How can I narrow the type of property of a class in an aliased conditional expressions?
Short: In a class method I want to do a type narrowing check such as this._end === null && this._head === null, but I want to first assign the result of this check to a variable before using it in a type narrowing if clause. It's not working as expected.
Not short: Let's say we have a Queue class:
class Queue {
private _length: number = 0;
private _head: Node | null = null;
private _end: Node | null = null;
public enqueue(node: Node): boolean {
this._length += 1;
const isQueueEmpty = this._end === null;
if (isQueueEmpty) {
this._head = node;
this._end = node;
return true;
}
this._end.setLink(node); // TypeScript doesn't understand that this._end passed the null check
// in the if statement and writes that this._end can be null.
this._end = node;
return true;
}
}
If you write a check in the enqueue function directly in the if condition, then TypeScript understands that this._end is not null:
public enqueue(node: Node): boolean {
this._length += 1;
if (this._end === null) {
this._head = node;
this._end = node;
return true;
}
this._end.setLink(node); // Now TypeScript understand that this._end passed the null check
this._end = node;
return true;
}
But the code turns out to be less descriptive.
Moreover, in general, I need to say that both properties of the class passed the null check. Specifically: this._head and this._end.
I wrote the following method:
type QueuePointerKeys = '_head' | '_end';
private _isQueueEmpty(): this is this & { [K in QueuePointerKeys]: null } {
return this._end === null;
}
And it doesn't work. If you put it in the enqueue method, then:
public enqueue(node: Node): boolean {
this._length += 1;
if (this._isQueueEmpty()) {
this._head = node; // TypeScript writes that a variable of type null cannot be assigned the type Node
this._end = node; // TypeScript writes that a variable of type null cannot be assigned the type Node
return true;
}
this._end.setLink(node); // TypeScript doesn't understand that this._end passed the null check
// in the if statement and writes that this._end can be null.
this._end = node;
return true;
}
How do I use a class method such as _isQueueEmpty for type narrowing? That works similarly to this._end === null && this._head === null used directly as the condition of the if statement?
Control Flow Analysis of Aliased Conditions and Discriminants was only recently added, and it has limitations, including:
Narrowing through indirect references occurs only when the conditional expression or discriminant property access is declared in a const variable declaration with no type annotation, and the reference being narrowed is a const variable, a readonly property, or a parameter for which there are no assignments in the function body.
Since this._end is not a readonly property, it's not going to work for you. I confirmed that this is your problem by assigning this._end to a local const variable, and then using it for the condition as well as the setLink call:
interface Node {
setLink(node: Node): void
}
class Queue {
private _length: number = 0;
private _head: Node | null = null;
private _end: Node | null = null;
public enqueue(node: Node): boolean {
this._length += 1;
const end = this._end
const isQueueEmpty = end === null;
if (isQueueEmpty) {
this._head = node;
this._end = node;
return true;
}
end.setLink(node);
this._end = node;
return true;
}
}
In fact, there is an open issue addressing your specific case: Control flow analysis of aliased conditions is not able to narrow object properties #46412, as well as Allow non-readonly properties to be used in aliased conditional expressions #44972.
Let's hope they can find solutions for these soonish 🤞🏾