It appears that Typescript infers field types based solely on the private variable of a field, rather than taking into account the getter's return type union (1) or inferring from the getter itself (2):
test('field type inference', () => {
class A {
x_: number;
// 1: No type checking for the getter
get x(): number | undefined {
if (this.x_ === 1) return undefined;
return this.x_;
}
set x(v: number | undefined ) {
this.x_ = +v;
}
}
const a = new A();
a.x = 1;
// 2: Inferred type is number (based on x_, not getter)
const x: number = a.x;
console.log(a.x) // outputs 'undefined'
})
Questioning if this behavior is specified or expected?
Note that even with strictNullCheck, these issues are not caught. It only detects missing initialization and setter errors.
An updated example to fix warnings with strictNullCheck:
test('field type inference', () => {
class A {
x_: number;
get x(): number | undefined {
if (this.x_ === 1) return undefined;
return this.x_;
}
set x(v: number | undefined ) {
this.x_ = +(v ?? 0);
}
constructor(value: number) {
this.x_ = value;
}
}
const a = new A(2);
a.x = 1;
const x: number = a.x;
console.log(a.x)
});