Stuck on an issue?

Lightrun Answers was designed to reduce the constant googling that comes with debugging 3rd party libraries. It collects links to all the places you might be looking at while hunting down a tough bug.

And, if you’re still stuck at the end, we’re happy to hop on a call to see how we can help out.

Type guard should infer the type of parent object when applied on a property

See original GitHub issue


🔍 Search Terms

Type guard, parent object, infer, inference

✅ 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

Typescript should be able to infer the type of an object, if a type guard was checked on a property of the said object. Currently, Typescript does correctly infer the type of the property, but not its parent object.

📃 Motivating Example

The following example is very common in data oriented design.

interface Type1 { list: string[] }

interface Type2 { list: { [key: string]: string } }

declare const obj: Type1 | Type2;

if(Array.isArray(obj.list)) {
    const list: string[] = obj.list; // WORKS
    const map: { [key: string]: string } = obj.list; // ERROR, as expected
    const objCasted: Type1 = obj; // ERROR, unexpectedly
} else {
    const map: { [key: string]: string } = obj.list; // WORKS
    const list: string[] = obj.list; // ERROR, as expected
    const objCasted: Type2 = obj; // ERROR, unexpectedly

The following example works and that is good because it is an equally common case in this type of design.

interface Type3 { type: 'type3', data: boolean }

interface Type4 { type: 'type4', data: string }

declare const obj2: Type3 | Type4;

if(obj2.type === 'type3') {
    const objCasted: Type3 = obj2; // WORKS
} else {
    const objCasted: Type4 = obj2; // WORKS

So I believe the type guards should work the same way. As far as I see, this does not cause any inconsistency in the language or the type system. It is an improvement without any downsides.

💻 Use Cases

See the full example.

Issue Analytics

  • State:open
  • Created 3 years ago
  • Reactions:33
  • Comments:12 (4 by maintainers)

github_iconTop GitHub Comments

RyanCavanaughcommented, Jan 19, 2021

I want to take another look at this; seems like we might have the right machinery in place now

bartenracommented, Mar 30, 2022

Here is a workaround for my use case:

function assertField<T extends { [field in Field]?: T[field] }, Field extends keyof T & string,>(
  obj: T,
  field: Field
): obj is T & { [field in Field]: NonNullable<T[field]> } {
  if (!obj[field]) false;
  return true;

type Slot = { id?: string }

function needsId(slot: { id: string }) {

function test(slot: Slot) {

  if ( {;  // string | undefined

    // needsId(slot)  ERROR

  if (assertField(slot, 'id')) {;  // string

    needsId(slot);  // WORKS


Read more comments on GitHub >

github_iconTop Results From Across the Web

Why typescript typeguard doesn't work for an internal property ...
Type 'DatabaseResponse' is not assignable to type 'MainResponse'. Types of property 'settings' are incompatible. Type 'string | null' is not ...
Read more >
How to use type guards in TypeScript - LogRocket Blog
The in type guard checks if an object has a particular property, using that to differentiate between different types. It usually returns a ......
Read more >
Typescript Instanceof Type Guard - TekTutorialsHub
Typescript instanceof operator checks if a value is an instance of a class/constructor function. It acts as TypeGuard & infers type in ...
Read more >
Type narrowing - mypy 0.991 documentation
callable() like in callable(obj) will narrow object to callable type ... Mypy can also use issubclass() for better type inference when working with...
Read more >
What are type guards in typescript? - Tutorialspoint
The 'in' type guard determines if an object contains a specific attribute, which is then used to distinguish between distinct types. It ...
Read more >

github_iconTop Related Medium Post

No results found

github_iconTop Related StackOverflow Question

No results found

github_iconTroubleshoot Live Code

Lightrun enables developers to add logs, metrics and snapshots to live code - no restarts or redeploys required.
Start Free

github_iconTop Related Reddit Thread

No results found

github_iconTop Related Hackernoon Post

No results found

github_iconTop Related Tweet

No results found

github_iconTop Related Post

No results found

github_iconTop Related Hashnode Post

No results found