How to use '! operator' with 'in operator' in TypeScript?

Question

I do programming with strikNullCheks option.

When I use ! operator with in operator, I got a following error. (This is exactly what happens when I use a payload of Redux's Action.)

Is there any way to solve the problem?

Thanks you for reading :)

interface Interface {
    AB?: {
        a: number;
    } | {
        b: number[];
    }
}

function test(ab: Interface['AB']) {
    const ab2 = ('a' in ab!) ? [ab!.a] : ab!.b;
    => error: Property 'a' does not exist on type '{ a: number; } | { b: number[]; }'. 
              Property 'a' does not exist on type '{ b: number[]; }'.
    => error: Property 'b' does not exist on type '{ a: number; } | { b: number[]; }'.
              Property 'b' does not exist on type '{ a: number; }'
}

example

Titian Cernicova-Dragomir · Accepted Answer · 2018-04-26 09:06:36Z

3

The in type-guard (and generally type-guards) will not work on expressions. So the expression ab! will not result in the narrowing of ab. For the type-guard to work, the guarded expression must be a simple parameter/variable/field. The simplest solution is to guard against null explicitly. This is probably a good idea since the field is optional anyway:

function test(ab: Interface['AB']) {
    const ab2 = !ab ? null : ('a' in ab) ? [ab.a] : ab.b;
}

answered Apr 26, 2018 at 9:06

Titian Cernicova-Dragomir

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1 Comment

left click Over a year ago

It helped. Thank you :D

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