Partial<T, Keys> make only some properties (Keys) optional

[thernstig]

Suggestion

Extend https://www.typescriptlang.org/docs/handbook/utility-types.html#partialtype from Partial<T> to Partial<T, Keys>. If Keys is not specified, it defaults to all keys.

🔍 Search Terms

partial

I already found an old, automatically closed issue, here #25760. But I feel like it is warranted to create a new issue as this should be possible to implement without a backwards-incompatible change.

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

See above.

📃 Motivating Example

interface Todo {
  title: string;
  description: string;
}
 
function updateTodo(todo: Todo, fieldsToUpdate: Partial<Todo, 'title'>) {
  return { ...todo, ...fieldsToUpdate };
}
 
const todo1 = {
  title: "organize desk",
  description: "clear clutter",
};
 
const todo2 = updateTodo(todo1, {
  description: "throw out trash",
});

// fails: 'description' is required to be updated
const todo3 = updateTodo(todo1, {
  title: "groceries",
});

💻 Use Cases

I often have types where I want all properties to be non-optional, but e.g. when reading them for the first time in an API call, or reading a file, some are optional and then I immediately set them to something if they were not included.

[MartinJohns]

Just pointing out, Partial<T> is a straight forward simple mapped type. Partial<T, K> is a vastly more complex conditional type for a rare use case.

[thernstig]

Just pointing out, Partial<T> is a straight forward simple mapped type. Partial<T, K> is a vastly more complex conditional type for a rare use case.

How do you know it is rare? Juding from #25760 and how upvotes usually work, I'd say it is fairly much upvoted and in effect, not rare. And it is included in https://github.com/sindresorhus/type-fest#utilities as SetOptional.

[fatcerberus]

18 upvotes on a nearly 6-year-old feature request (and no dupes other than this one) in a repo as heavily trafficked as TS isn't really that much.

[thernstig]

+75 upvotes, #25760 (comment) in addition to the issue being closed Jul 30, 2018 (meaning many possibly miss it or do not care upvoting), I do not necessarily agree this is in the lows. VS Code, an arguably even higher trafficked repo, automatically moves 20+ upvote issues into their backlog.

But if there is a real reason why this is a an unwanted feature, maybe due to other ways to achieve this, I can see is not necessary.

[RyanCavanaugh]

We don't add utility types to the lib. Twitter thread on why

[fatcerberus]

tbf, this is a request to enhance an existing utility type. But I figured it would fall under that rationale regardless.

[RyanCavanaugh]

It wouldn't be possible to change Partial like that; it's homomorphic but the keys-based version by definition isn't.

[thernstig]

Got it. Though "utility types" are already present (https://www.typescriptlang.org/docs/handbook/utility-types.html):

TypeScript provides several utility types to facilitate common type transformations. These utilities are available globally.

But I am assuming the definition here is that you do not add utility types, that can be constructed from built-in utility types?

[MartinJohns]

They don't add utility types unless they're required for emit purposes. Most of the utility types present were added before they made this decision, and removing them would be a big unnecessary breaking change.