The Delete in JavaScript

So, why can we delete the properties of an object:

var x = {a:1};d elete x.a; TRUEX.A; Undefined

But you cannot delete a variable:

var x = 1;delete x; False;x; 1

You cannot delete a function:

function X () {};d elete x; False;typeof x; "Function"

• Note: Delete returns false only if a property cannot be deleted .

• Each property has 0 to many, such as internal properties--*readonly,dontenum,dontdelete and internal**. You can think of them as labels--a property may or may not have a particular intrinsic property. In today's discussion, we are interested in dontdelete.
• When variables and functions are declared, they become variable objects (Variable object)-Either the active object (in the function code) or the Global object (in the global Code)-the properties that accompany the internal property Dontdelete generated. However, any explicit/implicit assignment of properties does not generate Dontdelete. And that's essentially why we can delete some properties and not remove other reasons.

var GLOBAL_OBJECT = this;/* ‘foo‘是全局对象的一个属性，    它通过变量声明而生成，因此拥有内部属性DontDelete    这就是为什么它不能被删除*/var foo = 1;delete foo; // falsetypeof foo; // "number"/* ‘bar‘是全局对象的一个属性，    它通过变量声明而生成，因此拥有DontDelete子    这就是为什么它同样不能被删除*/function bar() {};delete bar; // falsetypeof bar; // "function"/* ‘baz‘也是全局对象的一个属性，    然而，它通过属性赋值而生成，因此没有DontDelete    这就是为什么它可以被删除*/GLOBAL_OBJECT.baz = "baz";delete GLOBAL_OBJECT.baz; // truetypeof GLOBAL_OBJECT.baz; // "undefined"

1.5, built-in and Dontdelete | Build-ins and Dontdelete

So that's why all this happens: a special intrinsic property of the property controls whether the property can be deleted. Note: Some properties of the built-in object have intrinsic properties dontdelete, so they cannot be deleted; Special arguments variables (as we know, properties of activated objects) have dontdelete; the length (return parameter length) property of any function instance also has Dontdelete:

(function () {    //Cannot delete ' arguments ' because there is dontdelete    delete arguments;//false;    typeof arguments; "Object"    //cannot delete the length of the function because there is dontdelete function    f () {};    Delete f.length; false;    typeof F.length; "Number"}) ();

Properties associated with the function arguments also have dontdelete and cannot be deleted

(function (foo,bar) {    delete foo;//False    foo;//1    Delete bar;//False    bar;//"Bah"}) (1, "bah");

1.6, undeclared variable assignment | Undeclared assignments

• As you may recall, an undeclared variable assignment becomes a property of the global object unless it is found elsewhere within the scope chain. Now we know the difference between attribute assignment and variable declaration-the latter generates Dontdelete and the former does not-that is why the undeclared variable assignment can be removed.

var GLOBAL_OBJECT = this;/* 通过变量声明生成全局对象的属性，拥有DontDelete */var foo = 1;/* 通过未声明的变量赋值生成全局对象的属性，没有DontDelete */bar = 2;delete foo; // falsedelete bar; // true

Note: Internal properties are determined when the property is generated, and subsequent assignments do not change the intrinsic properties of an existing property. It is important to understand this distinction.

/* ‘foo‘创建的同时生成DontDelete */function foo() {};/* 之后的赋值过程不改变已有属性的内部属性，DontDelete仍然存在 */foo = 1;delete foo; // false;typeof foo; // "number"/* 但赋值一个不存在的属性时，创建了一个没有内部属性的属性，因此没有DontDelete */this.bar = 1;delete bar; // true;typeof bar; // "undefined"


总结：

• Both variable and function declarations are properties of the active (Activation) Global object.
• The--dontdelete property has internal properties, one of which is responsible for determining whether a property can be deleted.
• variables, function declarations in global Code or function code generate properties that have dontdelete.
• The function parameters are also properties of the activated object and have dontdelete.

• Delete the properties in the object:Delete object . member

  You can only delete your own members

  only var the declared global variable does not let Delete

  Use window. or window[""] an increased global member can Delete

The Delete in JavaScript