Implicit Function Parameter Types won't Work in JavaScript #65086

🔎 Search Terms

implicit function lambda parameter types javascript checkjs ts-check

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about implicit function parameter type detection

⏯ Playground Link

https://www.typescriptlang.org/play/?filetype=js#code/DYUwLgBMD20NYFcAOEC8EAUB6AVDiAAmAJ5IgQDeAdggLYBGIATgL4Q5YQCWAJgJRoAfBABEVAIa0QIgNwAoGPGRpMvAamEiAUtAAWVCABFo0mRCycA6tCZwAznIXgI9BHa5UQduwBloAcy4AYxUKOQgIiH9wAFFaJBhiEBBsPEISMkoaBmY2Dm5+SnDIkqZwBCYDMUlTYoiWOQbXd09vP0CggDposDiE6CTydAw1IVEdfSMTWXNOMqQbMDsIZiYbCABiAHYABh2ANkcMDHVhMJLmjy9fAODu2PjE5JURwo1xvQNjU1mIAHcbPZGnwTvI5EFgOJvBAAEJuK5tW4hc6RHp9J4pXD4IikcjUOiMVjsTijFElCJlMAVKoSKQiOoQBoNJyQejAFSeP6w+GtG4dUFyNn3XqPAbPYajd7aT5TH4WCDzRbLVbrbZ7Q5yY6nIoXYDC9FioaqN6aCZfaZmeUA2wOFggvjyIA

💻 Code

let lookup = (/** @type {number} */ id) => "name";
lookup = (id) => "John Doe"; // Works

let businessLogic = {
    getEmployee(/** @type {number} */ id) {
        return "name";
    }
}
businessLogic.getEmployee = (id) => "John Doe"; // reports error #7006

(() => {
    businessLogic.getEmployee = (id) => "John Doe"; // works
})();

class BusinessLogic {
    getEmployee(/** @type {number} */ id) {
        return "name"
    }
}

let bl = new BusinessLogic();
bl.getEmployee = (id) => "John Doe"; // reports error #7006

(() => {
    bl.getEmployee = (id) => "John Doe"; // works
})();

🙁 Actual behavior

The parameter type inference of functions don't work under very specific circumstances:

• A function within an object-variable is being assigned
• The function is being assigned in a JavaScript file with either checkJs enabled or // @ts-check
• The function is being assigned outside of a function body

In these cases, the language server reports the error Parameter 'xxx' implicitly has an 'any' type. instead of detecting the type of the parameter from the type declarations.

🙂 Expected behavior

In the example code, the type of the id parameter should always be detected as number instead of reporting errors.

Additional information about the issue

No response