TypeScript 5.5: The Features You'll Actually Use

TypeScript 5.5 came out and added a bunch of features. Some are useful. Most you'll never touch.

Here's what actually matters.

Inferred Type Predicates (Finally!)

This was annoying in TypeScript 5.4:

const numbers = [1, 2, null, 3, undefined, 4];

// TypeScript doesn't know this filters out nulls
const validNumbers = numbers.filter(n => n !== null && n !== undefined);

// Still thinks it could be null/undefined
validNumbers[0].toFixed(2); // Error: Object is possibly 'null'

You had to write a type predicate:

function isNotNull<T>(value: T | null | undefined): value is T {
  return value !== null && value !== undefined;
}

const validNumbers = numbers.filter(isNotNull);
validNumbers[0].toFixed(2); // Works!

TypeScript 5.5 figures it out automatically:

const numbers = [1, 2, null, 3, undefined, 4];

const validNumbers = numbers.filter(n => n !== null && n !== undefined);

validNumbers[0].toFixed(2); // Works! TypeScript knows it's number[]

Real Use Case: Filtering API responses. You get data with optional fields, filter out the nulls, TypeScript now knows they're gone.

interface User {
  id: string;
  email: string;
  phone?: string;
}

const users: User[] = await fetchUsers();

// Get only users with phone numbers
const usersWithPhones = users.filter(u => u.phone !== undefined);

// TypeScript knows phone exists here
usersWithPhones.forEach(u => {
  sendSMS(u.phone); // No more "possibly undefined" error
});

Regular Expression Syntax Checking

TypeScript now checks your regex patterns at compile time.

Before:

const pattern = /[a-z/; // Typo: missing closing bracket
// No error until runtime

Now:

const pattern = /[a-z/; // Error: Unterminated character class

Catches typos before you deploy. Small thing, but saves debugging time.

Real Example: We had a bug in production where a regex for validating Nigerian phone numbers had a typo. Took 2 hours to find. TypeScript 5.5 would've caught it immediately.

Performance Improvements

TypeScript 5.5 is faster. Like, noticeably faster.

Our largest project:

• TypeScript 5.4: 45 seconds to compile
• TypeScript 5.5: 28 seconds to compile

That's 17 seconds saved every time you build. Adds up when you're building 50 times a day.

The Stuff You Probably Won't Use

Control Flow Narrowing for Constant Indexed Accesses

Sounds fancy. Rarely useful.

function getLength(obj: { a: string } | { a: string; b: string }) {
  if ('b' in obj) {
    return obj.b.length; // Now works
  }
}

Cool, but how often do you write code like this?

JSDoc @import

Lets you import types in JSDoc comments. If you're using JSDoc instead of TypeScript files, you have bigger problems.

Should You Upgrade?

Yes, if:

• Starting new project
• Want faster builds
• Tired of writing type predicates

Maybe, if:

• Existing project is stable
• Team is comfortable with current version
• No urgent need for new features

No, if:

• Using old dependencies that break with 5.5
• In middle of big feature
• Team just learned TypeScript

Migration Tips

Upgrading is usually smooth, but:

1. Update all TypeScript-related packages:

npm install -D typescript@5.5 @types/node@latest

2. Check your build:

npm run build

3. Fix any new errors: TypeScript 5.5 is stricter in some cases. You might get new errors. Usually easy fixes.

4. Update your IDE: Make sure VS Code is using the right TypeScript version.

Real Project Impact

We upgraded 3 projects to TypeScript 5.5:

Project 1 (E-commerce site):

• Build time: 45s → 28s
• Found 2 regex bugs
• Removed 15 type predicates

Project 2 (Admin dashboard):

• Build time: 32s → 21s
• No breaking changes
• Cleaner filter code

Project 3 (API server):

• Build time: 18s → 12s
• Fixed 3 type errors we didn't know existed
• Happier developers

What We're Using

At Raspib, we've upgraded all new projects to TypeScript 5.5.

Using:

• Inferred type predicates (everywhere)
• Regex checking (caught 2 bugs already)
• Performance improvements (faster builds = happier devs)

Not using:

• Advanced control flow stuff (too niche)
• JSDoc imports (we use .ts files)

Common Issues

Issue 1: Stricter Null Checks

Some code that worked in 5.4 might error in 5.5:

// Might error now
const value = possiblyNull?.property;
doSomething(value); // Error if doSomething doesn't accept undefined

Fix:

const value = possiblyNull?.property ?? defaultValue;
doSomething(value);

Issue 2: Regex Patterns

If you have invalid regex, you'll find out now:

const pattern = /[a-z/; // Error!

Fix your regex or escape properly.

Bottom Line

TypeScript 5.5 is a solid upgrade. Faster builds, better type inference, catches more bugs.

No major breaking changes. Upgrade when convenient.

The inferred type predicates alone make it worth it. So much cleaner code.

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Need TypeScript help?

We build TypeScript projects for Nigerian businesses. APIs, dashboards, mobile backends.

📞 +234 905 162 3555
📧 info@raspibtech.com
📍 Lagos Island

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