Understanding delete

简述

        我们都知道无法通过delete关键字针对变量和函数进行操作，而对于显示的对象属性声明却可以进行，这个原因需要深究到js的实现层上去，让我们跟随 Understanding delete 来探究一番，另外本文并不考虑浏览器的兼容性实现问题。

理论

            为什么我们可以这样：

  var o = { x: 1 };
  delete o.x; // true
  o.x; // undefined

         却无法这样

var x = 1;
  delete x; // false
  x; // 1

               其实，这要涉及到执行上下文的概念，而每个执行上下文都对应一个变量对象VO，在全局上下文中VO就是全局对象window，在函数上下文中，VO也是活动地向AO，而在eval中的代码在执行时，其执行上下文也就是调用eval的上下文。

         在上下文中定义的变量，函数声明以及函数的入参和AO特有的arguments对象等等，都属于VO（AO）的属性。而对于VO这样的实体对象而言，它也有自己的元数据，也就是在ES5中对象的数据特性：[[configurable]],[[enurable]],[[value]],[[writable]]。而对于VO的属性，默认的[[configurable]]是false，这样就无法针对这些变量使用delete操作。而对于显示的对象属性赋值，比如obj.name = “a”,对于name属性的[[configurable]]特性是true，因此可以删除。

var GLOBAL_OBJECT = this;

  /*  `foo` is a property of a Global object.
      It is created via variable declaration and so has DontDelete attribute.
      This is why it can not be deleted. */

  var foo = 1;
  delete foo; // false
  typeof foo; // "number"

  /*  `bar` is a property of a Global object.
      It is created via function declaration and so has DontDelete attribute.
      This is why it can not be deleted either. */

  function bar(){}
  delete bar; // false
  typeof bar; // "function"

  /*  `baz` is also a property of a Global object.
      However, it is created via property assignment and so has no DontDelete attribute.
      This is why it can be deleted. */

  GLOBAL_OBJECT.baz = 'blah';
  delete GLOBAL_OBJECT.baz; // true
  typeof GLOBAL_OBJECT.baz; // "undefined"

           另外，函数的length属性也是不可以删除的。

           而对于未初始化的变量赋值，我们知道未初始化的变量默认为全局变量，VO的属性确定是在进入上下文阶段，因此未初始化变量并不会成为VO的属性，[[configurable]]仍未true，可以删除。

/* `foo` is created as a property with DontDelete */
  function foo(){}

  /* Later assignments do not modify attributes. DontDelete is still there! */
  foo = 1;
  delete foo; // false
  typeof foo; // "number"

  /* But assigning to a property that doesn't exist,
     creates that property with empty attributes (and so without DontDelete) */

  this.bar = 1;
  delete bar; // true
  typeof bar; // "undefined"

            凡是都有例外，对于delete操作也难免。上述提到的第三种上下文--eval上下文，有个特殊的行为，就是在eval中声明的变量，函数可以在调用上下文中删除。

(function(){
    eval('var foo = 1;');
    foo; // 1
    delete foo; // true
    typeof foo; // "undefined"
    
    eval('var foo = 1;');
    foo; // 1
    delete foo; // true
    typeof foo; // "undefined"

  })();

ES5严格模式

         ES5的严格模式与上述提到的行为不同，它不准许delete删除函数入参，变量和函数，以及函数对象的length。删除未声明的 变量也会抛出语法错误SyntaxError。

(function(foo){

  "use strict"; // enable strict mode within this function

  var bar;
  function baz(){}

  delete foo; // SyntaxError (when deleting argument)
  delete bar; // SyntaxError (when deleting variable)
  delete baz; // SyntaxError (when deleting variable created with function declaration)

  /* `length` of function instances has { [[Configurable]] : false } */

  delete (function(){}).length; // TypeError

  delete i_dont_exist; // deleting undeclared variable (or in other words, unresolved Referece) throws SyntaxError
})();

总结

•   需要知道有哪几种上下文，每个上下文对应一个VO
•   上下文中定义的函数、变量、入参、arguments等都是VO的属性，[[configurable]]为false
•   eval上下文的特殊性
•   未声明变量并不是VO的属性，[[configurable]]为true
•   删除宿主对象属性时需小心，可能有意外发生，取决于js引擎的具体实现