if

Conditional statement: You can judge a statement before it is executed. If the condition is true, the statement is executed. If the condition is not true, the statement is not executed

Chestnut 🌰

var score = prompt("Please enter your final grade (0-100)")
if(score > 100 || score < 0 || isNaN(score)){
  alert("Invalid input");
}else{
  if(score == 100){
    alert("Three Days at Disney");
  }else if(score >= 80){
    alert("Make a hand.");
  }else if(score >= 60){
    alert("Test volume");
  }else{
    alert("A stick"); }}// As long as there is one statement executed, there is no further execution
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switch

Also called conditional branch statements

switch(true) {case score >= 60:
    console.log("Qualified");
    break;
  default:
    console.log("Unqualified");
    break;
}
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  • Will be executed oncecaseThe value of the following expressionswitchThe value of the final conditional expression is compared.
  • If the comparison value is true, the subsequent code is executed and can be usedbreakUsed to quitswitchstatements
  • If all comparisons result in false, only executedefault

whilecycle

Judge before you execute

  • Evaluate the expression first
  • If the value is true, the body of the loop is executed
  • After the body of the loop completes, we continue to evaluate the expression
  • If the value is true, the body of the loop continues
  • Until the value is false, the loop is terminated

Chestnut 🌰

while (true) {
  var score = prompt("Please enter your final grade (0-100)");
  if (score >= 0 && score <= 100) {
    break;
  }
  alert("Please enter a valid score!!");
}
if (score == 100) {
  alert("Three Days at Disney");
} else if (score >= 80) {
  alert("Make a hand.");
} else if (score >= 60) {
  alert("Test volume");
} else {
  alert("A stick");
}
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for

while (true) {
forInitialize the expression; Conditional expression; Update expression){statement ···}for(var i = 0; i < 10; i++){
  alert(i);
}
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  1. Execute the initialization expression to initialize the variable
  2. Execute the conditional expression to determine whether to execute the loop
  3. Execute the update expression and continue to execute the conditional expression after the update expression is executed

Chestnut 🌰

var num = prompt("Please enter an integer greater than 1:");
if (num <= 1) {
  alert("This value is not valid");
} else {
  var flag = true;
  for (var i = 2; i < num; i++) {
    if (num % i == 0) {
      If num is divisible by I, then the value of num is not prime. If num=11, then I is a direct number from 2 to 10.
      flag = false; }}// The for loop fetches all numbers where num divisible into I
  if (flag) {
    alert(num + "Prime number!!");
  } else {
    alert(num + "Not prime."); }}Copy the code

nestedforcycle

The inner for loop executes before the outer for loop executes

// Scenario: Print a prime number between 2 and 100
for (var i = 2; i <= 100; i++) {
  var flag = true;
  for (var j = 2; j < i; j++) {
    if (i % j == 0) {
      flag = false; }}if (flag) {
    console.log(i); }}Copy the code