Cannot reassign undefined to the parameter of a function parameter destructuring with default value

🔎 Search Terms

"function parameter destructuring", "destructured parameter", "named parameter", "undefined"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "Why Method Bivariance?"

⏯ Playground Link

https://www.typescriptlang.org/play/?target=0&ts=5.7.0-dev.20241018#code/GYVwdgxgLglg9mABGOAnAtgQwDYAVOqboCmUxqAYuNPGABQEDmAXIgEZxzbGZIA+icABNiwGGGJDEAXkRRUIYgEpEAbwBQiLYiaI+fWcNHjJAbnUBfdetCRYCRCIDO8kNBCpJ+QiTKVq9vSqOqiMMnIKxIgWrMFMrBxcPPyCYCJiElIWKhoAkLr6hmnGmeZWNgG0jsQuCu6eQgDyAA6BON5EpORUdrR0caHhrlExaiGMAPwJnNy80Tnq+YOFqekmQmXWtjQOzq71XgSdfj07YACM-eNDkTpO7DPJeqslktGx49NJcwJGGW-ZNSLAoGF7-DaWLaVXY1fZQDyHHxdfy9BAAJiuulGAxYD2+KT+62i4RxrGG8yBSzCK0JpUhQA

💻 Code

function normalParameterFunction(arg: boolean | undefined = true) {
    arg ||= undefined;
}

function destructuredParameterFunction({ arg = true }: { arg: boolean | undefined }) {
	arg ||= undefined;
}

function destructuredOptionalParameterFunction({ arg = true }: { arg?: boolean }) {
	arg ||= undefined;
}

🙁 Actual behavior

The normalParameterFunction works fine, but in destructuredParameterFunction and destructuredOptionalParameterFunction, I cannot assign undefined to arg.

Type 'undefined' is not assignable to type 'boolean'.

🙂 Expected behavior

I know that if I specify the default value to the arg, the arg will no longer be undefined, but maybe I can reassign it to the undefined.

I guess it should have the same logic as the following:

var arg: boolean | undefined = true;
arg ||= undefined;

Additional information about the issue

• I don't know if it's really a bug or if it's a bad habit to change the value of a function parameter, but the function without parameter destructuring work as expected. Also, I hate recreate a new variable and come up with a new name in order to change the value of a function parameter, even though assigning the parameter itself directly doesn't modify the variable outside the function.

• The above code is a real-world use case that aims to set arg to undefined when it is a falsy value. In fact, arg ||= undefined is not needed, just arg = undefined will trigger the issue, but in this case, the original value of arg is ignored and has no practical meaning.

• You can also pass the type check in disguise with the following code:

function destructuredParameterFunction1({ arg = true as boolean | undefined }: { arg: boolean | undefined }) {
	arg ||= undefined;
}

function destructuredParameterFunction2({ arg }: { arg: boolean | undefined } = { arg: true }) {
	arg ||= undefined;
}

Inizia riproducendo i tre casi nel TypeScript Playground collegato e confrontali con gli esempi equivalenti di variabile locale e parametro presenti nell’issue. Traccia il modo in cui i valori predefiniti influenzano il tipo del parametro destrutturato; il lavoro è completo quando il comportamento è stato risolto in modo coerente con l’aspettativa documentata ed è coperto da un test di regressione.