变量与环境：作用域是怎么管起来的

// env.js —— 作用域环境的极简实现
class Environment {
  constructor(parent = null) {
    this.vars = new Map()
    this.parent = parent // 指向外层作用域，形成链
  }
  define(name, value) { this.vars.set(name, value) }
  has(name) {
    let env = this
    while (env) { if (env.vars.has(name)) return true; env = env.parent }
    return false
  }
  get(name) {
    let env = this
    while (env) {
      if (env.vars.has(name)) return env.vars.get(name)
      env = env.parent
    }
    throw new Error('未定义的变量: ' + name)
  }
  assign(name, value) {
    let env = this
    while (env) {
      if (env.vars.has(name)) { env.vars.set(name, value); return }
      env = env.parent
    }
    this.vars.set(name, value) // 找不到就建在顶层
  }
}

const globalEnv = new Environment()
globalEnv.define('x', 10)
const inner = new Environment(globalEnv)
inner.define('y', 20)
console.log('读 y（本层） =', inner.get('y'))
console.log('读 x（外层） =', inner.get('x'))
console.log('has(x) =', inner.has('x'), '| has(z) =', inner.has('z'))