Type narrowing for `if (!Array.isArray(a)) {` fails to eliminate a readonly array

[brad4d]

🔎 Search Terms

"Array.isArray", readonly, narrow, "!Array.isArray"

🕗 Version & Regression Information

• This is the behavior in every version I tried from the latest back to 4.9.5, and I reviewed the FAQ for entries about Array.isArray, narrowing, and readonly arrays.

⏯ Playground Link

https://www.typescriptlang.org/play?#code/GYVwdgxgLglg9mABMAFAQwFyLCAtgIwFMAnRAH0WMLQBMEAbAT2zyOIG0BdASkQG8AUIkQxgiFAEIAgsWJpGAOhgBnGXMbpuvQcOEQEyuPUIL6cAObpEAakRpuAbiGIAvgLcDQkWAkSXMLAQk5IFsXNrOouLSsvJKqrEa9hG6iPpghsamFla29k7Cbm5AA

💻 Code

function f(a: number | readonly number[]) {
  if (!Array.isArray(a)) {
    console.log(a + a); // error here
  }
}

function g(a: number | number[]) {
  if (!Array.isArray(a)) {
    console.log(a + a); // no error here
  }
}

🙁 Actual behavior

TypeScript reports "Operator '+' cannot be applied to types 'number | readonly number[]' and 'number | readonly number[]'."

for the a + a expression inside

function f(a: number | readonly number[]) {
  if (!Array.isArray(a)) {
    console.log(a + a); // error here
  }
}

🙂 Expected behavior

Expect that narrowing variable a of type number | readonly number[] using if (!Array.isArray(a)) { should narrow the type of a to number.
That is what happens when a is of type number | number[].

Additional information about the issue

No response

[Andarist]

Duplicate of #53395

This function is defined as (method) ArrayConstructor.isArray(arg: any): arg is any[]. A readonly array doesn't satisfy the return type of this predicate (a readonly array has no .push method and more) and thus it's not eliminated from the union.

[typescript-bot]

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