Make object type indexable

[dupasj]

🔍 Search Terms

"object record"
"object index"
"record index"
"property access"
"object unknown"
"guard object"

✅ Viability Checklist

• 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 isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

Explanation of the solving problem

When working with unknown, a very common runtime pattern to ensure an object-like structure is:

obj !== null && typeof obj === 'object'

However, TypeScript currently narrows this to object, which is not indexable and then prevents any property access.
(I checked with noUncheckedIndexedAccess enabled)

Actual workaround

The workaround that actually working for now:

const isRecord = (obj: unknown): obj is Record<string,unknown> => {
  return obj !== null && typeof obj === 'object'
}

const processUnknownObjectWithAssertion = (obj: unknown) => {
  if (!isRecord(obj)) return;
  obj.test; // Works because obj is Record<string,unknown>, then obj.test is unknown
}

Feature request

This leads to extra type check when it's sometime not expected. Would be nice to add an options to type obj !== null && typeof obj === 'object' as Record<string,unknown>.

Resource

Feel free to test the see the snippet here.

📃 Motivating Example

Current behavior:

const processUnknownObject = (obj: unknown) => {
  if (obj === null) return;
  if (typeof obj !== 'object') return;

  obj.test;  ❌ Error: Property 'test' does not exist on type 'object'
}

Expected behavior:

const processUnknownObject = (obj: unknown) => {
  if (obj === null) return;
  if (typeof obj !== 'object') return;

  obj.test; // Works because obj is Record<string,unknown>, then obj.test is unknown
}

💻 Use Cases

1. What do you want to use this for?
   Make the use of TS easier and more configurable.

2. What shortcomings exist with current approaches?
   Add an option in the tsconfig to guard obj !== null && typeof obj === 'object' as Record<string,unknown>.

3. What workarounds are you using in the meantime?
   Manually use a type assertion function:

const isRecord = (obj: unknown): obj is Record<string,unknown> => {
  return obj !== null && typeof obj === 'object'
}

[MartinJohns]

Essentially you want #38801.