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js & void & undefined & null All in One

js & void & undefined & null All in One

The void operator evaluates the given expression and then returns undefined.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void


void function test() {
  console.log('boo!');
  // "boo!"
  return "undefined  === void 0 && null == void 0";
}();

try {
  test();
} catch (e) {
  console.log(e);
  // ReferenceError: test is not defined
}

/*

"boo!"

ReferenceError: test is not defined

*/


void 2 == '2';
// false

void (2 == '2');
// undefined

void


void 0;
// undefined

undefined === void 0;
// true

null === void 0;
// false

null == void 0;
// true


undefined == null;
// true

// undefined == false;
false

null == false;
// false


const log = console.log;

let varVoid = void 0;

log(`var void =`, varVoid);
// var void = undefined

undefined

const log = console.log;

let varDefault;

log(`var default =`, varDefault);
// var default = undefined

null

const log = console.log;

let vaNull = null;

log(`var null =`,  vaNull);
// var null = null

html

<body>
  <header>
    <h1>void(js expression)</h1>
  </header>
  <main>
    <article>
      <section>
        <div>
          <a href="javascript:void(0);">
            Click here to do nothing
            <span>javascript:void(0);</span>
          </a>
        </div>
        <div>
          <a href="javascript:void(1);">
            Click here to do nothing
            <span>javascript:void(1);</span>
          </a>
        </div>
        <div>
          <a href="javascript:void(true);">
            Click here to do nothing
            <span>javascript:void(true);</span>
          </a>
        </div>
        <div>
          <a href="javascript:void(document.body.style.backgroundColor='green');">
            Click here for green background
          </a>
        </div>
        <div>
          <a href="javascript:void(document.body.style.backgroundColor='white');">
            Click here for init background
          </a>
        </div>
      </section>
    </article>
  </main>
  <footer>
    <p>copyright&copy; xgqfrms 2020</p>
  </footer>
  <!-- js -->
  <script>
    const log = console.log;
  </script>
</body>

https://codepen.io/xgqfrms/pen/poyYGZw

js void 0 & undefined All In One

TypeScript void

interface Animal {
  speak(): void;
}

class Dog implements Animal {
  food: string = '';
  // override 字类覆盖父类方法
  constructor(food: string) {
    // super();
    this.food = food;
  }
  speak() {
    console.log("wang!");
  }
  eat(food: string) {
    // ?? => !== null && !== void 0 ? : 
    // void 0 === undefined
    console.log(food ?? this.food);
  }
}
const puppy = new Dog('meat');
puppy.speak();
puppy.eat('🐶');

"use strict";
class Dog {
    // override 字类覆盖父类方法
    constructor(food) {
        this.food = '';
        // super();
        this.food = food;
    }
    speak() {
        console.log("wang!");
    }
    eat(food) {
        // ?? => !== null && !== void 0 ? : 
        // void 0 === undefined
        console.log(food !== null && food !== void 0 ? food : this.food);
    }
}
const puppy = new Dog('meat');
puppy.speak();
puppy.eat('🐶');

image

void anything === undefined

void 0 === undefined;
// true

// void 0;
// undefined
void 1;
// undefined
// oid {};
// undefined
void [];
// undefined

void;
// Uncaught SyntaxError: Unexpected token ';'

https://gist.github.com/xgqfrms/f7196dc9f76ba13a80163fb6f910ed3b

refs

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posted @ 2020-09-27 12:37  xgqfrms  阅读(178)  评论(5)  编辑  收藏  举报