TypeScript Declaration Merging in 2026: Augmenting Third-Party Modules Without Losing Type Safety

Short summary

A technical guide to TypeScript declaration merging for augmenting third-party module types without losing type safety. It covers interface and namespace merging mechanics, module-scoped vs global augmentation, and the use of ambient .d.ts files with declare module syntax. The article includes code examples showing how to extend existing interfaces and explains which constructs support merging versus those that don't.

• •Declaration merging lets you extend third-party types in isolated .d.ts files without forking dependencies
• •Interfaces and namespaces merge automatically; classes and type aliases do not
• •Use module-scoped augmentation over global augmentation to avoid namespace pollution

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