Variable types don't seem to be filtered by comparing them via an array includes statement.

filterByArrIncludes.ts

abstract class SomeNode {
    public readonly termType!: string;
}

class NamedNode extends SomeNode {
    public readonly termType: "NamedNode";

    constructor() {
        super();
        this.termType = "NamedNode";
    }
}

class Literal extends SomeNode {
    public readonly termType: "Literal";

    constructor() {
        super();
        this.termType = "Literal";
    }
}

class BlankNode extends SomeNode {
    public readonly termType: "BlankNode";

    constructor() {
        super();
        this.termType = "BlankNode";
    }
}

class Collection extends SomeNode {
    public readonly termType: "Collection";

    constructor() {
        super();
        this.termType = "Collection";
    }
}

type SomeTerm = NamedNode | BlankNode | Literal | Collection | undefined;

function subset(x: NamedNode | BlankNode): void {
    console.log(x);
}

function example(x: SomeTerm): void {
    // if (!x || x.termType === "Literal" || x.termType === "Collection") does work however
    if (!x || ["Literal", "Collection"].includes(x.termType)) {
        return;
    }
    subset(x); // This should be valid
}

example(new NamedNode());
example(new BlankNode());
example(new Collection());