TypeScript 4.8 has been released with a number of correctness and consistency improvements that affect how intersection and union types work. The changes are used by TypeScript to narrow types.

TypeScript is a superset of JavaScript that adds optional static types which can be checked by the TypeScript compiler to catch common errors in your programs. TypeScript can use this information to help you avoid about mistakes like typos, missing arguments, or forgetting to check for null and undefined.

The improvements to managing correctness and consistency improvements under --strictNullChecks in the latest release affect how intersection and union types work. The team says that unknown is close in spirit to the union type {} | null | undefined because it accepts null, undefined, and any other type. TypeScript now recognizes this, and allows assignments from unknown to {} | null | undefined.

Another change means that NonNullable can use an intersection with {}, because {} & null and {} & undefined are discarded, meaning that {} intersected with any other object type simplifies to that object type. This is an improvement because intersection types like this can be reduced and assigned to, while conditional types currently cannot. The developers say that the changes also support "sensible improvements in control flow analysis and type narrowing".

Another improvement in the new release fixes a long-standing bug that until now has caused problems when some changes occurred in files in  --watch mode and editor scenarios. This sometimes manifested as displaying stale or inaccurate errors that might show up that require restarting tsc or VS Code. Developers reported the problems more on Unix systems, especially after saving a file with vim or swapping branches in git.

TypeScript 4.8 should no longer manifest the problem thanks to reworking of how it decides when to use a new watcher.

Other improvements include faster running of "find-all-references" in the editor. Because of a change to make TypeScript smarter as it aggregates references, the team says you should see improvements of about 20 percent in the time taken.

TypeScript 4.8 is available now.

• Ian Elliot is the author of Just JavaScript: An Idiomatic Approach; JavaScript Async;  Just jQuery: The Core UI; Just jQuery: Events, Async & AJAX and JavaScript Bitmap Graphics with Canvas. All these titles are part of the I Programmer Library published by I/O Press.

More Information

TypeScript On NuGet

TypeScript On GitHub

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