Inferred Type Predicates

It's finally here! While working with TypeScript, I assume we all have been in a situation where we have to write a type predicate to help TypeScript understand the type of a variable. For most cases this is fine, but it can be a pain to write these while using the filter method.

Now with the TypeScript 5.5 release, TypeScript introduces a new feature called Inferred Type Predicates. This feature allows TypeScript to infer type predicates automatically. This means that TypeScript can now understand the type of a variable or return value based on the conditions that are checked in the code.

Functions that previously required a Type Predicate can now be simplified. This is a great improvement for developers, as it reduces the amount of boilerplate code needed to write type predicates.

 
type Person = { name: string };
const persons: (Person | null)[] = [];
 
const personNames = persons.filter((person) => !!person).map((person) => person.name); // ^? Person

 
type Person = {name:string};
const persons: (Person|null)[] = []
 
const personNames = persons
  .filter(person => !!person)
  .map(person => person.name)
                 ~~~~~~ 'person' is possibly 'null'.
                 // ^? Person | null

Take a look at the following TypeScript playground examples to see the difference between TypeScript 5.5.0-beta and TypeScript 5.4.5.

Additional resources

• Announcing TypeScript 5.5 RC by Daniel Rosenwasser - the official announcement of the feature
• The Making of a TypeScript Feature: Inferring Type Predicates by Dan Vanderkam - why and how Dan implemented this feature
• Type Predicate Inference: The TS 5.5 Feature No One Expected by Matt Pocock - a detailed explanation of the feature (with examples)