Boolean type is not inferred correctly when a variable is declared with `let`

[GervinFung]

🔎 Search Terms

boolean

🕗 Version & Regression Information

Seems like this issue is easily reproduced for v3, v4 to v5

I personally had not encounter this issue as I use const when declaring variables almost all the time, might have been an intended design instead?

⏯ Playground Link

https://www.typescriptlang.org/play/?target=99&jsx=0&ts=5.8.3#code/DYUwLgBAhhC8EDMrAM4ggegxAlgOwRACciQATaFCAIgCMB7e0KPagKBi1wONIqirUkqENWgIwxAO5QiZKgApaAV0goAFvWXAKpALZR8lCAyYgW3FJKhkAlGzahItOBADkbzNnyES5Y9RWRPgA5uwuXFY4wMDcvnwBQaHsjuAQAMauAAxecbz+AjR4ynq0xOyZkWDRsT75-ILFpeVAA

💻 Code

let a = false // inferred as "boolean"
a // inferred as "false" afterwards (but should remain as boolean instead)

let b = '' // inferred as "string"
b // still inferred as "string"

let c = 0 // inferred as "number"
c // still inferred as "number"

🙁 Actual behavior

given let a = false

the typeof variable a should be boolean but instead was false

🙂 Expected behavior

given let a = false

the typeof variable a must be boolean, as it's inconsistent with other types such as ''/0 and it's inferred as string/number respectively

Additional information about the issue

No response

[MartinJohns]

This is working as intended. boolean is treated as a union of false and true.

You get the same behavior with other literal types, e.g.:

let b: 'a' | 'b' = 'b'
b // here typed as "b"

[GervinFung]

@MartinJohns that explanation make sense, but wouldn't it be better if it's

let t: T | U = somethingT
t // typed as T | U, instead of T

cause now the variable t re-assignable, it can be either T or U, but the compiler says the typeof t is T, which in a way, is not accurate. I know u can reassign it with any value that matches the type U

[MartinJohns]

No, that would not be better. Assignment narrows the type, this is intentional.

Your suggestion would make this code fail:

let t: string | number = "123"
t.toLowerCase()

When I assign a string, I expect to be able to use it as a string. Think of the type annotation as an information what type is allowed to be assigned, not what it always is at every moment.