what exactly is the difference between doing abc.p = undefined and delete abc.p

I have an object referred to by the variable a开发者_如何学运维bc

Besides the return value of the expressions, what exactly is the difference between doing abc.p = undefined and delete abc.p

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> abc.p = undefined
undefined

> 'p' in abc
true

> delete abc.p
true

> 'p' in abc
false

Assigning a member to undefined doesn't make the member go away. It still occupies a place in the object (but its value is undefined). delete-ing it will actually remove the member.

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In this context, they serve the same purpose.

var abc = {};

abc.p = "hello";
alert(typeof abc.p);//string
delete abc.p;
alert(typeof abc.p);//undefined

abc.p = "hello world";
alert(typeof abc.p);//string
abc.p = undefined;
alert(typeof abc.p);//undefined

Here is a detailed explanation on delete: Understanding Delete by Kangax

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abc.p = undefined will not remove the property, i.e. it's still listed when iterating over the object. delete abc.p will remove it.

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abc.p = undefined will keep the property to the object but its value will be undefined.

The delete abc.p will remove the property completely from the object.

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One exception is when using "with" statement:

abc.p = undefined;
with(abc) {alert(p);} // shows: "undefined"

delete abc.p;
with(abc) {alert(p);} // an error occurs: "ReferenceError: p is not defined"