Property does not exist on type 'never' reported when it shouldn't

[hanginwithdaddo]

TypeScript Version: 3.0.3

Search Terms:
property does not exist on type 'never'

The code fragment below without any changes produces the following error when
using TypeScript 3.0.3/TSLint 5.11.0:

(44,47): Property 'b' does not exist on type 'never'.
Replacing the "if isNotDefined(options)" line in ClassB.modify() with either of the lines above it that are commented out stops this error from appearing.

Code fragment:

export function myIsDefined<T>(value: T | undefined | null): value is T {
  return <T>value !== undefined && <T>value !== null;
}

export function myIsNotDefined<T>(value: T | undefined | null): value is T {
	return <T>value === undefined || <T>value === null;
}

export interface ClassAModifyResult {
	a: boolean
}

export interface ClassBModifyResult extends ClassAModifyResult {
	b: boolean
}

export class ClassA {
	constructor(options?: any) {
		this.modify(options);
	}

	public modify(options?: any): ClassAModifyResult {
		var result = {} as ClassAModifyResult;
		if ( myIsNotDefined(options) )
			return result;

		result.a = true;
		return result;
	}
}

export class ClassB extends ClassA {
	constructor(options?: any) {
		super(options);
	}

	public modify(options?: any): ClassBModifyResult {
		// if ( !myIsDefined(options) )
		// if ( options === undefined || options === null )
		if ( myIsNotDefined(options) )
			return {} as ClassBModifyResult;

		var result = super.modify(options) as ClassBModifyResult;
		result.b = this._modifyBIfNecessary(options.b);
		return result;
	}

	protected _modifyBIfNecessary(value?: boolean): boolean {
		return true;
	}
}

I have seen other cases where using a method like myIsNotDefined() causes other issues for either TypeScript or TSLint.

Example:

	someMethod(value?: any | null): void {
		if ( myIsNotDefined(value) )
			return;

		if ( value.member === 'abc' )
			doSomething();
	}

With the code fragment above an error may appear stating that 'value' maybe undefined or null at the line that reads "if ( value.member === 'abc' )". Replacing the call to 'myIsNotDefined(value)' with either '!myIsDefined(value)' or 'value === undefined || value === null' stops the error from appearing.

[AlCalzone]

I think the problem is your definition of myIsNotDefined, specifically the return type (options is T) in combination with any.

if ( myIsNotDefined(options) ) {
    // Here, options is any
    return {} as ClassBModifyResult;
}
// Here, options is whatever remains after excluding any from the type, which results in never

When you define the function argument differently, the error changes.

[AlCalzone]

This works on the playground with strictNullChecks:

function isDefined<T>(a: T | null | undefined): a is T {
	return a !== null && a !== undefined;
}

function foo() {
	var a = window.getComputedStyle(null as any);

	if (isDefined(a) && isDefined(a.color)) {

		var match = a.color.match(/\d+/g);

		match[0]; // Error: Object is possibly null
		if (isDefined(match)) {
			match[0]; // works
		}
	}
}

[hanginwithdaddo]

... correct ... the isDefined() approach works, its just the isNotDefined() which is not working... sorry.

[hanginwithdaddo]

I misunderstood the use of type guards. Closing this ticket.

[AlCalzone]

isNotDefined also works if you don't combine it with ||:

function isNotDefined<T>(a: T | null | undefined): a is (null | undefined) {
	return a === null || a === undefined;
}

function foo() {
    var a = window.getComputedStyle(null as any);

    // works:
    if (isNotDefined(a)) {
        a; // null
    } else if (isNotDefined(a.color)) {
        a.color; // null
    }

    // doesn't work!
    if (isNotDefined(a) || isNotDefined(a.color)) {
        a; // CSSStyleDeclaration
        a.color; // string
    }
}

I'm not sure if that is a design limitation or a bug, since it is the exact opposite check of isDefined(a) && isDefined(a.color).

[hanginwithdaddo]

That is odd. Thanks for the additional notes. I'm just using isDefined() and !isDefined().

[AlCalzone]

@RyanCavanaugh while the original post seems to be a question, what about the behavior in #27041 (comment)? Is that a bug or limitation somehow?

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.