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);

リンクされた TypeScript Playground の例と、JavaScript クラス内の代入 this.maybeNum = Math.max(this.maybeNum) から始めます。その代入が後続の呼び出し takesString(this.maybeNum) にどのような影響を与えるかを追跡し、その後、期待される number | null 型または noImplicitAny 診断に対して修正を検証します。