Cannot annotate a field in constructor as null using jsdoc @type

TypeScript Version: 4.0.2

Search Terms: jsdoc null type implicit any

Code

class Foo {
    constructor () {
        /** @type {number} */
        this.bar = 42

        /** @type {null} */
        this.foo = null
    }
}

Expected behavior:

Expected to have a class with two fields, bar of type number & foo of type null

I don’t know why it doesn’t understand null; using string | null works fine.

Actual behavior: Member ‘foo’ implicitly has an ‘any’ type.

Playground Link: https://www.typescriptlang.org/play?useJavaScript=true#code/MYGwhgzhAEBiD29oG8BQ0PWPAdhALgE4Cuw+8h0AFAJQrqaMD0AVC9AAL4CeADgKYocxALYAjfoQC+0FkwaMM+ABYBLCADoxYSgF5oAFgBMqBYtbsufQcmEgQMuWcYr1GgGaJo+uyDNTUKSA

Related Issues:

[sandersn]

I can’t speak for the serializer code, since it’s much newer than getWidenedTypeForAssignmentDeclaration. However, the latter treats a lone null the same whether it comes from a type annotation or an initialiser – it’s regarded as an insufficient signal of intent and the code reports the error that @a-tarasyuk reports.

The correct fix is to treat an explicit type annotation differently from a null initialiser. However, I can’t think of a good reason for declaring a property of type null. @Raynos can you explain what you were using it for? Until we have a good, commonly used reason, I don’t feel like improving these semantics.

Meanwhile, I’m not sure what to do about the serialiser – it prioritises the type annotation for historical reasons. Ideally it should use the same rules as getWidenedTypeForAssignmentDeclaration, and one easy but expensive way to make that happen is to always request the actual type instead of short-circuiting on type annotation.

[Raynos]

@Raynos can you explain what you were using it for?

This workflow is for converting an existing JavaScript class into a JSDoc --checkJS // @ts-check class.

• I have a large existing class I want to introduce type safety to.
• The constructor is the first method in the class block
• A field is null in the constructor but is then updated to be some concrete object in some asynchronous method further down in the class
• I want to add a type annotation to every this field in the constructor so I can get explicit type checking
• Added @type {null} is valid for the constructor to type check and remove the red errors.
• There should now be an error in some other method further down the class where assigning a concrete object to null is a type error
• I can find that error and then update the type annotation in the constructor to be @type {null | MyThing}

I don’t need a @type {null} but its a useful temporary annotation to remove red errors linearly in my text editor / tsc CLI as I’m going through a untyped file.