const array = []
// ^? - const array: any[]
#Why is an empty array typed as `any[]`?
feral nacelleBOT
carmine lily
My experience says that an empty array is inferred as never[], but suddenly today I am getting any[] everywhere I try, which is troubling.
I can even downgrade to TS 4.9.5 and still get any[] both in VSCode and using tsc
strict / noImplicitAny = true, without errors.
compact summit
TS does some inference with unannotated array literals where the type adapts as you use it:
feral nacelleBOT
const x = [];
// ^? - const x: any[]
x.push(0);
console.log(x)
// ^? - const x: number[]
crystal urchin
Huh, I was fairly certain it only happened with let.
compact summit
If you don't ultimately do anything that infers the type, it'll error
feral nacelleBOT
function foo() {
const x = [];
// ^
// Variable 'x' implicitly has type 'any[]' in some locations where its type cannot be determined.
return x;
// ^
// Variable 'x' implicitly has an 'any[]' type.
}
I'm sure I've had errors where it wouldn't let me assign, because it was typed as never[]. Maybe that only occurs in certain situations.
compact summit
Huh, I was fairly certain it only happened with `let`.
It does similar things with let, I think. Doesn't even need to be arrays.
crystal urchin
I'm sure I've had errors where it wouldn't let me assign, because it was typed a...
If you force some type of inference to happen then yeah, eg:
feral nacelleBOT
const x = [] satisfies unknown[]
// ^? - const x: never[]
let x;
// ^? - let x: any
if(Math.random() > 0.5) {
x = "foo"
} else {
x = 0;
}
console.log(x);
// ^? - let x: string | number
Yeah I'm aware of the let's ability to refine its type as you use it, but I wasn't aware it happens with const arrays.
carmine lily
Hmm, exporting it makes it into never[]
feral nacelleBOT
export const array = []
// ^? - const array: never[]
Generics are a big place it'll infer never[]
feral nacelleBOT
const ex = Promise.resolve([]);
// ^? - const ex: Promise<never[]>
export let array = []
// ^? - const array: never[]
Makes sense I support
Alright thanks @compact summit , learned something new.
And @crystal urchin
compact summit
Yeah, this is pretty niche. Not sure I've ever used this behavior on purpose, but it's useful for making more JS code migratable to typescript with more type safety.
crystal urchin
I could've sworn this behavior never existed with const arrays but maybe I just remembered wrong.