All primitives can be spread in objects according to MDN but TypeScript prevents it about typescript HOT 15 CLOSED

Are you absolutely sure? Because then it should work. That it does work when you use a type assertion indicates the type is not as you think it is.

string | null is the inferred type that VSCode showed me. But after rebasing my branch, TypeScript does not complain anymore, I can’t reproduce the error. I don’t know what happened but that’s good news.

I’ve finally understood TypeScript’s behaviour: you can actually spread any primitives, including null and undefined, since it’s allowed in JavaScript, but there must be a case where you spread an object:

const x = new Date().getSeconds() > 30 ? true : null;
const y = { ...(x && {}) };  // okay

const x = new Date().getSeconds() > 30 ? false : null;
const y = { ...(x && {}) };  // error

const x = new Date().getSeconds() > 30 ? true : undefined;
const y = { ...(x && {}) };  // okay

const x = new Date().getSeconds() > 30 ? false : undefined;
const y = { ...(x && {}) };  // error

const x = new Date().getSeconds() > 30 ? true : 0;
const y = { ...(x && {}) };  // okay

const x = new Date().getSeconds() > 30 ? false : 0;
const y = { ...(x && {}) };  // error

const x = new Date().getSeconds() > 30 ? true : '';
const y = { ...(x && {}) };  // okay

const x = new Date().getSeconds() > 30 ? false : '';
const y = { ...(x && {}) };  // error

So thank you all for your helpful comments. I’m closing this issue now.

from typescript.

The inferred type of someCondition is string | null:

Are you absolutely sure? Because then it should work. That it does work when you use a type assertion indicates the type is not as you think it is.

from typescript.

For null and undefined it's intentional: #57501. I suspect for the other types as well. Spreading them makes no sense and indicates a logic error.

from typescript.

What JavaScript allows at runtime and what TS allows at compile time are intentionally very different things.

Since these are no-ops, there's no reason to ever write e.g. { ...true } literally instead of just {}. And if you have something like boolean | Foo where it's only sometimes an object that you're spreading in, then that's likely to indicate a bug in your code, so TS errors accordingly.

from typescript.

Related #38469

from typescript.

Thanks for the quick feedback.

The use case is this (we have a lot of instances of it in the codebase at my current company):

const someObject = {
  someProperty,
  ...(someCondition && { someConditionalProperty })
};

It is perfectly valid in JavaScript but in TypeScript we have to write this instead, which is a little awkward:

const someObject = {
  someProperty,
  ...(someCondition ? { someConditionalProperty } : {})
};

from typescript.

This checks without error:

declare const someProperty: { x: string };
declare const someCondition: boolean;
declare const someConditionalProperty: { y: string };
const someObject = {
  someProperty,
  ...(someCondition && { someConditionalProperty })
};

What are your actual types?

from typescript.

The inferred type of someCondition is string | null:

import { useSelector } from 'react-redux';

const someCondition = useSelector(state => state.someCondition);

What is weird is that the spread does type check without error if I use a type assertion:

import { useSelector } from 'react-redux';

const someCondition = useSelector(state => state.someCondition) as string | null;

from typescript.

You seem to be reducing these examples so far that they stop working or making sense. I'd really like an actual example of this problem occurring to make sure nothing weird is happening.

from typescript.

I’ve finally understood TypeScript’s behaviour: you can actually spread any primitives, including null and undefined, since it’s allowed in JavaScript, but there must be a case where you spread an object:

Yes, as mentioned in this comment: #20153 (comment)

If you try to spread something that's known to always be a primitive (except string), then it indicates an error in your logic. Why would you want to spread that value? But if it may be an object (like in a union) it's allowed.

from typescript.

Exactly. We may improve the error message so that developers won’t be confused anymore like I was.

Instead of this message which is too restrictive

error TS2698: Spread types may only be created from object types.

what do you think of this message?

error TS2698: Spread types may only be created from types that include the object type.

Is it the standard wording in type theory to express the idea?

from typescript.

For things that are harmless but appear wrong, we've had good feedback (i.e. lack of complaint) about messages like "Spread appears unintentional because this value is not possibly an object". Open to discussion.

from typescript.

I like this wording too. To proceed forward, should I reopen this issue, create another issue, or do nothing?

from typescript.

Just a PR is fine

from typescript.

Done.

from typescript.