js语句使用介绍

js语句使用介绍

/**
 * created by administrator on 2017/12/24.
 * 1.顺序语句
 * 2.选择语句  if else  swtich case
 * 3.循环语句  for   while   do...while
 */

//选择语句 if
//if(条件成立){执行代码}
var a = 100;
if (a > 0 && a < 101){
    console.log("a > 0");
}else {
    console.log("go out of if");
}

a = 75;
if (a >=80){
    console.log("优秀");
}else if(a >= 70){
    console.log("良");
}else if(a >= 60){
    console.log("及格");
}else{
    console.log("不及格");
}
console.log("======================");

//swtich
var index = 5;

switch (index) {
    case 1:
        console.log("1");
        break;
    case 2:
        console.log("2");
        break;
    case 3:
        console.log("3");
        break;
    case 4:
        console.log("4");
        break;
    case 5:
        console.log("5");
        break;
    default:
        console.log("default");
        break;
}
console.log("======================");

//循环结构  while  for   do...while
a = 10;
while(a > 0 ){
    console.log(a);
    a--;
}
console.log("======================");

//编写程序1+2+3+...100的和
//第一种
var sum = 0;
var i = 1;
while(i <= 100){
    sum += i;
    i++;
}
console.log(sum);
//第二种
for( sum = 0 , i =1; i <= 100; i++ ){
    sum += i;
}
console.log(sum);

console.log("======================");
//计算100以内所有的奇数的和
//第一种
sum = 0;
i = 1;
while(i <= 100){
    if ((i % 2) == 0){
        i++;
        continue;
    }
    sum += i;
    i++;
}
console.log(sum);

//第二种
for ( sum = 0, i = 1; i<=100; i++){
    if ( (i % 2) == 0){
        continue;
    }
    sum += i;
}
console.log(sum);