it is not written, a penetration will occur
Penetration: no matter whether the next case is satisfied or not, the code of the next case will be executed
4. The effect of downward penetration will start from the first satisfied case and continue until a break is encountered
5. default can be written or not
If default is written, the code will execute if all conditions are not met
If you don't write default, no code will execute if all conditions are not met
4, The difference between if statement and switch statement
1. if condition judgment can judge the scope
if statement is recommended when you need unit judgment
2. switch can only judge accurately
When you need to judge accurately and judge more, it is recommended to use the switch statement
5, Small case
/*
According to a number from 1 to 12, give feedback on the number of days in the month
Determine whether February is 28 days or 29 days by a year's variable
*/
// 1. Prepare a number
var month = prompt('Please enter a 1 ~ 12 The number between represents the month')
var year = prompt('Please enter a four digit number for the year') - 0
// Mathematical operations without addition can be converted into numbers
// Minus 0 does not change the original size
month = month - 0
// Write with switch penetration
switch (month) {
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
console.log(month + ' There are 31 days in the month')
break
case 4:
case 6:
case 9:
case 11:
console.log(month + ' There are 30 days in the month')
break
case 2:
// Judging whether it is a normal year or a leap year by year
if (year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
console.log(month + ' There are 29 days in the month')
} else {
console.log(month + ' There are 28 days in the month')
}
break
default:
console.log('Please enter a 1 ~ 12 Number between')
}