Closed
Description
🔎 Search Terms
Optional Properties accept undefined
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the documentation
⏯ Playground Link
💻 Code
{
type T = {
DELETED?: string
}
let t1: T = {};
console.log( Object.keys(t1) ); // []
console.log( "DELETED" in t1 ); // false
let t2: T = {DELETED: undefined}; // WRONG
console.log( Object.keys(t2) ); // ["DELETED"]
console.log( "DELETED" in t2 ); // true
}
{
function foo(_a: number, _b?: string) {
console.log(arguments);
}
console.log("test 1");
foo(3); // [3]
foo(3, undefined); // WRONG [3, undefined]
}🙁 Actual behavior
In JS, there is a difference in behavior between properties that :
- doesn't exist (i.e. without value), and returns
undefinedwhen accessed. - exists, with
undefinedas a value.
TS enables to declare a property optional with the "?" symbol (so normally corresponding to case 1.). However, this notation also allows case 2., which shouldn't be the case:
{
type T = {
DELETED?: string
}
let t1: T = {};
console.log( Object.keys(t1) ); // []
console.log( "DELETED" in t1 ); // false
let t2: T = {DELETED: undefined}; // WRONG
console.log( Object.keys(t2) ); // ["DELETED"]
console.log( "DELETED" in t2 ); // true
}
{
function foo(_a: number, _b?: string) {
console.log(arguments);
}
console.log("test 1");
foo(3); // [3]
foo(3, undefined); // WRONG [3, undefined]
}🙂 Expected behavior
When assigning a value to a type with optional fields, TS should forbid undefined values except if explicitly authorized :
type T1 = {field?: string};
type T2 = {field?: string|undefined};
let t1: T1 = {field: undefined} // should raise an error.
let t2: T2 = {field: undefined} // okAdditional information about the issue
I get this would likely be a breaking change... but this is a bug.
Maybe a flag or a way to change TS behavior on this matter could help keeping retro-compatibility ?