// 使用 let 声明变量（推荐）
let name = "john";

// 使用 const 声明常量
const pi = 3.14159;

// 不推荐使用 var（旧语法）
var age = 30;

let str = "hello";
let num = 42;
let bool = true;
let undef = undefined;
let n = null;
let sym = symbol("id");

// 算术运算符
let sum = 5 + 3;
let difference = 10 - 4;
let product = 6 * 7;
let quotient = 20 / 4;

// 比较运算符
let isequal = (5 === "5"); // false（严格相等）
let isnotequal = (5 != "5"); // false

// 逻辑运算符
let andresult = (true && false); // false
let orresult = (true || false); // true
let notresult = !true; // false

let age = 18;

if (age >= 18) {
console.log("you are an adult");
} else if (age >= 13) {
console.log("you are a teenager");
} else {
console.log("you are a child");
}

// 三元运算符
let status = (age >= 18) ? "adult" : "minor";

// for 循环
for (let i = 0; i < 5; i++) {
console.log(i);
}

// while 循环
let j = 0;
while (j < 5) {
console.log(j);
j++;
}

// do...while 循环
let k = 0;
do {
console.log(k);
k++;
} while (k < 5);

// 函数声明
function greet(name) {
return `hello, ${name}!`;
}

console.log(greet("alice")); // 输出: hello, alice!

// 箭头函数（es6）
const multiply = (a, b) => a * b;
console.log(multiply(3, 4)); // 输出: 12

// 获取元素
const element = document.getelementbyid("myelement");

// 修改内容
element.textcontent = "new content";

// 修改样式
element.style.color = "red";

// 添加事件监听器
element.addeventlistener("click", function() {
alert("element clicked!");
});

fetch('https://api.example.com/data')
.then(response => response.json())
.then(data => console.log(data))
.catch(error => console.error('error:', error));

function delay(ms) {
return new promise(resolve => settimeout(resolve, ms));
}

delay(2000).then(() => console.log("2 seconds have passed"));

async function fetchdata() {
try {
const response = await fetch('https://api.example.com/data');
const data = await response.json();
console.log(data);
} catch (error) {
console.error('error:', error);
}
}

fetchdata();