How to Fix "Cannot Find Module or Type Declarations for Side-Effect Import" in TypeScript

noUncheckedSideEffectImports in TypeScript 5.6

TypeScript 5.6 introduced the noUncheckedSideEffectImports compiler option to make this behavior stricter.

When enabled, TypeScript checks whether side-effect imports can be resolved.

This means an import such as:

import './styles.css'

must be resolvable from TypeScript's perspective.

If TypeScript cannot find the module or an appropriate declaration, you may see:

Cannot find module './styles.css'
or its corresponding type declarations.

The option is useful because it can catch genuine mistakes, but it also means TypeScript needs additional information for non-TypeScript assets such as CSS.

Why Doesn't @types/mapbox-gl Fix the Error?

You might think installing the Mapbox TypeScript definitions should solve the problem:

npm install -D @types/mapbox-gl