Of course! TypeScript 5.5, released in June 2024, is a feature-packed update that brings significant performance improvements, new type predicates, and several other enhancements that make JavaScript development safer and more efficient.

Here is a comprehensive guide to the key features and changes in TypeScript 5.5.

1. The inferred Keyword in Template String Types

This is a powerful new feature for type-level string manipulation. It allows you to "capture" parts of a string template literal type during inference.

The Problem Before:
Previously, extracting parts of a string was cumbersome and often required complex recursive conditional types.

// Old way: Using a complex conditional type
type ExtractName<T> = T extends `Hello, ${infer Name}` ? Name : never;
type OldName = ExtractName<"Hello, Alice">; // type OldName = "Alice"

The New Way with inferred:

The new inferred syntax is more intuitive and declarative. It feels like destructuring at the type level.

// New way: Using the `inferred` keyword directly in the template
type NewName<T> = T extends `Hello, ${inferred Name}` ? Name : never;
type Name = NewName<"Hello, Alice">; // type Name = "Alice"

You can capture multiple parts at once:

type SplitAtUnderscore<S> = S extends `${infer First}_${infer Rest}` ? [First, Rest] : [S];
type Parts = SplitAtUnderscore<"foo_bar">; // type Parts = ["foo", "bar"]

Why it's better: It's syntactically cleaner and makes complex string parsing types much easier to write and read.

2. Regular Expression Syntax Checking

TypeScript can now validate your regular expression patterns at compile time! If you provide a string that is not a valid regex to APIs like RegExp constructor, TypeScript will give you an error.

// This is a valid regex
const validRegex = new RegExp("[a-z]+"); // OK
// This is NOT a valid regex! TypeScript 5.5 will catch this.
const invalidRegex = new RegExp("[a-z"); 
// Error: "Unterminated character class."

Why it's better: Catches a common class of bugs earlier, right in your editor, without having to run your code.

3. Isolated Declarations

This is a major feature for library authors and large monorepos. --isolatedDeclarations allows declaration files (.d.ts) to be generated in parallel, significantly speeding up build times.

What it does: It enables a build process where multiple .ts files can have their .d.ts declaration files generated simultaneously by different processes (e.g., using tsc --declaration --emitDeclarationOnly in a parallelized build tool).

How to use it: Add the flag to your tsconfig.json.

{
 "compilerOptions": {
 "declaration": true,
 "isolatedDeclarations": true
 }
}

Why it's better: Dramatically faster build times for projects that generate declaration files, especially in CI/CD pipelines.

4. Performance and Optimizations

TypeScript 5.5 includes massive performance gains in several key areas:

• Faster Incremental Builds (--build): The --build mode (used with project references) now avoids saving state to disk that is unnecessary for the next build, leading to 2-3x faster incremental build times in many scenarios.


• --watch Mode: The --watch mode has been rewritten to be more responsive and use less CPU.


• --isolatedDeclarations: As mentioned above, this enables parallelization, which is a huge performance win.


• Memory Usage: Reduced memory usage for object types, leading to faster type-checking and lower overall memory footprint.

5. Preserved Narrowing in Closures Following Last Assignments

TypeScript's control flow analysis has been improved to preserve narrowed types in closures, if the variable is not reassigned after the closure is created.

function validateInput(input: string | number) {
 if (typeof input === "string") {
 // input is narrowed to 'string' here
 setTimeout(() => {
 // In TypeScript 5.4 and earlier, `input` would revert to `string | number` here.
 // In TypeScript 5.5, it correctly remains 'string'.
 console.log(input.toUpperCase()); // OK!
 });
 }
}

Why it's better: More intuitive and correct type narrowing in asynchronous and callback-heavy code.

6. ECMAScript Set Methods

TypeScript now includes type definitions for the new ECMAScript Set methods, which are useful for mathematical set operations.

const firstSet = new Set([1, 2, 3]);
const secondSet = new Set([2, 3, 4]);
// Union: all elements from both sets
const union = firstSet.union(secondSet); // Set<1, 2, 3, 4>
// Intersection: elements present in both sets
const intersection = firstSet.intersection(secondSet); // Set<2, 3>
// Difference: elements in first set but not in second
const difference = firstSet.difference(secondSet); // Set<1>
// Symmetric Difference: elements in either set but not in both
const symmetricDifference = firstSet.symmetricDifference(secondSet); // Set<1, 4>
// Check if one set is a subset of another
const isSubset = firstSet.isSubsetOf(secondSet); // false

Why it's better: Provides type-safe access to these useful utility methods on Set objects.

7. Other Notable Changes

• Array.prototype.flatMap is No Longer readonly: The return type of flatMap is now writable, aligning with the actual ECMAScript spec behavior.


• Type Inference for Array.from: Improved inference when using Array.from with a mapFn argument.


• Experimental Support for get and set Accessors in Object.groupBy: Type definitions for this new JavaScript feature.

How to Get Started

• Install via npm:

npm install -D typescript@latest

npx tsc --init # to create a new tsconfig.json with the latest settings

• Update your tsconfig.json: Ensure your target is at least "ES2024" to get the full benefit of new lib definitions (like the Set methods).

{
 "compilerOptions": {
 "target": "ES2024",
 "lib": ["ES2024", "DOM", "DOM.Iterable"],
 "module": "ESNext",
 // ... other options
 }
 }

Summary

TypeScript 5.5 is a robust release focused on:

• Developer Experience: New inferred syntax and regex validation.


• Performance: Massive build speed improvements.


• Correctness: Better control flow analysis and updated lib definitions.