TypeScript should respect "instanceof" in logical expressions

Hello!

Thank you for your hard work.

Please, consider this code:

class Foo { 
    foo: string;
}
class Bar {
    bar: string;
}

function foo(value: Foo | Bar) {
    const isFoo = value instanceof Foo;
    const isBar = !isFoo && value.bar; // <-- causes error
}

The line above throws an error:

Property 'bar' does not exist on type 'Foo | Bar'.
Property 'bar' does not exist on type 'Foo'.

However, if I replace the logical variable with the expression directly, it works:

const isBar = !(value instanceof Foo) && value.bar;

The isFoo variable in my example is a boolean constant, it's value can't change, so I believe it should be statically analyzable.

Typescript don't track related variables.

Duplicate of #12184

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

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TypeScript should respect "instanceof" in logical expressions #31291

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TypeScript should respect "instanceof" in logical expressions #31291

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j-oliveras commented on May 7, 2019

JakeTunaley commented on May 8, 2019

typescript-bot commented on May 11, 2019

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