单例模式

singleton_pattern.js

/**
 * 单例模式 
 * 避免重复创建相同对象
 * example: 
 * let a = new TestClass();
 * let b = new TestClass();
 * a === b // false
 * 如何a===b 为 true
 */

// ----------- achieve first -----------
let TestClass = {
  say: function () {
    console.log('say something!');
  }
}
let a = TestClass;
let b = TestClass;

console.log(a===b); // 指向同一引用
// ----------- achieve second -----------
function TestClass2() {
  if (TestClass2.unique) return TestClass2.unique;
  TestClass2.unique = this;
}
TestClass2.prototype.say = function () {
  console.log('say something!');
}
let c = new TestClass2();
let d = new TestClass2();
console.log(c === d);
// ----------- achieve third -----------
const TestClass3 = (function () {
  let instance;
  function TestClass3() {
    
  }
  TestClass3.prototype.say = function () {
    console.log('say somthing!');
  }

  if (instance === undefined) instance = new TestClass3;

  return instance;

})();
let e = TestClass3;
let f = TestClass3;
console.log(e === f);