Typescript - assign null to variable

Question

I have a class like this:

export class Signal {
    method: (d: any) => void;
    otherMethod: (d: any) => void;


    public resetMethods(): void {
        this.method = null;
        this.otherMethod = null;
    }
}

unfortunately this will not compile anymore, until some previous version didn't give problems, now in the compilation phase I get the following error:

 Type 'null' is not assignable to type '(d: any) => void'.

for the structure of my code it is important to make those properties "null" and reassign them later, how can I remedy the compiler's complaints?

score 2 · Accepted Answer · answered Jan 16 '20 at 13:34

2

type Nullable<T> = T | null
export class Signal {
    method: Nullable<(d: any) => void> = null;

    public resetMethods(): void {
        this.method = null;
    }
}

Create custom type Nullable, very useful

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answered Jan 16 '20 at 13:34

Medet Tleukabiluly

11,662
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The other answers were also doable but i loved this one!! TNX – Plastic Jan 16 '20 at 13:41
2

`Nullable` is more general, nice one. – T.J. Crowder Jan 16 '20 at 13:44

score 1 · Answer 2 · answered Jan 16 '20 at 13:23

1

As the code is right now, you have to declare the fields as optional anyway, since they are not assigned a value. And it that case you could assign undefined to make TS happy:

export class Signal {
    method?: (d: any) => void;
    //    ^
    otherMethod?: (d: any) => void;
    //         ^


    public resetMethods(): void {
        this.method = undefined;
        this.otherMethod = undefined;
    }
}

If you really want/need to assign null, then you can use a union type:

export class Signal {
    method?: ((d: any) => void) | null;
    otherMethod?: ((d: any) => void) | null;


    public resetMethods(): void {
        this.method = null;
        this.otherMethod = null;
    }
}

answered Jan 16 '20 at 13:23

Felix Kling

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1,143

mmmmmm `(d: any) => void) | null;` so it wouldn't be like saying that the return type of the method is void or null ?? – Plastic Jan 16 '20 at 13:28
@T.J.Crowder: Without `?` typescript still complains because the fields don't have an initial value. If they do get a value in the constructor but it's left out in this example, then it's not needed. – Felix Kling Jan 16 '20 at 13:33
1

@Plastic: No, because of the parenthesis. – Felix Kling Jan 16 '20 at 13:33
Yeah, I was assuming the constructor definitely assigns it. :-) – T.J. Crowder Jan 16 '20 at 13:43

score 0 · Answer 3 · answered Jan 16 '20 at 13:25

Define a type for your "methods":

type MyMethod = (d: any) => void;

Then either declare them with that | null:

method: MyMethod | null;

Or give yourself a convenience type:

type NullableMyMethod = MyMethod | null;

and use that

method: NullableMyMethod;

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Typescript - assign null to variable

3 Answers3