Summary of TypeScript Performance

This is a summary of official TypeScript performance.

TL;DR

• Use interface over type for merging multiple types and a base types for serveral subtypes instead of a large union types.
• Annotate types especially for return types.
• Use project references for non-trivial size codebase.
• Setup include and exclude properly in tsconfig or jsconfig.
• Enabled flags that help speed up TypeScript: incremental, skipLibCheck, strictFunctionType, isolatedModules.
• If you working with 3rd party tools, read their performance suggestions carefully.
• Investigating issue with disabling editor plugin, run tsc with diagnotics and trace.
• Keep TypeScript up to date.

Writing Easy-to-Compile Code

Preferring Interfaces Over Intersections

Why:

When compose two or more types, extending types with interfaces / extends is suggested, cause intersection types recursively merge properties while interface create a single flat object type.

Using Type Annotations

Why:

Anonymous types by type inference may costly especially return types (export function).

Preferring Base Types Over Unions

Why:

A large union types (for function parameter) can cause a real problems in compilation speed, an alternative is a base type with common members and serveral subtypes, e.g. HtmlElemnt and DivElement ... ImgElement.

Using Project References

Why:

Split a non-trivial size codebase into serveral independent projects with project references can avoid loading too many files in single compilation and easier to organize difference layout strategies for certain codebase. (example)

Configuring tsconfig or jsconfig.json

Specifying Files

Why:

Exclude "heavy" folders like node_modules and include only needed folder may avoid walking through unnecessary folders which can slow down compilations. To avoid this, setup either files / include and exclude.

For best pratice, include only input source code folders, exclude any unrelated files and folder like artifacts, test, and dependency folder.

Controlling @types Inclusion

Why:

TypeScript automatically includes every @types package in node_modules, this slow down both compilation and editing scenarios.

This can solve by specifying empty field for types and include types manually.

Incremental Project Emit

Why:

--incremental flag allow TypeScript save last compilation state which help TypeScript re-check / re-emit the smallest set of files. Incremental is enabled by default when using composite flag for project references.

Skipping d.ts Checking

Why:

TypeScript performs a full re-check of all d.ts files which typically unnecessary. skipDefaultLibCheck can skip type-checking of the d.ts that TypeScript ships with, and skipLibCheck can skip checking all d.ts in a compilation.

Using Faster Variance Checks

Why:

Without --strictFunctionType, type parameter is check by structure comparison of type, member by member. Read more about variance in tsconfig and handbook.

Configuring Other Build Tools

TypeScript may working with other build tools, please make sure you read up about performance in your choice.

Concurrent Type-Checking

Why:

Serveral tools like fork-ts-checker-webpack-plugin or awesome-typescript-loader provides running type-checking in a saparate process which can reduce waiting time.

Isolated File Emit

Why:

Features like const enum and namespace needing to check other files inorder to generate output which make emit slower even more conflict with tool like Babel. isolatedModules can tell us which feature might not be supported.

Investigating Issues

Disabling Editor Plugins

Why:

Editor experiences can be impacted by plugins. If you have any troubles, try disabling plugins or reading their guides.

extendedDiagnotics

Why:

If the compile time over your expecting, try run TypeScript with --extendedDiagnotics to print out the compiler spending time info. For the detail about each fields, read this.

showConfig

Why:

tsconfig.json can extends other configuration files, running with showConfig will print the final calculate version.

traceResolution

Why:

With traceResolution, the reason of each file was included in a compilation is emit verbosely. If any files than shouldn't include, please review the include / exclude settings or even more like types, typeRoots or paths.

Running tsc Alone

Why:

Using 3rd party build tools with TypeScript may run into slow performance. Make sure you compare the diagnotics information and configuration between tsc and the build tool.

Upgrading Dependencies

Why:

Sometimes TypeScript's type-checking can be impacted by computationally intensive .d.ts files. This is rare, but can happen. Upgrading to a newer version of TypeScript (which can be more efficient) or to a newer version of an @types package (which may have reverted a regression) can often solve the issue.

Common Issues

Misconfigured include and exclude

Suggestion config for include and exclude

{
  // include only source files
  "include": ["src"],
  // prevent deeper node_modules and dot files
  "exclude": ["**/node_modules", "**/.*/"]
}

Filing an Issue

If your project is already properly and optimally configured, you may want to file an issue.

A few steps you can follow before file an issue:

• Make sure you're not hitting a resolved issue by testing your problem with the nightly version.
• Profiling the compiler by several flag like --trace-ic, generateCpuProfile.
• If the issue is about editing performance, please collecting a TSServer log in Visual Studio Code.