1.if...else... 语句

if(表达式){
  语句
}else if(表达式){
  语句
}else{
  语句
}

const a = 10; 
if (a > 10) { console.log(a); } 
// 此处就是一个简单的if else 更多的使用反而是下面这种 if (time < 10) {
   greeting = "Good morning"; 
} else if (time < 20) {
   greeting = "Good day";
} else {   
   greeting = "Good evening"; }

2.switch 语句

const food = "sushi"; 

switch (food) { 
  case "sushi": 
     console.log("Sushi is originally from Japan."); 
     break; 
  case "pizza": 
     console.log("Pizza is originally from Italy."); 
     break; 
  default: 
    console.log("I have never heard of that dish.");
    break; 
}
// Expected output:"Sushi is originally from Japan."

3.while 循环 (当...时)

语法: while (表达式) {语句}

首先判断表达式的真假

当表达式为真,执行语句,执行完再判断表达式的真假

当表达式为假,执行后面的语句

let n = 0;

while (n < 3) {
  n++;
}

console.log(n);

4.for 循环

语法: for (语句1 ; 表达式1 ; 语句3) {循环体}

首先执行语句1

然后判断语句2

如果为真,执行循环体,然后执行语句3

如果为假,直接退出循环,执行后面的语句

let str = '';

for (let i = 0; i < 9; i++) {
  str = str + i;
}

console.log(str);
// Expected output: "012345678"

5.break和continue

break 退出所有循环

let i = 0;

while (i < 6) {
  if (i === 3) {
    break;
  }
  i = i + 1;
}

console.log(i);
// Expected output: 3

continue 退出当前一次循环

let text = ''; 

for (let i = 0; i < 10; i++) { 
  if (i === 3) { 
    continue; 
  } 
    text = text + i; 
  } 
    console.log(text); 
  // expected output: "012456789"

6.label

语法:

foo:{
    console.log(1);
    break foo;
    console.log('本行不会输出');
}
console.log(2)
// expected output: "1" "2"

let str = ''; 

loop1: 
  for (let i = 0; i < 5; i++) {
    if (i === 1) {
      continue loop1; 
     } 
     str = str + i; 
   } 
   
     console.log(str); 
// expected output: "0234"

面试:{foo:1}表示foo为lable语句,其值为1