JavaScript basic conditional statement 2.0

Time:2020-10-25

5. Conditional statement

a. If statement

Grammar:If (conditional expression) / / conditional expression: either true or false

{

The code to be executed if the condition is satisfied;

}

If else statement

If (conditional expression)

{

Meet the conditions and execute the code;

}else {

Code executed if the condition is not met;

}

be careful: else cannot be used alone

 

practice:Grade n [90 ~ 100] a

         [80~90)     B

                           [70~80)     B

         [60~70)     B

[0 ~ 60) fail

code:Method 1

var n = 95;
  if(n>=90 && n<=100) {

    alert("A");

  }else if(n>=80 && n<90){

    alert("B");

  }else if(n>=70 && n<80){
    
    alert("C");

  }else if(n>=60 && n<70){

    alert("D");

  }else {

    Alert ("fail");

  }

Method 2

if(n<60) {

    Alert ("fail");

  }else if(n<70) {

    alert("D");

  }else if(n<80) {

    alert("C");

  }else if(n<90) {

    alert("B");

  }else if(n<100) {

    alert("A");

  }

b. Switch statement

Syntax: switch (n){

Case X1: Code 1…; break;

Case x2: Code 2…; break;

Case X3: Code 3…; break;

     ……

Case X1: Code 1…; break;

}

be careful:Case is followed by a specific value and cannot be a range [enumeration]

Switch (variable), not like if statement and condition

Limitations:Switch is not flexible enough and not as widely used as if, but it is reasonable to exist.

 

code:

var n = 10;

switch(n) {

  case 1: alert("hello world"); break;

  Case 2: alert; break;

  Case 3: alert; break;

  default:alert Break; break; break; break; break;

}

C. While statement

Grammar:

While (condition){

Loop body [execute when conditions are met]

}

Note: this can easily lead to infinite loops

 

The finite cycle condition is as follows

(1) , initial variable

(2) The condition is a bounded range

(3) In the loop body, operate on the initial variable

 

Debug — > view language execution order:

(1) . breakpoints need to be set

(2) Debugging tool with browser [F12]

(3) And [F10] step by step

(4) And [F8] runs to the next breakpoint. If there is no breakpoint, it ends directly

code:

Var I = 10; // initial variable

While (I > 0) {// the condition is a bounded range

  Debugger; // set breakpoint

  console.log (the "condition holds" + I);

  I --; // operate on the initial variable

}

d. Do while statement

Grammar: do{

code;

Variable operation

}While (condition);

code:

var i = 50; 
do {
    Alert ("the score is" + I + "unqualified");
    i+=2;
}while(i<60);