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TypeScript assertion signatures #10527

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DanielRosenwasser opened this issue Oct 4, 2019 · 6 comments · May be fixed by #10543

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@DanielRosenwasser
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commented Oct 4, 2019

TypeScript 3.7 will support an asserts Identifier is Type syntax in function annotation positions (much like with type predicates).

declare function assertIsFoo(x): asserts x is Foo;

function assertIsDefined<T>(x: T): asserts x is NonNullable<T> {
    if (x == null) throw "oh no";
}

See more at the original pull request: microsoft/TypeScript#32695

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commented Oct 4, 2019

Hey @DanielRosenwasser! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack community that typically always has someone willing to help. You can sign-up here for an invite."

@nicolo-ribaudo

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commented Oct 4, 2019

Will it be released in 3.7?

@tanhauhau

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commented Oct 6, 2019

anyone working on this? can I work on this issue?

@nicolo-ribaudo

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commented Oct 6, 2019

Yes 👍

@DanielRosenwasser

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commented Oct 7, 2019

Will it be released in 3.7?

Yes!

@DanielRosenwasser

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commented Oct 7, 2019

For the record, the grammar is something like

asserts [No LineTerminator here] Identifier is Type

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