In this post we will go into a bit deeper understanding of how an object is structured, enumerating properties of an object, property descriptors, ...
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They may also be Symbols.
It is also worth noting that Arrays are objects, and that while numbers are effectively converted to strings, in practice they are not, unless some operation is performed which requires the string value to be exposed.
So it may be better to say that they are referred to by strings and Symbols, rather than actually being those.
Couldn't have put that in a better way myself. Yes, after the introduction of Symbols in ES6, an object key can be either a string or a Symbol. Also, all keys in the square bracket notation are converted to string unless they're Symbols. Since this post was targeted to an audience who've just started with JS, I was trying not to get into Symbols for now :)
Let's consider x[0].
Here 0 probably actually won't be converted to a string, at least until you try to look at it in a way that requires presenting it as a string.
So, I think it's worth distinguishing between "X is Y" and "X is presented as Y in some situations." :)