1. Statements
Statement: JavaScript programs are executed in lines, which means line-by-line execution. In general, each line is a statement
The difference between a statement and an expression:
- Statements are meant to do something, and generally do not require a return value
* A semicolon indicates the end of a statement
* Multiple statements can be written on one line
* There can be nothing before the semicolon, which the JavaScript engine treats as empty statements
- Expression is to get a return value, it must return a value
* Expression does not require a semicolon end
* Once a semicolon is added after an expression, the JavaScript engine treats the expression as a statement, which results in some meaningless statements
var a = 1 + 3 ; var b = 'abc';//A statement ends with a semicolon, and a semicolon indicates the end of a statement. Multiple statements can be written on one line.
;;;//Represents three empty statements.
'abc';//A statement simply produces a value and has no practical meaning.
var a = 1 + 3;// 1+3 is called an expression and refers to a calculation to get the return value
Ii. if structure
if statement: mainly to judge, first to determine the Boolean value of an expression, and then to execute different statements according to the authenticity of the Boolean value
Grammar:
If (expression) {
Code block;
}
// 1. If a>b, output pairs;
var a = 30,b = 20;
if(a > b){
console.log('Yes');
}
// 2. Ask an adult?
var user = 'So-and-so',age = 20;
if(age >= 18){
console.log('So-and-so' + age + 'Aged Adult')
}
var userless = prompt("Please enter your name"),ages = parseInt(prompt("Please enter your age"));
if(ages >= 18){
alert(userless + ages + 'Aged Adult')
}
3. if-else statements
if-else: two branches, only one
Grammar:
If (expression) {
Code block 1;
}else {
Code block 2;
}
// 1. Ask an adult?
var user = prompt("Please enter your name"),age = parseInt(prompt("Please enter your age"));
console.log(typeof(user));
if(age >= 18){
alert(user + age + 'As an adult, he can take charge of himself');
}else{
alert(user + age + 'Juvenile, can't go to Internet cafe')
}
// 2. Judging the maximum value of two numbers and outputting a larger value requires manual input by the user
var num1 = Number(prompt()),num2 = Number(prompt());
console.log(typeof(num1,num2));
if(num1 > num2){
alert('The larger value is' + num1);
}else{
alert('The larger value is' + num2);
}
4. Ternary Expressions
Both branches execute only one, in which case a ternary expression can be used
// To determine the maximum value of two numbers and output the larger value, the user needs to input it manually
var num1 = Number(prompt('Please enter a number'));
var num2 = Number(prompt('Please enter another number'));
var result = num1 > num2 ? num1 : num2;
alert('The larger number is:' + result);
//Or var result = num1 > num2? Alert ('larger number is: '+ num1'): alert ('larger number is:' + num2);
// Judging adulthood
var user = prompt("Please enter your name"),age = parseInt(prompt("Please enter your age"));
console.log(typeof(user));
var cn = age >= 18 ? alert(user + age + 'As an adult, he can take charge of himself') : alert(user + age + 'Juvenile, can't go to Internet cafe');5. if-else if-else statement
if statement: a branch
if-else statement: two branches
if-else if...-else: multiple branches
Ultimately, only one branch will be executed
Grammar:
If (condition 1) {
Condition 1 establishes block 1 of code being executed;
} Else if (condition 2) {
Condition 2 establishes block 2 of code being executed;
} Else if (condition 3) {
Condition 3 establishes block 3 of code being executed;
}...
else {
None of the above conditions are valid for the code block n being executed;
}
// Grade: <60E 60-69D 70-79C 80-89B 90-99A 100S
var degree = Number(prompt('Please enter your score (100 full)'));
if(!isNaN(degree)){
if(degree === 100){
alert('Your rating is: S');
}else if(degree >= 90 && degree <= 99){
alert('Your rating is: A');
}else if(degree >= 80 && degree <= 89){
alert('Your rating is: B');
}else if(degree >= 70 && degree <= 79){
alert('Your rating is: C');
}else if(degree >= 60 && degree <= 69){
alert('Your rating is: D');
}else if(degree < 60){
alert('Your rating is: E');
}
}else{
alert('Error Entering Results!')
}6. switch-case
switch-case syntax:
Switch (a specific condition) {
case value 1:
If the value judged is equal to the value 1, the code block 1 is executed.
break;
case value 2:
If the value judged is equal to 1, the code block 2 is executed.
break;
case value 3:
If the value judged is equal to 1, the code block 3 is executed.
break;
... ...
default: Code block executed when values are not equal;
}
// Known rank, judge score interval
var degree = prompt('Please enter your rating( S/A/B/C/D/E)');
switch(degree){
case 'S':alert('100');
break;
case 'A':alert('90-99');
break;
case 'B':alert('80-89');
break;
case 'C':alert('70-79');
break;
case 'D':alert('60-69');
break;
case 'E':alert('60 Following');
break;
default:alert('Input error please re-enter!');
}
- break: jump out of the whole statement
- switch-case statement: multiple branches, only one execution
- In general, if series is used in range judgment
- switch when making a judgement on the specific value
7. Practice Work
1. Judging leap years
// Leap years: divisible by 4, but not by 100; or divisible by 400 (if-else)
var year = parseInt(prompt('Please enter a year'));
if(!isNaN(year)){
if(year % 4 == 0 && year % 100 != 100 || year % 400 == 0){
alert(year + 'Is a leap year');
}else {
alert(year + 'Is a normal year');
}
}else {
alert('Input error, please re-enter');
}
/*Or use if-else-if-else
if(!isNaN(year)) {
if(year % 4 == 0 && year % 100 != 0) {
console.log(year + 'Is a leap year';
}else if(year % 400 == 0) {
console.log(year + 'Is a leap year';
}else {
console.log(year + 'It is a normal year';
}
}else {
alert('Enter the correct year (positive integer)!'.
} */2. User enters a number to determine if it is odd or even
//User enters a number to determine if it is odd or even
var num = parseInt(prompt('Please enter an integer'));
if(!isNaN(num)){
if(num % 2 == 0){
alert(num + 'Is Even');
}else{
alert(num + 'is odd');
}
}else{
alert('Input error, please re-enter!');
} 3. Display the corresponding days based on the month entered by the user (February ignores leap years and outputs 28 days directly)
//Display the corresponding days based on the month entered by the user (February ignores leap years and outputs 28 days directly)
var month = prompt('Please enter a month');
console.log(month);
switch(month){
case '1':alert(month + 'Month has 31 days');
break;
case '2':alert(month + 'Month has 28 days');
break;
case '3':alert(month + 'Month has 31 days');
break;
case '4':alert(month + 'Month has 30 days');
break;
case '5':alert(month + 'Month has 31 days');
break;
case '6':alert(month + 'Month has 30 days');
break;
case '7':alert(month + 'Month has 31 days');
break;
case '8':alert(month + 'Month has 31 days');
break;
case '9':alert(month + 'Month has 30 days');
break;
case '10':alert(month + 'Month has 31 days');
break;
case '11':alert(month + 'Month has 30 days');
break;
case '12':alert(month + 'Month has 31 days');
break;
default:alert('Input error, please re-enter');
}