【经典】class编译为es5

class User {
  constructor(name) {
    this.name = name;
  }
  getName() {
    return this.name;
  }
}
var u = new User('Jack')
u.getName()
"Jack"

es5写法

function User(name) {
    this.name = name
}
User.prototype.getName = function getName() {
    return this.name
}
var u = new User('Mary')
u.getName()
"Mary"

注： 若写为匿名函数，
User.prototype.getName = function () {
return this.name
}

常见错误写法！

function User(name) {
    this.name = name
    const _this = this
    this.getName = function getName() {
        return _this.name
}
}
var u = new User('Lucy')
u.getName()

区别在于keys

Object.keys(u)
(2) ["name", "getName"]
// 正确
Object.keys(u)
["name"]