--allowImportingTsExtensions lets TypeScript imports name source files with extensions such as .ts, .mts, and .tsx. In a monorepo, that can suit code handled directly by a bundler or TypeScript runtime—but the flag does not link packages, select a runtime resolver, or make emitted JavaScript able to load TypeScript files.
Despite the title’s TypeScript 6.0 framing, this is not a new 6.0 feature: the TypeScript TSConfig reference marks the option as available since TypeScript 5.0. TypeScript 6.0 is relevant as the current-era context for choosing a module setup, not as the option’s introduction. TypeScript’s option reference and 6.0 release notes make that distinction clear.
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What the flag changes—and what it does not
Without this option, TypeScript generally does not allow TypeScript-specific extensions in import specifiers. Enabling it permits imports such as import { helper } from "./helper.ts". The same idea applies to .mts and .tsx files. The TSConfig reference describes the option as allowing TypeScript files to import one another with TypeScript-specific extensions.
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When a monorepo setup can use it
The key question is whether your development or build path consumes TypeScript source directly, or whether it emits JavaScript that must run later. The first case can fit this option; the second needs output-compatible imports.
Bundler or direct TypeScript runtime
A bundler that transpiles TypeScript in memory, or a direct TypeScript runtime or loader, can resolve and process source imports without depending on emitted JavaScript containing a .ts path. TypeScript’s module documentation names Node, Deno, and Bun as direct TypeScript runtime examples, and ts-node and tsx as transpiling loaders. The exact support and configuration depend on the host you use. TypeScript’s module theory guide explains these distinctions.
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For a no-emit check paired with a bundler, use a resolver mode that reflects that bundler. A minimal example is:
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{
"compilerOptions": {
"noEmit": true,
"allowImportingTsExtensions": true,
"module": "ESNext",
"moduleResolution": "bundler"
}
}This example assumes a bundler is responsible for executing or bundling the TypeScript imports; tsc checks the project but does not produce runnable JavaScript. TypeScript documents moduleResolution: "bundler" for bundlers and Bun. If your actual host is Node or another runtime, select the corresponding module and resolution settings rather than assuming this flag makes their behavior match. The module theory guide covers resolver selection.
JavaScript-emitting build
By default, TypeScript restricts allowImportingTsExtensions to configurations using noEmit or emitDeclarationOnly. The reason is practical: generated JavaScript ordinarily cannot resolve a path that still ends in .ts. Enabling the flag alone does not convert TypeScript import paths into JavaScript paths.
TypeScript 5.7 introduced rewriteRelativeImportExtensions, which rewrites relative .ts, .tsx, .mts, or .cts specifiers to their JavaScript equivalents in emitted output. If you are considering this route, check the precise option combination and defaults for the compiler version installed in the repository; do not rely on the permission flag alone. TypeScript’s module documentation explains why extension rewriting matters.
Keep workspace package imports separate from source-file imports
There are two different import relationships in a monorepo:
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- Within a package: a relative import such as
./helper.tsnames a source file. This is where the extension permission may be relevant. - Across packages: an application imports a sibling package by its package name, such as
@acme/shared. Workspaces should link that sibling intonode_modulesso TypeScript and the runtime or bundler resolve it as a package.
For example, a workspace-managed application should depend on the sibling package through its declared package name and import that name in code. Use npm, Yarn, or pnpm workspaces to establish the package link; the exact workspace configuration varies by package manager.
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TypeScript advises against using paths aliases to point directly at sibling package source when the goal is to check how consumers will resolve a published package. Such an alias can bypass the package’s metadata and hide resolution problems. Workspace package resolution tests the actual package boundary instead. TypeScript’s ESM and Node.js guide discusses workspaces and package resolution.
Choose by build path, resolver, and package boundary
| Setup question | What to check | Why it matters |
|---|---|---|
| Does the tool execute or bundle TypeScript directly? | Confirm the bundler, runtime, or loader resolves TypeScript source imports. | The flag permits the specifier; the host must make it work. |
| Does the build emit JavaScript? | Check whether relative extensions are rewritten, or use specifiers that work in output. | Unrewritten .ts paths can remain in emitted JavaScript. |
| Which host resolves modules? | Set TypeScript’s module-resolution mode to match that host. | Node, bundlers, Bun, and other environments do not necessarily follow identical rules. |
| Is the import within one package or across packages? | Use relative source imports within a package and workspace package names across package boundaries. | A direct source alias can bypass package metadata and fail to model consumer resolution. |
| Is the package private or intended for publication? | For a package meant for consumers, validate its package metadata and resolution path as a consumer would. | Repository-only links or aliases do not by themselves prove the published package will resolve. |
What TypeScript 6.0 adds to the context
TypeScript 6.0 is described in its release notes as a transition release toward TypeScript 7.0, with its own module-related changes. It did not introduce allowImportingTsExtensions; that option is documented as released in TypeScript 5.0. Treat the 6.0 version label as context for current configuration decisions, not as the feature’s origin. TypeScript 6.0 release notes provide the release context.
The same release notes report that some projects improved build time by 20–50% after setting the types option appropriately. That reported improvement concerns automatically included type packages; it is not a result of allowImportingTsExtensions and is not a guaranteed gain for monorepos. The release notes describe the separate change.
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