常用语句

If语句

条件语句:

if (condition1) {
    statement1
} else if (condition2) {
    statement2 
} else {
    statement3
}

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如果条件1 成立，则执行条件1的语句，然后结束。
如果条件1不成立，则判断条件2。
如果条件2成立，则执行条件2的语句，然后结束。
如果条件1 成立，则不会再去判断条件2。

var num1 = 40;
			
if (num1 < 20){
  console.log("num1 < 20");
} else if (num1 == 30) {
  console.log("num1 == 30");
} else {
  console.log("num2 > 20");
}

实验: 分数

var studentname = "lisi";
var score = 80;
if ( score >= 60){
    console.log("恭喜你及格了!")
} else if (score < 60){
    console.log("请继续努力！ 加油！")
} else if (score >70){
    console.log("你很棒！")
} else{
   console.log("你太优秀了！")
}

实验效果

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do-while语句

do {
   statement
} while (expression);

var num = true;

do {
var num1 = 100;
console.log("test");

} while (num);

while语句

while(expression) {
    statement
}

var num = 1;
			
while ( num < 10){
      console.log(num);
      num++;

}

实验效果

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for语句

for (initialization; expression; post-loop-expression){
    statement
}

for-in语句

for (property in expression) {
     statement
}

/*var num = 10;

for (var i=1; i<num; i++){
    console.log(i);
}*/


for ( var name in [1,10] ){
    console.log(name);

}

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break和continue语句

break 和 continue 语句用于在循环中精确地控制代码的执行

break : 跳出循环
continue：跳出本次循环后面继续

with语句

with 语句的作用是将代码的作用域设置到一个特定的对象中

with (expression)  {
    statement;
}

with (false){

var num1 = 20;
}	


document.write(num1);

switch语句

switch 语句与 if 语句的关系最为密切.

 switch (expression) {
      case value: 
          statement;
          break;
      case value: 
          statement;
          break;
      case value: 
          statement;
          break;
      case value: 
          statement;
          break;
      default: 
          statement;
}


var  name = "zhangsan";


switch(name) {
case "lisi":
console.log("lisi");
break;

case "zhangsan":
console.log("zhangsan");
break;

default:
console.log("name error!");
}

实验效果

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函数

function functionName(arg0, arg1,...,argN) {
    statements
}

var num1 = 20;
var num2 = 30;

if (num1 < 20)	{
console.log("num1 < 20");
}  else if (num1 > 20){
console.log("num1 > 20");
} else {
console.log("num1 == 20");
}




function test(num){
if (num < 20)	{
console.log("num < 20");
}  else if (num > 20){
console.log("num > 20");
} else {
console.log("num == 20");
}
}

test(15);
test(40);



function print(num1,num2,num3){
  console.log(num1 + num2 + num3);

}

print(1,2,4);

实验代码

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