ES6 WeakMap类封装

Question

I'm trying to understand why I need to use WeakMaps to create private class members, instead of just using a normal variable. They both create encapsulation with closures, and module imports.

(function encapsulation() {
  const my_var = 'My secret info';
  const my_var2 = new WeakMap();

  class Test {
    constructor() {
      my_var2.set(this, 'My secret info 2');
      console.log(my_var); // My secret info
      console.log(my_var2.get(this)); // My secret info 2
    }
  }

  const t = new Test();
})();


console.log(my_var); // undefined
console.log(my_var2); // undefined

// Same result!

Answer 1

The problem with an ordinary variable like my_var is that it will only save data for a single instantiation of the class:

 const Test = (function encapsulation() { let my_var = 'My secret info'; class Test { constructor(param) { my_var = param; } getInfo() { return my_var; } } return Test; })(); const t1 = new Test('foo'); const t2 = new Test('bar'); console.log(t1.getInfo()); // the above returns 'bar'... uh oh, but we passed in 'foo' to `t1`! Our data is lost! console.log(t2.getInfo()); // 'bar' 

Thus, the need for a WeakMap , to hold separate data for each instantiation :

 const Test = (function encapsulation() { const my_var2 = new WeakMap(); class Test { constructor(param) { my_var2.set(this, param); } getInfo() { return my_var2.get(this); } } return Test; })(); const t1 = new Test('foo'); const t2 = new Test('bar'); console.log(t1.getInfo()); // 'foo', as expected console.log(t2.getInfo()); // 'bar', as expected