The Complete Guide to Fixing TypeScript Errors in Next.js Projects

The Magic Command That Finds All Your Type Errors

Before we dive into fixes, you need to know about this super useful command that will show you exactly what's wrong with your TypeScript code:

npx tsc --noEmit --skipLibCheck

This command is like having a TypeScript detective that goes through your entire project and tells you exactly what's broken. Here's what each part does:

• npx tsc - Runs the TypeScript compiler
• --noEmit - Just checks for errors, doesn't actually compile anything
• --skipLibCheck - Skips checking library files (saves time and focuses on your code)

The Most Common TypeScript Errors (And How to Fix Them)

1. Missing Module or Type Declarations

This happens when TypeScript can't find a file you're trying to import. Usually, it's because:

• The file doesn't exist
• You've got the wrong path
• You forgot to export something

The Fix: Create the missing file or fix the import path. For example, if you're missing a types file:

// Create src/types/index.ts
export interface MyInterface {
  id: string;
  name: string;
}

2. Type Mismatches in Props

This is super common with React components. You're passing a value that doesn't match what the component expects.

The Fix: Check your component's prop types and make sure you're passing the right values. Instead of an empty string, use one of the allowed values:

// Instead of this:
<AdminHeader variant="" />

// Do this:
<AdminHeader variant="default" />

3. Missing Properties on Objects

This means you're trying to access a property that isn't defined in your type or interface.

The Fix: Add the missing property to your interface:

interface Alert {
  id: string;
  name: string;
  // Add the missing property
  comments?: Comment[];
}

4. Function Return Type Mismatches

This happens when a function is expected to return a Promise but you're not returning one.

The Fix: Make your function async or return a Promise:

// Instead of this:
const handleSubmit = (data: FormData) => {
  // do something
}

// Do this:
const handleSubmit = async (data: FormData): Promise<void> => {
  // do something
}

5. Accessing Properties on 'unknown' Types

This usually happens when you're working with API data or any variables that TypeScript can't figure out the type for.

The Fix: Add proper type assertions or create interfaces for your data:

// Quick fix with type assertion:
const price = (data as any).price;

// Better fix with proper typing:
interface PriceData {
  price: number;
  change: number;
}

const priceData = data as PriceData;
const price = priceData.price;

My Step-by-Step Process for Fixing TypeScript Errors

1. Run the magic command to see all errors at once
2. Start with the easiest ones first - missing imports, typos, etc.
3. Group similar errors together - if you have 10 errors about the same missing property, fix the interface once
4. Work on one file at a time - don't jump around, it gets confusing
5. Test as you go - run the command again after each fix

Advanced Fixes for Tricky Situations

Union Types and Literal Types

Sometimes you need to be super specific about what values are allowed:

// Instead of:
instrumentType: "traditional"

// Use:
instrumentType: "traditional" as const

// This ensures TypeScript knows it's exactly "traditional", not just any string

Module Resolution Issues

If you're getting weird module resolution errors, try updating your tsconfig.json:

{
  "compilerOptions": {
    "moduleResolution": "bundler", // Try this instead of "node"
    "allowSyntheticDefaultImports": true,
    "esModuleInterop": true
  }
}

When All Else Fails

If you're really stuck, here are some debugging tricks:

• Check your imports - Make sure you're importing from the right place
• Look at similar working code - Find a similar component that works and compare
• Use the TypeScript playground - Test small pieces of code in isolation
• Check for typos - Seriously, like 30% of TypeScript errors are just typos

The Bottom Line

Fixing TypeScript errors is mostly about being methodical and patient. Use that magic command I showed you, work through errors one by one, and don't be afraid to create proper interfaces for your data. Your future self (and your teammates) will thank you for writing type-safe code!

The key is to not get overwhelmed. Start with the command, identify the patterns in your errors, and tackle them systematically. Before you know it, you'll be a TypeScript error-fixing pro!