TypeScript Isolated Declarations: Real-World Performance Gains in Monorepo Build Times

Short summary

TypeScript's `--isolatedDeclarations` flag parallelizes declaration emit across monorepo packages, replacing sequential processing that slows builds with package count. Enable it in tsconfig.json with composite project references, then add explicit type annotations to exports. Teams report 3–15× build speedups; real example shows 47 seconds → 3.2 seconds in an 18-package repo, scaling linearly with CPU cores.

• •Enable `--isolatedDeclarations` in tsconfig.json with composite project references
• •Add explicit return type and object type annotations to all exported functions and constants
• •Expect 3–15× faster builds; gains scale linearly with available CPU cores and package count

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