String union type Array.includes is problematic.

[pouyakary]

Bug Report

🔎 Search Terms

.includes + union

🕗 Version & Regression Information

Version 4.3.5

Playground link with relevant code

💻 Code

type MyType =
    "X" | "Y" | "Z"

const myArray: MyType[ ] = [
    "X", "Y", "Z"
]

let something = "string"

myArray.includes( something )

🙁 Actual behavior

TypeScript gives error on something, while if I knew it existed in the myArray, I wouldn't have used the .includes.

🙂 Expected behavior

My expectation is that I can send whatever string to .includes( ) and when getting back a true be sure that string is of type MyType. Then how else do you check if your string belongs to a union string type than having the code above? So I think the case of .includes( union type) it should accept all the primitive types that array excepts.

[MartinJohns]

Another duplicate of #26255 / #14520.

[typescript-bot]

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