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;
}

Start by reproducing the three cases in the linked TypeScript Playground and compare them with the equivalent local variable and parameter examples in the issue. Trace how default values affect the destructured parameter’s type; done means the behavior is resolved consistently with the documented expectation and covered by a regression test.