Type 'null' is not assignable to type 'boolean | undefined' when using optional

[alan-agius4]

TypeScript Version: 3.0.1

Search Terms:
null checks
Type 'null' is not assignable

Code

interface Foo {
    prop?: boolean;
}

const bar: Foo = {
    prop: null
};

I am not sure, if it's working as expected with TypeScript 3, but I didn't find any reference of this change.

With strictNullChecks, when having an optional property and one assigns a null it now emits an error.

I tried asking on gitter, but no one confirmed if it's a bug or not.

Expected behavior:
no error.

Actual behavior:

Type 'null' is not assignable to type 'boolean | undefined'

[MartinJohns]

null is a different type and value from boolean and undefined. undefined === null would also result to false. So if you want this to run without an error, you have two options:

• Assign undefined, not null, because the type says it accepts either boolean or undefined.
• Change the type to include null, e.g. boolean | undefined | null (or prop?: boolean | null).

[alan-agius4]

Thanks for your response.

I know that null is different from undefined. But in Typescript 2.9 and previous versions in this particular usecase null and undefined used to be treated the same.

My question is, is this a bug or the change is intentional and undocumented?

[MartinJohns]

This results in absolutely the same behavior in TypeScript 2.9.1, 2.8.4, 2.7.1, 2.6.2, 2.5.3 and 2.4.2 (I just checked it).

Can you provide a minimal test case that reproduces the behavior as you claim it to be?

Most likely you're thinking of a project where strictNullChecks are disabled, in which case neither undefined nor null are considered types and they're interchangeably when assigning. Or maybe your actual use case differs and you run into the strict class initialization that was introduced with 2.7? https://github.com/Microsoft/TypeScript/wiki/What%27s-new-in-TypeScript#strict-class-initialization

[RyanCavanaugh]

This hasn't changed recently and the error is expected under strictNullChecks

[MartinJohns]

This is still not a bug. undefined and null are different values, and with strictNullChecks enabled different types. Optional properties only refer to undefined, even if it feels wrong to you.

[ollyde]

@MartinJohns sorry I was commenting on the wrong library 🙈