TypeScript doesn't respect declared function argument type definition

[jez9999]

Bug Report

🔎 Search Terms

argument type inconsistent respect function definition

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about 4.8.4

⏯ Playground Link

Playground link with relevant code

💻 Code

class Foo {
    constructor() {
        this.bar = 123;
    }
    bar: number;
}

function isSpecified<T>(input: undefined | null | T): input is T {
    return ((typeof input !== "undefined") && (input !== null));
}

function variableAsArg(inputVar?: number) {
    const fooInstance = new Foo();

    if (isSpecified(inputVar)) {
        console.log(`Var is specified: ${inputVar}`);
        fooInstance.bar = inputVar;
    }
    else {
        console.log("var is unspecified")
    }
}

function variableAsConst() {
    const fooInstance = new Foo();

    const inputVar: null | undefined | number = undefined;
    if (isSpecified(inputVar)) {
        console.log(`Var is specified: ${inputVar}`);
        fooInstance.bar = inputVar; // <-- Error
    }
    else {
        console.log("var is unspecified")
    }
}

const inputVar: null | undefined | number = undefined;
//variableAsArg(inputVar);
variableAsConst();

🙁 Actual behavior

When a variable is declared in the same block and assigned undefined, even when its type is explicitly declared to have | number, its type is treated as null | undefined rather than number | null | undefined when passed to a function. I guess TS thinks it's being helpful here, but it seems to me like annoying inconsistent behaviour that breaks my type guard function when I've explicitly told TS that the variable type should be considered | number. Because of this behaviour, in that circumstance, my isSpecified function no longer acts as a type guard against null | undefined and I get an error from TS.

🙂 Expected behavior

TS should always respect the explicit type that the variable is declared to be whether it's a function argument or declared in the same code block as a const.

[MartinJohns]

Assignment narrows the variable. This is working as intended. You can use the new satisfies operator instead.

[jez9999]

Could you give an example as to how satisfies fixes this problem?

[MartinJohns]

Nevermind, I talked without verifying. satisfies does not help. But you can use a type assertion to prevent narrowing: const inputVar = undefined as null | undefined | number;

[jez9999]

Yep, that does fix it. I still find it kind of odd that assignment narrow the type of a variable whose type is explicitly declared, but oh well. I mean in C#, int? foo = 2 wouldn't turn the variable into an int under any circumstances.

[MartinJohns]

C# doesn't have union types, so the argument is moot.

[RyanCavanaugh]

This used to not be the case, and people found that behavior very annoying and inconsistent, so we changed to this other behavior which is apparently also very annoying and inconsistent. Can't win 'em all 😅

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.