TypeScript type modification when using defaults and object destructuring assignment

Question

I ran into a strange case today using TypeScript (v3.5.1) that's left me puzzled. I defined an interface with an optional property but TypeScript allowed me to set the default to whatever I want when using object destructuring:

interface IFoo {
    letter?: "a" | "b" | "c";
}

const foo: IFoo = {};

const { letter = 1 } = foo;

console.log(letter); // --> 1
                     // Type signature of `letter`: "a" | "b" | "c" | 1
                     // Why does TS modify the type to include 1?

Run this code in the TypeScript playground.

Using some type inspection I noticed that TypeScript is modifying the expected signature of "a" | "b" | "c" to "a" | "b" | "c" | 1. I was expecting to get a type error when I tried to default letter to 1. Am I missing something here? Thanks!

Your expectation isn't warranted since you are declaring a new local variable. Furthermore, the behavior you expect would be detrimental. — Aluan Haddad
– Aluan Haddad, Commented Aug 16, 2019 at 1:00

SpencerPark · Accepted Answer · 2019-08-16 00:35:39Z

2

The destructuring statement is introducing a new variable. It doesn't yet have a type unless one is assigned or in this case inferred.

Looking at the downcompiled code this becomes even more apparent:

// const { letter = 1 } = foo;
var _a = foo.letter, letter = _a === void 0 ? 1 : _a;

Or to clean it up a bit:

const letter = foo.letter === undefined ? 1 : foo.letter;

letter is either foo.letter or 1 if the former is undefined.

answered Aug 16, 2019 at 0:35

SpencerPark

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1 Comment

Pace Over a year ago

Yep. You are creating a new variable named letter. There is no rule that it has to have the same type as foo.letter

Fernando Flores · Accepted Answer · 2023-12-27 19:10:50Z

0

That is the default behavior. If you want to prevent that and get an error, you should type the destructuring:

const { letter = 1 } : IFoo = foo; // "IFoo" added, this should show the error: Type '1' is not assignable to type '"a" | "b" | "c"'

answered Dec 27, 2023 at 19:10

Fernando Flores

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