Spread Operator Doesn't Keep Object Type

[GeoMarkou]

Bug Report

🔎 Search Terms

spread operator interface object assign type

🕗 Version & Regression Information

Always

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Spread Operator

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Person {
    firstName: string;
    surname: string;
}

const father:Person = {
    firstName: 'John',
    surname: 'Smith'
};

// This is a "Person"
const daughter = Object.assign({}, father);

// This is not a "Person", it just happens to have all the same properties
const son = { ...father };

🙁 Actual behavior

Using the spread operator to clone an object doesn't retain the object type. This isn't a huge problem, but it makes the Intellisense harder to read, and annoys me. I also feel like it would use more memory / CPU validating another object type instead of just referencing the original.

🙂 Expected behavior

Using the spread operator to clone the object should keep the same type.

[IllusionMH]

Current behavior is more in line with what actually happens because const a = { ...b } in not always same type.
However would be nice to see proposed optimization when possible.

class WithPrivate {
    #hidden: string = 'hidden';
    visible: number = 1;
}

const withPrivate = new WithPrivate();

const noPrivate = { ...withPrivate };
const noPrivateError: WithPrivate = { ...withPrivate };

type SetterOnly= {
    set prop(val: string);
    regular: number;
}

declare const setterOnly: SetterOnly;

const noSetter = { ...setterOnly };
const noSetterError: SetterOnly = { ...setterOnly };

Playground

[GeoMarkou]

I see - thank you for your explanation.

In the examples given, doesn't that mean Object.assign is actually wrong? MDN says

The Object.assign() method copies all enumerable own properties from one or more source objects to a target object.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign

In your example the #hidden field is not enumerable, but Typescript lets you use Object.assign anyway. If the set prop represents a setter via Object.defineProperty it also has the same issue of not being copied (but it is "triggered" whatever that means).

https://www.typescriptlang.org/play?ts=4.5.0-dev.20210929#code/MYGwhgzhAEDqCWAXAFgBQE7wG5kQU2gG8AoaM6AYmXgBMa8A7ALmgkUwYHNoBeaAcmp1G-ANylyWeBHgAjEHhYMArgFtZedL2gBGcQF9ixYAHsGbaAHckaTDnzaGeS3BsZsuPAAoAlONPmiNAMJu72eACi6Ogm6EwIKGGe2oTQAHQZ1ol2yfr+ZhYhSfgAglDwnMwJth4OfADysgBWeMCIaZAylV6E+gA0Vm45+H5GiACeAA4EAMp4iPjo9Qwg4ykSZBDz0JMxk144ICxsHJyj5NDoeJzK4HHBahroBkYBFlsLmsurTHOfSys1nwSBcrjc7iwAAx9DY7PYsfj8Yh5V4FIIhP6LKIxLTA9IZD6Lb5rFEY+ZY6KxNK7EyTbT8ADMYlRgWCJkxmjKXQY2kaLTaHXK3V6A0JX0BozJ-y5FQY1L29IATGIgA

class WithPrivate {
    #hidden: string = 'hidden';
    visible: number = 1;
}

const withPrivate = new WithPrivate();
const noPrivateError:WithPrivate = { ...withPrivate };
const noPrivateAssign:WithPrivate = Object.assign({}, withPrivate);

type SetterOnly = {
    set prop(val: string);
    regular: number;
}

const setterOnly:SetterOnly = {
    regular: 0,
    prop: ''
};

const noSetterError = { ...setterOnly };
noSetterError.prop = '3';

const noSetterAssign = Object.assign({}, setterOnly);
noSetterAssign.prop = '2';

[MartinJohns]

@GeoMarkou The issue with Object.assign is missing #9726.

[icecream17]

Linking to a similar comment: #36554 (comment)