Does using undefined as a property key guarantee the result will be undefined?

In a situation where an object property is retrieved through a variable i.e.:

myObject[someField]

there is the possibility of someField (which is a string) to be undefined (the result of a potentially uninitialized string value). My experiments show that for all types of objects I can think of the result is undefined i.e.:

anyObject[undefined] === undefined

Is this a well known behavior, something I can rely on? Can't seem to find something in related documentation and my alternative would be to re-write the above as

someField ? myObject[someField] : undefined;

but would really prefer the succinct way if there is a guarantee that undefined is returned whenever we try to access the property undefined.

No, accessing obj[undefined] will not always return undefined. Like any value used as property name, undefined will get coerced to a string (unless it is a symbol), so it will actually access a property named "undefined". obj[undefined] is equivalent to obj["undefined"] or obj.undefined. It will return the property value if such a property exists, e.g. when obj = {undefined: true};.

You should indeed write

someField != null ? myObject[someField] : undefined;

if someField: undefined | string.