I want to have a function to test if a class variable is not null and use it in follow up function calls. But got TS plaints. Encapsulate this validate function because I need to call it in many of my methods.

class A {
  point: Point | null
  validatePoint() {
    if (!this.point) throw new Error()
  }

  update(p: Point | null) {
    this.validatePoint()
    // ts plains this.point can be null
    doSomething(this.point)
  }
}

I want to have a function to test if a class variable is not null and use it in follow up function calls. But got TS plaints. Encapsulate this validate function because I need to call it in many of my methods.

class A {
  point: Point | null
  validatePoint() {
    if (!this.point) throw new Error()
  }

  update(p: Point | null) {
    this.validatePoint()
    // ts plains this.point can be null
    doSomething(this.point)
  }
}

• javascript
• typescript

Share Improve this question Follow edited Jul 5, 2020 at 18:43 halfer 20.4k1919 gold badges109109 silver badges202202 bronze badges asked Nov 24, 2019 at 7:50 MengoMengo 1,27711 gold badge1212 silver badges2828 bronze badges

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4 Answers 4

Sorted by: Reset to default 6

Typescript 3.7 introduces assertions in control flow analysis:

class A {
    point: Point | null = null;
    validatePoint(): asserts this is { point: Point} {
        if (!this.point) throw new Error()
    }

    update(p: Point | null) {
        this.validatePoint()
        // now ts is sure that this.point can't be null
        doSomething(this.point)
    }
}

Playground

One option is to use a type guard instead, and throw in update instead of validatePoint:

class A {
    point: Point | null;
    validatePoint(p: Point | null): p is Point {
        return Boolean(this.point);
    }
    update(p: Point | null) {
        if (!this.validatePoint(this.point)) {
            throw new Error();
        }
        doSomething(this.point);
    }
}

But using a separate method for something like this seems a bit overkill:

update(p: Point | null) {
    if (this.point === null) {
        throw new Error();
    }
    doSomething(this.point);
}

If you have to put it in a separate function, then another option is for validatePoint to return the validated point (since this sort of thing can't be used to narrow the type of a parent object IIRC):

class A {
    point: Point | null;
    validatePoint() {
        if (this.point === null) {
            throw new Error();
        }
        return this.point;
    }
    update(p: Point | null) {
        const validatedPoint = this.validatePoint();
        doSomething(validatedPoint);
    }
}

You should be explicitly checking for null using an if(this.point === null) statement. Although I don't think you need a separate method for this, it seems a bit overkill.

In update, you validate if this.point is null.

If this.point is null, an error is thrown and doSomething is not invoked. If this.point is not null, that means this.point is Point type.

So it is not possible that this.point is null, when doSomething is invoked. That's why TS plains like that.