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Version: 2.1.x

Troubleshooting ts_project failures

ts_project is a thin wrapper around the tsc compiler from TypeScript. Any code that works with tsc should work with ts_project with a few caveats:

  • ts_project always produces some output files, or else Bazel would never run it. Therefore you shouldn't use it with TypeScript's noEmit option. If you only want to test that the code typechecks, use tsc directly. See examples/typecheck_only
  • ts_project needs to know which .ts sources exist. If you have a tool which produces an "opaque" folder of .ts files and you cannot predict what they will be, then you can use tsc directly.
  • Your tsconfig settings for outDir and declarationDir are ignored. Bazel requires that outputs are written beneath bazel-out/[target architecture]/bin/path/to/package.
  • Bazel expects that each output is produced by a single rule. Thus if you have two ts_project rules with overlapping sources (the same .ts file appears in more than one) then you get an error about conflicting .js output files if you try to build both together. Worse, if you build them separately then the output directory will contain whichever one you happened to build most recently. This is highly discouraged.

You can often create a minimal repro of your problem outside of Bazel. This is a good way to bisect whether your issue is purely with TypeScript, or there's something Bazel-specific going on.

Getting unstuck

The basic methodology for diagnosing problems is:

  1. Gather information from Bazel about how tsc is spawned, like with bazel aquery //path/to:my_ts_project.
  2. Reason about whether Bazel is providing all the inputs to tsc that you expect it to need. If not, the problem is with the dependencies.
  3. Reason about whether Bazel has predicted the right outputs.
  4. Gather information from TypeScript, typically by adding flags to the args attribute of the failing ts_project, as described below. Be prepared to deal with a large volume of data, like by writing the output to a file and using tools like an editor or unix utilities to analyze it.
  5. Reason about whether TypeScript is looking for a file in the wrong place, or writing a file to the wrong place.

Verbose mode

Running your build with --@aspect_rules_ts//ts:verbose causes the ts_project rule to enable several flags for the TypeScript compiler. This produces a ton of output, so you'll probably want to redirect the stdout to a file that you can analyze with power tools.

Problems with Persistent Workers

When Worker Support is enabled, we run tsc in a "watch mode" using the Bazel Persistent Worker feature. See the supports_workers attribute for docs on enabling this feature.

Non-deterministic behavior

Persistent workers risk leaking state from one compilation to another, and you may still encounter such bugs, for example:

  • removing a required types package from deps but the compilation still succeeds
  • outputs created by a previous compilation are still produced even though the source file is deleted

You can confirm that it's a worker bug by running bazel shutdown and trying again. If that resolves the issue, it means that some state was leaking.

Please check for issues with persistent workers: persistent workers label

Not reproducible ts_project worker bugs

Not reproducible ts_project, a.k.a. state, bugs has been a challenge for anyone to diagnose and possibly, fix in ts_project. Not knowing what state the worker has been at when it falsely failed, or what went wrong along the way is hard to know. For this, we introduced support for --worker_verbose flag which prints a bunch of helpful logs to worker log file.

If you find yourself getting yelled at by ts_project falsely on occasion, drop build --worker_verbose to the .bazelrc file. In addition to --worker_verbose, set extendedDiagnostics and traceResolution to true in the tsconfig.json file to log additional information about how tsc reacts to events fed by worker protocol.

"compilerOptions": {
"extendedDiagnostics": true,
"traceResolution": true

Next time, the ts_project yields false negative diagnostics messages, collect the logs files output_base and file a bug with the log files.

To collect log files run the command below at the workspace directory and attach the logs.tar file to issued file.

tar -cf logs.tar $(ls $(bazel info output_base)/bazel-workers/worker-*-TsProject.log)

This will help us understand what went wrong in your case, and hopefully implement a permanent fix for it.

Which files should be emitted

TypeScript emits for each file in the "Program". --listFiles is a tsc flag to show what is in the program, and --listEmittedFiles shows what was written.

Upgrading to TypeScript 4.2 or greater can be helpful, because error messages were improved, and new flags were added.

TS 4.1:

error TS6059: File '/private/var/tmp/_bazel_alex.eagle/efa8e81f99c35c1227ef40a83cd29a26/execroot/examples_jest/ts/test/index.test.ts' is not under 'rootDir' '/private/var/tmp/_bazel_alex.eagle/efa8e81f99c35c1227ef40a83cd29a26/execroot/examples_jest/ts/src'. 'rootDir' is expected to contain all source files.
Target //ts/src:src failed to build

TS 4.2:

error TS6059: File '/private/var/tmp/_bazel_alex.eagle/efa8e81f99c35c1227ef40a83cd29a26/execroot/examples_jest/ts/test/index.test.ts' is not under 'rootDir' '/private/var/tmp/_bazel_alex.eagle/efa8e81f99c35c1227ef40a83cd29a26/execroot/examples_jest/ts/src'. 'rootDir' is expected to contain all source files.
The file is in the program because:
Matched by include pattern '**/*' in 'tsconfig.json'

The --explainFiles flag in TS 4.2 also gives information about why a given file was added to the program.

Module not resolved

Use the --traceResolution flag to tsc to understand where TypeScript looked for the file.

Verify that there is actually a .d.ts file for TypeScript to resolve. Check that the dependency library has the declarations = True flag set, and that the .d.ts files appear where you expect them under bazel-out.

Source maps missing

TypeScript source maps can be configured in various ways which may effect compatibility with Bazel or other tools.

First, the tsconfig compilerOptions.sourceMap and associated ts_project(source_map) must be set to True to enable source maps. If these are misaligned the the ts_project(validate) will report an error.

Second, if compilerOptions.inlineSources is set not set to true then the .ts source files must be manually included alongside the compiled files to ensure they are present at runtime.

The recommended approach is using compilerOptions.inlineSources set to true to place the original TypeScript source code inline within the files, or compilerOptions.inlineSourceMap set to true to place the full sourcemap and original TypeScript source code within .js files. This way no extra bazel configuration is required to ensure the TypeScript source code is available when debugging.

NPM package type-checking failures

Strict dependencies and non-hoisted packages can cause type-checking failures when a package does not correctly declare TypeScript related npm dependencies. If a package exposes a dependency via TypeScript (such as publicly exporting a type from a dependency) then that dependency must be declared in the package.json dependencies in order for dependents to compile. Outside rules_ts with hoisted packages this may not be exposed if the missing dependency is declared in a parent or root package.json, however with strict dependencies in rules_js and bazel this will more likely be an issue.

Common solutions:

  1. TypeScript skipLibCheck will avoid type-checking within dependencies where this errors may be occurring.
  2. PNPM packageExtensions can be used to correct the dependencies of packages (normally by adding @types/* to the dependencies of the package).

Example type-checking errors due to use of devDependencies:

A React component from a library which does not declare @types/react as a dependency (JSX error):

src/index.tsx(40,10): error TS2786: 'X' cannot be used as a JSX component.
Its instance type 'X' is not a valid JSX element.
Type 'X' is missing the following properties from type 'ElementClass': setState, forceUpdate, props, state, refs

A React component extending a component from a library which does not declare @types/react as a dependency (JSX error):

src/index.tsx(55,12): error TS2786: 'Y' cannot be used as a JSX component.
Its instance type 'Y' is not a valid JSX element.
Type 'Y' is missing the following properties from type 'ElementClass': setState, forceUpdate, props, state, refs

Use of a library which does not declare @types/express as a dependency (type error):

src/index.ts(2,25): error TS7016: Could not find a declaration file for module 'express'. '/bazel-out/.../node_modules/express/index.js' implicitly has an 'any' type.
If the 'express' package actually exposes this module, consider sending a pull request to amend ''
  • Undeclared type-only dependencies (often @types/*). Type-checking is done at compile-time so type-only packages are normally npm devDependencies. However many packages contain type-definitions which expose those type-only package as part of their TypeScript API, this makes the types required for any downstream TypeScript compilation but the use of devDependencies means those packages are not available downstream.


error TS5033: Could not write file 'bazel-out/x64_windows-fastbuild/bin/setup_script.js': EPERM: operation not permitted, open 'bazel-out/x64_windows-fastbuild/bin/setup_script.js'.

This likely means two different Bazel targets tried to write the same output file. Use --listFiles to ask tsc to show what files are in the program. Try --explainFiles (see above) to see how they got there.

You may find that the program contained a .ts file rather than the corresponding .d.ts file.

Also see - it's possible that TypeScript is resolving a .ts input where it should have used a .d.ts from another compilation.

Webpack resolution failures when dropping ts-loader

The compilerOptions.paths property in tsconfig.json is often used for module names or simplifying import statements. A tool such as ts-loader enables Webpack to understand such paths.

When typescript compilation is moved to a separate step under rules_ts this understanding within webpack may be lost since it only sees JavaScript inputs.

Possible solutions:

  • tsconfig-paths-webpack-plugin webpack plugin for tsconfig paths (module names or just simplified import statements)
  resolve: {
plugins: [new TsconfigPathsPlugin({ configFile: 'tsconfig.json' })]
  • Use pnpm workspaces and npm_package/npm_link_package in between the ts_project rule and the webpack rule, so that the loader finds files under the node_modules tree like it would with third-party npm packages.