instanceof type guard does not narrow type of this

Here's an example which demonstrates (tested on 1.6.2 and current playground):

class Foo {
    a: number;

    foo(): string {
        if (this instanceof Bar) {
            return this.b;  // not okay - `this` typed as Foo
        }
        return '';
    }

    bar(): string {
        let that = this;
        if (that instanceof Bar) {
            return that.b;  // okay - `that` typed as Bar
        }
        return ''
    }
}


class Bar extends Foo {
    b: string;
}

I think this is all related to polymorphic this #4910 which isn't in 1.6.2 (and playground is 1.6.2). Before that, you couldn't change the this type.

Kitson Kelly (@kitsonk)

Type guards currently only affect parameters and local variables, but I agree it seems reasonable to also have them apply to this. Kitson Kelly (@kitsonk) No, not related to the polymorphic this feature, the restriction is still there in current builds.

This should be working as expected in master.

Navigation Menu

Repository navigation

instanceof type guard does not narrow type of `this` #5534

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

instanceof type guard does not narrow type of this #5534

Description

Activity

kitsonk commented on Nov 5, 2015

ahejlsberg commented on Nov 5, 2015

mhegazy commented on May 6, 2016

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

instanceof type guard does not narrow type of `this` #5534