Definitelytyped: [@types/lodash] Lacking type inference after chaining

[Semigradsky]

const x = _.chain([1]).value();

Type of x is ArrayLike<number>. But it should be number[];

/cc @aj-r

[aj-r]

This change was made as part of an effort to improve the performance of the lodash types. If this is a big problem then we can try to find a solution, but it may not be straightforward. What issue is this causing?

cc @sandersn

[IlyaSavich]

e.g.

function someFunction(): string[] {
    return _.chain(['A', 'B', 'C']).map((letter) => letter.toLowerCase()).value();
}

[Zjaaspoer]

I'm also having issues with the new typing since 4.14.125.

When lodash is returning an ArrayLike<number> which you assign to a number[] you get the error:

Type 'ArrayLike<number>' is missing the following properties from type 'number[]': pop, push, concat, join, and 25 more.

[aj-r]

@sandersn I wonder if we should add a new chain type for arrays - it could inherit from Collection<T> but value() would return T[] instead of ArrayLike<T>.

Or possibly modify Collection<T>.value() to return T[], even though that's not technically correct all of the time, it's probably correct 99% of the time.

[Semigradsky]

_.chain({ 0: 1, length: 1 }).map(_.id).value() // [1]
_.chain({ 0: 1, length: 1 }).filter(Boolean).value() // [1]
_.chain({ 0: 1, length: 1 }).sort().value() // {0: 1, length: 1}
_.chain({ 0: 1, length: 1 }).value() // {0: 1, length: 1}

[nfour]

@sandersn I wonder if we should add a new chain type for arrays - it could inherit from Collection<T> but value() would return T[] instead of ArrayLike<T>.

Or possibly modify Collection<T>.value() to return T[], even though that's not technically correct all of the time, it's probably correct 99% of the time.

We have to manually type cast to solve this now, so that kind of change sounds like less casting to me! Ship it!

[crashmstr]

This change would require me to make changes to several dozen .chains that are expected to return T[], but now are typed as ArrayLike<T>. I would consider the change that caused this to be a "breaking" change.

[sandersn]

@aj-r We could add Array (or some similar name) but I don't think subtyping would work. We'll have to duplicate all the Collection functions over to Array.

Might be better just to optimistically return Array.