What is incremental compilation?

Incremental compilation is a powerful feature in TypeScript designed to significantly speed up compilation times by only re-compiling parts of your project that have changed since the last build, along with their direct dependencies. This drastically improves developer productivity, especially in large codebases.

What is Incremental Compilation?

At its core, incremental compilation is a strategy where a compiler reuses information from previous compilations. Instead of performing a full build of the entire project every time a change is made, it identifies which files have been modified, determines their impact on other files, and then recompiles only the affected parts.

TypeScript achieves this by generating a special build info file (typically .tsbuildinfo) alongside your compiled JavaScript. This file stores a snapshot of the project's state, including file hashes, timestamps, and dependency graphs. On subsequent compilations, the TypeScript compiler (tsc) consults this file to make informed decisions about what needs to be recompiled.

How it Works

• Initial Compilation: When you build your project for the first time with incremental compilation enabled, TypeScript performs a full compilation and generates a .tsbuildinfo file (or one per project reference).
• Change Detection: On subsequent builds, tsc first checks the timestamps or content hashes of source files (and potentially declaration files) against the information stored in the .tsbuildinfo file.
• Dependency Tracking: If a file has changed, tsc consults its internal dependency graph (also stored in .tsbuildinfo) to determine which other files might be affected by this change. For example, if a type definition in a module changes, any file importing that type will also need to be rechecked or recompiled.
• Partial Recompilation: Only the changed files and their direct/transitive dependents are recompiled. Unchanged files are skipped, and their previously generated JavaScript output is reused.
• Build Info Update: After the partial compilation, the .tsbuildinfo file is updated to reflect the new state of the project.

Benefits

• Faster Build Times: This is the primary advantage, leading to significantly reduced compilation times, especially for large projects with many files.
• Improved Developer Experience: Developers get quicker feedback after making changes, leading to a more fluid and less disruptive development workflow.
• Efficient Resource Usage: By avoiding unnecessary work, incremental compilation consumes fewer CPU cycles and less disk I/O.
• Optimized for Wach Mode: It works seamlessly with tsc --watch, providing near-instantaneous compilation results for small changes.

Enabling Incremental Compilation

You enable incremental compilation by setting the incremental option to true in your tsconfig.json file. TypeScript will then automatically generate and manage the .tsbuildinfo file in your output directory (or root directory if outDir is not specified).

{
  "compilerOptions": {
    "target": "es2017",
    "module": "commonjs",
    "incremental": true, /* Enable incremental compilation */
    "outDir": "./dist", /* Specify output directory */
    "tsBuildInfoFile": "./dist/.tsbuildinfo" /* Optional: specify build info file path */
  },
  "include": [
    "src/**/*.ts"
  ]
}

The tsBuildInfoFile option allows you to explicitly define the path for the .tsbuildinfo file, which can be useful for organizing your build artifacts.

When is it Most Effective?

• Large Codebases: The speed benefits are most pronounced in projects with hundreds or thousands of TypeScript files.
• Monorepos with Project References: Incremental compilation is foundational for efficient builds in monorepos that utilize TypeScript's project references feature, allowing individual sub-projects to be built incrementally.
• Frequent Small Changes: During active development, where developers often make small, iterative changes, incremental builds provide immediate feedback.