JS [笔记] Javascript 优良部分、糟糕与不良的部分

优良部份1~5

1. 宽松型别（loose typing）及易转型

https://codepen.io/Rouoxo/pen/XWzMgvY
(1) 易转型

    console.log(!!0); // false
    console.log(parseInt("169.99cm")); // 169
    console.log(parseFloat("169.99cm")); // 169.99
    console.log(String(false)); // "false"
    console.log(Object(169.99)); // Number {169.99}

(2) 判断Array

    // 利用Array.isArray()函式
    // 利用instanceof运算子判断是否为Array类别
    // 利用Object.prototype.toString()函式判断字串
    console.log(Array.isArray([])); // true
    console.log([] instanceof Array); // true
    console.log(Object.prototype.toString.call([]) == "[object Array]"); // true 好用
    var Vtuber = ["茸茸鼠", "阿骂"];
    if (typeof Vtuber === "object" && Vtuber.constructor === Array) {
      console.log("Vtuber是真正的阵列");
    }

2. Object Literals 物件实字

https://codepen.io/Rouoxo/pen/qBVrPaa?editors=0012

    function getPlayerObj(name, progress) {
      return {
        sayHi() {
          return `Hi, I am ${name}`;
        },
      };
    }
    console.log(getPlayerObj("OneJar", 23).sayHi()); // "Hi, I am OneJar"

3. Hoisting

https://codepen.io/Rouoxo/pen/jOaBGwR?editors=0011
var、具名函式可使用Hoisting
let 、const、匿名函式、=>(为匿名函式) 无法使用Hoisting
无法在函式宣告之前就使用

    console.log("eee+fff", eee() + fff()); //hoisting
    function eee(cry = 123) {
      return cry;
    }
    function fff(cry = 456) {
      return cry;
    }

4. Closure

https://codepen.io/Rouoxo/pen/oNoZGow?editors=0012

    var testModule = (function () {
      var counter = 0;
      return {
        incrementCounter: function () {
          return counter++;
        },
        // 语法糖 物件
        resetCounter() {
          console.log("counter value prior to reset: " + counter);
          counter = 0;
        },
      };
    })();

    //test
    testModule.incrementCounter();
    testModule.incrementCounter();
    testModule.incrementCounter();
    testModule.resetCounter(); //counter value prior to reset: 3

5. Prototype

prototype 似继承给拿走的变数使用

    let Crash = function () {
      this.name = "Crash";
      this.age = 18;
      this.secret = {
        weight: 100,
        fn: function () {
          console.log(this); // {weight: 100, fn: ƒ}
        },
      };
    };

    let myCrash = new Crash();

    // 接着针对函式的原型添加属性
    Crash.prototype.cat = 3;
    Crash.prototype.dog = 4;
    console.log(Crash); // 只有原本的资料

    console.log("myCrash", myCrash); // 有新增的
    myCrash.secret.fn(); // {weight: 100, fn: ƒ}