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Type validations do not carry over when assigned to a const #48891
Description
Bug Report
🔎 Search Terms
- Object is possibly 'undefined' when it shouldn't be
- TypeScript fails to detect when a value is not undefined
- TypeScript fails to detect when a value is not null
- TypeScript fails to detect when a value is not nil
- TypeScript fails to detect non-falsy values
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about
⏯ Playground Link
Playground link with relevant code
💻 Code
type Obj = {
test1?: string
test2: string | null
test3: string | false
}
const obj: Obj = {
test1: "hello",
test2: "hello",
test3: "hello",
}
function fn(arg: string) {
/** blah */
}
// A - ✓
// expected: error
fn(obj.test1)
fn(obj.test2)
fn(obj.test3)
// B - ✓
// expected: no error
if (!!obj.test1) fn(obj.test1)
if (!!obj.test2) fn(obj.test2)
if (!!obj.test3) fn(obj.test3)
// C - ✓
// expected: no error
if (typeof obj.test1 === "string") fn(obj.test1)
if (typeof obj.test2 === "string") fn(obj.test2)
if (typeof obj.test3 === "string") fn(obj.test3)
// D - x
// expected: no error
// behavior: error
const ok1 = !!obj.test1
const ok2 = !!obj.test2
const ok3 = !!obj.test3
if (ok1) fn(obj.test1)
if (ok2) fn(obj.test2)
if (ok3) fn(obj.test3)
// E - x
// expected: no error
// behavior: error
const ok4 = typeof obj.test1 === "string"
const ok5 = typeof obj.test2 === "string"
const ok6 = typeof obj.test3 === "string"
if (ok4) fn(obj.test1)
if (ok5) fn(obj.test2)
if (ok6) fn(obj.test3)🙁 Actual behavior
TypeScript can tell when the value of a variable is not falsy inside of an if block where the validation is directly used as the condition. But it fails to carry it over when the outcome of the validation is assigned to a const and that variable is used as the condition of the if statement instead.
🙂 Expected behavior
As long as the outcome of the validation is assigned to a const, TypeScript should deduce that the value is not falsy.
Activity
MartinJohns commentedon Apr 30, 2022
It's working as intended. See #44730.
oguzgelal commentedon Apr 30, 2022
Martin Johns (@MartinJohns) Thanks for pointing out the PR! Exactly the issue I'm facing.
In my code sample above, could you help me understand why there are errors in D and E when the expression is assigned to a
constandobjis also const (and not re-assigned) ?MartinJohns commentedon Apr 30, 2022
Because of this part:
obj.test1(and the other two) don't refer to aconstvariable, areadonlyproperty or a parameter.oguzgelal commentedon Apr 30, 2022
Ah I see! Got it. Thank you very much!
For reference, making
ObjtypeReadonlysolved the issue.