TypeScript 5.5 introduces inferred type predicates, regex syntax checking, array filter improvements, and new Set methods—making code safer and more maintainable.
TypeScript has become an essential tool for modern web development. It is a language built on JavaScript that allows developers to declare and describe types. TypeScript 5.5, the latest iteration of Microsoft's type-infused superset of JavaScript, comes with new features compared to its previous update, 5.4, and introduces a better approach to writing code.
Writing types in our code helps explain intent and catch mistakes like typos, null/undefined issues, etc. Some notable features in version 5.5 include inferred type predicates, regular expression syntax checking, advancements in array filtering, and more.
Comparison to TypeScript 5.4
The new version empowers developers with better tools, smoother transitions, and improved code quality. Key improvements include:
- Inferred Type Predicates: Improves type tracking for variables, allowing better inference of types as they move through code. This enhancement catches more subtle errors during compilation, leading to safer code.
- Control flow narrowing for constant-indexed accesses: More accurate type narrowing when accessing properties dynamically.
- Regular expression syntax checking: Basic syntax validation on regular expressions improves performance and balances flexibility with correctness.
- Performance optimizations: Faster build and iteration times improve productivity during development.
- New EcmaScript Set methods: Union, intersection, difference, and symmetric difference methods for Sets make set operations more convenient and efficient.
New Features in Version 5.5
Improvement in Array Filter
Version 5.5 has enhanced type inference capabilities, particularly with the filter method. Previously, filtering an array to remove null or undefined values did not automatically narrow the type of the resulting array:
const nums = [1, 2, null, 4];
const filteredNums = nums.filter(num => num !== null);
// In 5.4: filteredNums is inferred as (number | null)[]
In the latest iteration, using a type predicate infers that filteredNums is number[], eliminating the need for further type checks:
const nums: (number | null)[] = [1, 2, null, 4];
const filteredNums = nums.filter((num): num is number => num !== null);
// filteredNums is correctly inferred as number[]
Object Key Inference Fixes
TypeScript now correctly infers the predicate type for object keys when using mapped types with certain type conditions, resolving issues that caused unexpected errors in previous releases.
Regular Expression Features
It is now possible to perform basic syntax checking on regular expressions while maintaining strict error handling for questionable escapes. The release strikes a balance between flexibility and correctness:
const regex55 = /\d{2,4}/; // Matches 2 to 4 digits
console.log(regex55.test("123")); // true
console.log(regex55.test("12345")); // true
console.log(regex55.test("1")); // false (less than 2 digits)
Set Methods
Version 5.5 introduces several new methods for Set objects that align with the latest ECMAScript standards:
- Union – combines two sets into one with all unique elements
- Intersection – creates a new set with elements present in both sets
- Difference – returns elements in the first set but not in the second
- Symmetric difference – finds elements in either set but not both
- isSubsetOf – checks if all elements of one set are in another
These methods streamline operations that previously required manual implementation with spread operators and filtering.
Conclusion
TypeScript 5.5, released in June 2024, brings meaningful updates that allow developers to work with optional types and complex structures more safely. The features discussed—array filter improvements, object key inference, regex enhancements, and Set methods—make the language more suitable for large-scale applications and improve compatibility with the broader JavaScript ecosystem.
Resources: TypeScript 5.5 Release, Official Documentation.