Allow using 'as const' for creating tuples for map.replace

[Nokel81]

Code change checklist

• Added/updated unit tests
• Updated /docs. For new functionality, at least API.md should be updated
• Verified that there is no significant performance drop (yarn mobx test:performance)

This is really helpful after #3287 since that PR made it so that this use case failed to type check.

I added a test to make sure that this doesn't regress.

[changeset-bot]

🦋 Changeset detected

Latest commit: d92353b

The changes in this PR will be included in the next version bump.

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mobx	Patch

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[kubk]

Thank you for the PR. If you explicitly specify the type, this TS error will also disappear:

const dispose = mobx.reaction(
  () => {
    const values: Array<[string, number]> = Array.from(srcValues, (value, index) => [value, index])
    return values;
  },
  (entries) => map.replace(entries)
)

Considering that there is no replace method in ES 6 Map it might be a better alternative to complicate already complex types :) But there is one more issue.

This example compiles successfully:

const srcValues: string[] = [];
const map = new Map<string, number>(
  Array.from(srcValues, (value, index) => [value, index] as const);
);

This one doesn't

const srcValues = mobx.observable.array<string>()
const map = mobx.observable.map<string, number>(
  Array.from(srcValues, (value, index) => [value, index] as const)
)

That is because TS Map constructor has the following definition:

interface MapConstructor {
    new(): Map<any, any>;
    new<K, V>(entries?: readonly (readonly [K, V])[] | null): Map<K, V>;
    readonly prototype: Map<any, any>;
}

And your PR fixes the issue 👍 So LGTM from me to merge this. Let's wait for other opinions.

[urugator]

Is there a similar issue with Set or Array or possibly object api?

[Nokel81]

Maybe, but it is less likely to come up since there is less of a need to use as const since the source array isn't required to be nested.

[kubk]

@urugator this as const TS pattern is quite common when there is a need to convert an array to ES6 Map. So I doubt there is such an issue with Set/Array

[kubk]

@Nokel81 Could you also add this line to the test to make sure it is also covered?

const srcValues = mobx.observable.array<string>()
const map = mobx.observable.map<string, number>(
  Array.from(srcValues, (value, index) => [value, index] as const)
)

[urugator]

Can we somehow wrap this up?

[kubk]

@Nokel81 We're ready to go once you fix #3344 (comment) and #3344 (comment)

[Nokel81]

@kubk Sorry for the delay, done

[kubk]

@Nokel81 Thank you 👍