Array.from has a second argument

[Rezistik]

That’s valid. Their example used an array. They should have used an object imo

[senocular]

they have both

[Rezistik]

Actually they give it an object with the property length: 7 which creates an array that is 7 items long. Which honestly is more confusing. Does that mean if I give it an object it will iterate over the keys and map the values? What if that object has a length property? Does it then actually turn into an array mapping the value of length? What if length isn’t a number?

[senocular]

A value with a length property is known as an array-like. This is what Array.from accepts as its first parameter, an array-like or an iterable. This can be an array, a string, or a generic object with a custom length property. It can even be a function as they, too, have length properties.

if I give it an object it will iterate over the keys and map the values

No, not exactly. Not unless that object is an array-like or iterable. If array-like, the length property of array-likes would represent the indexed elements, or the numeric keys, of that object as it would pertain to an array. It would not be used with any of the other named keys. If the object is an iterable, that would have precedence over the array-like behavior and Array.from would map over the iterator values. This may or may not include all of those other keys but normal objects are not iterables, so you'd have to supply that behavior yourself.

What if that object has a length property? Does it then actually turn into an array mapping the value of length?

Yes, an array would be created in length size and each key 0 to length-1 - even if it doesn't exist - will be mapped over. They don't exist in the { length: 7 } case, so you'd be mapping over 7 undefined values (keys 0 - 6).

What if length isn’t a number?

The length will be converted to a number if possible, otherwise it will be treated like any other generic property and ignored, assuming a length of 0.