JS编程核心：从变量到异步全解析

变量声明

• let：块级作用域
• const：常量（不可重新赋值）
• var：函数作用域（存在变量提升）

let a = 10;
const PI = 3.14;
var b = "hello";

数据类型

• 基本类型：Number, String, Boolean, null, undefined, Symbol
• 引用类型：Object（含Array, Function等）
• 类型检测：

  typeof 42; // "number"
  typeof [1,2]; // "object"
  Array.isArray([1,2]); // true

相等判断

• ==：隐式类型转换（1 == '1' → true）
• ===：严格相等（类型和值均需相同）

类型转换

Number("123"); // 123
String(123); // "123"
Boolean(0); // false

条件语句

if (score > 90) {
console.log("A");
} else if (score > 60) {
console.log("B");
} else {
console.log("C");
}

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循环语句

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

let arr = [10, 20, 30];
arr.forEach(item => console.log(item));

函数定义

function add(a, b) {
return a + b;
}

// 箭头函数
const multiply = (a, b) => a * b;

作用域与闭包

function outer() {
let x = 10;
return function inner() {
return x * 2; // 闭包访问外部变量
};
}

对象创建

let person = {
name: "Alice",
age: 30,
greet() {
console.log(`Hello, ${this.name}!`);
}
};

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原型链

function Animal(name) {
this.name = name;
}
Animal.prototype.speak = function() {
console.log(`${this.name} makes a sound.`);
};

Promise

fetchData()
.then(data => processData(data))
.catch(error => console.error(error));

async/await

async function loadData() {
try {
const data = await fetchData();
console.log(data);
} catch (error) {
console.error(error);
}
}

for (var i = 0; i < 3; i++) {
setTimeout(() => console.log(i), 100);
}