为什么object [undefined]在javascript中不起作用？

Question

I thought all values in between [ ] were converted to strings, why isn't undefined converted to "undefined" ?

var a={};
a[undefined];
Object.keys(a).length===0

Answer 1

I think you've basically found an edge case.

If I had a variable a that was undefined, calling myObject[a] should also return undefined , like you're referencing an object property that isn't there. Hardcoding undefined probably is equivalent to the interpreter as using an undefined variable.

Answer 2

You're assumption that all values between [] are converted to strings is incorrect.

That being said, what you're doing would work, if you actually assigned anything to your undefined key, but you aren't.

var a = {};
a[undefined] = 1;
Object.keys(a).length === 1;

Or even

var a = {};
a[undefined] = undefined;
Object.keys(a).length === 1;

If you don't assign a value to the key in a hash, the key is not retained.

EDIT

It's interesting how this language works. When you declare a variable without a value, JS initializes it with a value of undefined

var x;
console.log(x); //undefined

But when you attempt to declare a key on an object without assigning it to a value, then it is not initialized with a value of undefined ;

var x = {};
x[1];
Object.keys(x).length === 0;

Also, just for fun, it should be easy to see from the above that the following is true:

var x; //initializes to undefined
var y = {};
y[undefined] = x;
Object.keys(y).length === 1;