JavaScript basic syntax--conditional Statements & looping statements

Conditional statements

The conditional statement (Conditional statement) is one of the basic constructs in JavaScript, and the program executes or skips a branch depending on whether the expression is true or false, and the conditional statement can sometimes be called a "branch statement"

1. If & else if & else

The basic wording is as follows:

if (expression 1) {         //  If expression 1 is true, execute code block 1    Else if (expression 2) {  //  If expression 2 is true, execute code block 2    else {              //  Otherwise, execute code block 3     code block 3} 

Give me a chestnut:

varScore = 78;if(Score >= 90) {alert ("Score: Excellent");} Else if(Score >= 80) {alert ("Score: Liang");} Else if(Score >= 70) {alert ("Score: Medium");} Else if(Score >= 60) {alert ("Score: Poor");} Else{alert ("Score: Failed");}//popup Result: "Score: Medium"

2. Switch

The If & else if & else logic above is still a bit confusing, and to write multiple expressions, the structure is not very clear. So, with the switch statement

The basic wording is as follows:

Switch(n) { Case1://if N==1, execute code block 1code block 1;  Break;  Case2://if n==2, execute code block 2code block 2;  Break;  Case3://if n==3, execute code block 3code block 3;  Break; default://if n is not equal to the above, execute the code block ncode block N;  Break;}

Give me a chestnut:

varDay = 4;Switch(day) { Case0: Alert ("Sunday");  Break;  Case1: Alert ("Monday");  Break;  Case2: Alert ("Tuesday");  Break;  Case3: Alert ("Wednesday");  Break;  Case4: Alert ("Thursday");  Break;  Case5: Alert ("Friday");  Break; default: Alert ("Saturday");  Break;}//popup Result: "Thursday"

Looping statements

Similarly, a looping statement (looping statement) is also one of the basic constructs of JavaScript, which can execute a piece of code repeatedly until a given condition is not true.

JavaScript has 4 loop statements: While & Do/while & for & for...in

Each loop structure has a slightly different, be careful to distinguish, if you can determine the number of cycles in advance, with a for loop, otherwise, consider using while or do/while

where while and do/while are slightly different, while the loop is the first to judge after execution, and the Do/while loop is executed first after judgment. Therefore, Do/while is executed at least once, regardless of whether the condition is true or not.

PS:You do not have to add a semicolon at the end of the while, Do/while need to add semicolons at the end

Why don't you just give me a chestnut?

Summation: 1+2+3+...+100

1. While

// While Loop var sum1 = 0; var j = 1;  while (J <=) {    = sum1 + J;    J+ +;} Alert ("1 + 2 + 3 + ... + + =" + sum1);  // 5050

2. Do/while

// Do and Loop var sum2 = 0; var k = 1;  Do {    = sum2 + k;    K++ while (k <=); alert ("1 + 2 + 3 + ... + + =" + sum2);  // 5050

3. For

// For Loop var sum3 = 0;  for (Let i = 1; I <=; i++) {    = sum3 + i;} Alert ("1 + 2 + 3 + ... + + =" + sum3);  // 5050

4. for...in

// For ... in iterating over array elements var arr = [1,2,3,4,5,6,7,8,9,10];  for (var in arr) {    console.log (arr[i]);} // output array elements: 1,2,3,4,5,6,7,8,9,10

Where for/in is commonly used to traverse object member properties

// For ... in iterating over an array index  for (var in arr) {    console.log (key);} // output array index (properties of the Array object): 0,1,2,3,4,5,6,7,8,9

JavaScript basic syntax--conditional Statements & looping statements