TS2495: Type 'ArrayLike<T>' is not an array type or string type.

[electricessence]

for(let x of <ArrayLike<T>>source) { }

Error: TS2495: Type 'ArrayLike<T>' is not an array type or string type.

Why is this a thing? A for loop using ArrayLike<T> as a source should work for any ES target.

TS 2.6.1

[Jessidhia]

It definitely should not work for any ES target. It may happen to work if you are using typescript with downlevel iteration disabled, but that is a deliberate spec violation in the name of compatibility with old, non-polyfilled browsers.

The right hand side of for of is not ArrayLike, it's Iterable, from --lib es2015.iterable.

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EDIT: which means the message probably should be changed to say that the type is not Iterable 🤔

[electricessence]

@Kovensky
Isn't "ArrayLike" mean that it can be indexed (like an array), and that .length indicates the boundary of the indexes?
for of should still be constructed normally and work normally without error. Allow for some level of runtime failure.

If you look at ArrayLike<T>'s declaration it's pretty straight forward read only properties.

[Jessidhia]

yes, but for of does not care about .length or numerical properties.

for of only cares about the [Symbol.iterator]() property, and that it returns an iterator. Actual Arrays have a [Symbol.iterator]() that will return their elements in order; an arbitrary object that is ArrayLike potentially doesn't, such as the object I used in the TypeError screenshot.

If you do want to iterate over any ArrayLike, you can pass it as the argument to Array.from(), which converts both ArrayLike and other iterables into an ordinary Array (which is, again, itself iterable).

[electricessence]

Sorry still trying to wrap my head around why this is a thing.

Since the underlying JS code will work, just as it does with an array. So instead of an efficient for loop being generated under the hood, I now have to call Array.from()?

Maybe it's just my opinion, but this seems overly strict. And I normally prefer strict. :|

[electricessence]

for(let x of <ArrayLike<T>>source) { }

Can you provide a type alternative to ArrayLike<T> that will work? Given that source is potentially exactly that: an integer indexed (readonly) object?

[DanielRosenwasser]

Looks like a duplicate of #2862.

[electricessence]

Seems dupe-enough, but I still feel left uneasy because ArrayLike<T> should be enough to treat it like an array. :/ So now I need to use what type instead? Iterable<T>? Which is still not correct if what I really want is a simple for(var _i=0;... underneath. :/