Use allowJs to Mix TypeScript and JavaScript

Overview

Migrating a large JavaScript codebase to TypeScript doesn't have to be all-or-nothing. The allowJs compiler option lets you gradually adopt TypeScript by mixing .ts and .js files in the same project. This enables incremental migration, allowing teams to convert files one at a time while maintaining a working codebase.

This approach is essential for large migrations where a big-bang rewrite isn't feasible.

When to Use This Skill

• Migrating JavaScript projects to TypeScript
• Gradually adopting TypeScript in a JS codebase
• Working with mixed TypeScript/JavaScript teams
• Converting large existing projects
• Teams still learning TypeScript

The Iron Rule

Use allowJs to enable incremental migration. Convert files module by module, starting from leaf modules and working up the dependency graph.