About JavaScript Medium delete operation

See a question , That's true :

delete x;
return x;
  • 1

  • null

  • undefined

  • Error

    I fast Error、、、、, But it's wrong , Run it for a while , The answer is 1.

    Can't you delete Parameters in a function ? What can be delete Of ? What kind of situation can't be delete Of ?

    I went to the Internet to check

    Usually :delete The operator is used to delete the attributes of an object .

    grammar :delete expression

    If successful return true, Failure to return false, But in strict mode , If the property is not configurable ,delete It throws an exception .

    If delete Operator deleted successfully , The deleted attribute will disappear completely from the object it belongs to .

"use strict"
var obj={
Object.defineProperty(obj, "name", { writable: true,
enumerable: true,
configurable: false
delete obj.name;

obj.name After setting to non configurable delete, In strict mode, the following exception is thrown

Uncaught TypeError: Cannot delete property 'name' of #<Object>

delete Operators can also be used to delete implicitly declared global variables , It just doesn't work var Declared global variables

There was a problem in the experiment

 var name="aa";
delete name;


 var name1="aa";
delete name1;

The runtime is different , In the first example , Will report a mistake . because name It's a built-in property 、、、

And marked DontDelete Some of the object's properties cannot be delete.

/* foo It's normally declared , So with DontDelete Mark , So it can't be deleted ! */
var foo = 1;
delete foo; // false
console.log(typeof foo); // "number"
/* bar Is declared as a function , Same with DontDelete, Cannot be deleted . */
function bar(){}
delete bar; // false
console.log(typeof bar);// "function" /* Function var The variable of , Same with DontDelete, Cannot be deleted .*/
function f() {
var z = 44;
delete z; // returns false

It should be noted that some built-in objects hold this tag automatically , So it can't be deleted , For example, in a function arguments, And functions length attribute , And the incoming Parameters .

If the variable is delete 了 , If you take it again, you will report an error ,index.html:11 Uncaught ReferenceError: x is not defined,x( A variable ) Can not find .

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