JavaScript class property becomes `any` without warning when used in a function that is assigned to itself

🔎 Search Terms

jsdoc class null any

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about any

⏯ Playground Link

https://www.typescriptlang.org/play?strictNullChecks=true&useDefineForClassFields=true&filetype=js#code/KYDwDg9gTgLgBAYwDYEMDOa4DEITgbwCg4S4B6AKguNLgrgAEwUoUBbAgOwFc2AjYFAD8AXzhsUATwEA5XjVIUyCxBE5oYUbghjQAFBOnA5bAJQEVtGAAsAlmgB0h2bzgBeODyRIA3IUukNvYOaMAwALJSLmx6pn60Iv60oRFRxryxFrRWdo7O6RwekTZOKCB6QXlpJnEqiSqU1Nn0TCzsBBpQtpwA5mKdlkoqMCgA1sBoAMqa3T16neb49bQAZriZRNmBuQ4j41MzvRU7+TXxW+RkF9cAenf3D-cAtC9PnniCUNAANOLVrvY4ChOJI6oREkA

💻 Code

export class Foo {
    /**
     * @param {number?} maybeNum
     */
    constructor(maybeNum) {
        this.maybeNum = null;

        this.setMaybeNum();
    }

    setMaybeNum() {
        this.maybeNum = Math.max(this.maybeNum);
    }

    /**
     * @param {string} str
     */
    takesString(str) {}

    foo() {
        this.takesString(this.maybeNum);
        //               ^^^^^^^^^^^^^---- no error, maybeNum is any
    }
}

🙁 Actual behavior

this.maybeNum is any

🙂 Expected behavior

this.maybeNum is number | null or (since noImplicitAny is set) an error is emitted warning the user that this.maybeNum is any.

Additional information about the issue

It seems like the core issue here is this line:

this.maybeNum = Math.max(this.maybeNum);

Start with the linked TypeScript Playground example and the assignment this.maybeNum = Math.max(this.maybeNum) in the JavaScript class. Trace how that assignment affects the later takesString(this.maybeNum) call, then verify the fix against the expected number | null type or a noImplicitAny diagnostic.