深入理解JavaScript核心机制：作用域、原型链与异步编程

//块作用域
if (true) {
  let x = 1;
}
console.log(x); //ReferenceError: x is not defined，x只能在if块作用域内访问
//函数作用域
function foo() {
  let y = 2;
}
console.log(y); //ReferenceError: y is not defined，y只能在foo函数作用域内访问

function counter() {
  var count = 0;
  function add() {
    count++;
    console.log(count);
  }
  return add;
}
var counter1 = counter();
var counter2 = counter();
counter1();// 1
counter1();// 2
counter2();// 1
counter2();// 2

let emptyObj = {};
console.log(emptyObj.__proto__); //Object.prototype

function Person(name) {
  this.name = name;
}
Person.prototype.sayHi = function () {
  console.log(`Hi, my name is ${this.name}`);
};
let person1 = new Person('张三');
person1.sayHi(); //Hi, my name is 张三
let person2 = new Person('李四');
person2.sayHi(); //Hi, my name is 李四

let emptyObj = {};
console.log(emptyObj.toString()); //[object Object]，toString方法被Object.prototype继承
console.log(emptyObj.__proto__); //Object.prototype
console.log(emptyObj.__proto__.__proto__); //null，原型链到达了Object.prototype的上层

function loadData(callback) {
  setTimeout(() => {
    callback('This is the data');
  }, 2000);
}
function loadDataCallback(data) {
  console.log(data);
}
loadData(loadDataCallback); //2秒后输出This is the data

function loadData() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('This is the data');
    }, 2000);
  });
}
loadData().then((data) => {
  console.log(data);
});

console.log('start');
setTimeout(() => {
  console.log('2s later');
}, 2000);
console.log('end');
//输出start，end
//2s后输出2s later