A TypeScript IMPort loader for Node.js

This is an importer that runs Node.js programs written in TypeScript, using the official TypeScript implementation from Microsoft.

It is designed to support full typechecking support, with acceptable performance when used repeatedly (for example, in a test suite which spawns many TS processes).

There are quite a few TypeScript loaders and compilers available! Which one should you choose, and why did I need to create this one?

• swc is a TypeScript compiler implementation in Rust
• tsx is a zero-config TypeScript executer that aims to be a drop-in replacement for node, powered by esbuild.
• ts-node is probably the most established of these, with a huge feature set and support for every version of node and TypeScript you could possibly want.

How this differs:

• It uses the TypeScript implementation from Microsoft as its compiler. No shade towards swc and esbuild, they're fast and can do a lot, but the goal of tsimp is strict consistency with the "official" tsc program, and just using it is the simplest way to do that.
• It supports the --import and Module.register() behavior added in node v20.6, only falling back to warning-laden experimental APIs when that's not available.
• Type checking is enabled by default, so no need to run an extra tsc --noEmit step after running tests, using a persistent sock daemon and a generous amount of caching to make it performant.
• It's just a module loader, not a bunch of other things. So there's no repl, no bundler, etc. Pretty much all it does is make TypeScript modules in Node work.

Install tsimp with npm:

npm install tsimp

Run TypeScript programs like this in node v20.6 and higher:

node --import=tsimp/import my-typescript-program.ts

Or like this in Node versions prior to v20.6:

node --loader=tsimp/loader my-typescript-program.ts

Or you can use tsimp as the executable to run your program (but the import/loader is ~100ms faster because it doesn't incur an extra spawn call):

tsimp my-typescript-program.ts

Note that while tsimp run without any arguments will start the Node repl, and in that context it will be able to import/require typescript modules, it does not include a repl that can run TypeScript directly. This is just an import loader.

In Node v20.6 and higher, you can also load tsimp in your program, and from that point forward, TypeScript modules will Just Work.

Note that import declarations happen in parallel before the code is executed, so you'll need to split it up like this:

import 'tsimp'
// has to be done as an async import() so that it occurs
// after the tsimp import is finished. But any imports that the
// typescript program does can be "normal" top level imports.
const { SomeThing } = await import('./some-thing.ts')

By comparison, this won't work, because the imports happen in parallel.

import 'tsimp'
import { SomeThing } from './some-thing.ts'

CommonJS require() is patched as well. To use tsimp in CommonJS programs, you can run it as described above, or require() it in your program.

//commonjs
require('tsimp')
// now typescript can be loaded
require('./blah.ts')

In Node version 20.6 and higher, this will also attach the required loaders for ESM import support. In earlier Node versions, you must use --loader=tsimp/loader for ESM support.

Most configuration is done by looking to the nearest tsconfig.json file at or above the module entry point in the folder tree.

You can use a different filename by setting TSIMP_PROJECT=<filename> in the environment.

If there is a tsimp field in the tsconfig json file, then that will override anything else in the file. For example:

{
  "compilerOptions": {
    "rootDir": "./src",
    "declaration": true,
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,
    "inlineSources": true,
    "jsx": "react",
    "module": "nodenext",
    "moduleResolution": "nodenext",
    "noUncheckedIndexedAccess": true,
    "resolveJsonModule": true,
    "skipLibCheck": false,
    "sourceMap": false,
    "strict": true,
    "target": "es2022"
  }
  "tsimp": {
    "compilerOptions": {
      "skipLibCheck": true,
      "strict": false
    }
  }
}

Sourcemaps are always enabled when using tsimp, so that errors reference the approriate call sites within TypeScript code.

If the tsconfig.json file used by tsimp changes, then it will automatically expire its memory and disk caches, because new options can result in very different results.

However, while extends is fully supported (if tsc can load it, so can tsimp, because that's how it loads config), any extended config files will not be tracked for changes or cause the cache to expire.

When in doubt, tsimp --restart will reload everything as needed.

The ultimate resulting module style for tsimp must be something intelligible by Node, without any additional bundling or transpiling.

Towards that end, the module and moduleResolution settings are both hard-coded to NodeNext in tsimp, regardless of what is in tsconfig.json.

Also, the following fields are always hard-coded by tsimp:

• outDir Because tsimp isn't a build tool, but rather a module importer, it doesn't actually write the emitted JavaScript to disk. (Ok, technically it does, but only as a cache.) So, the outDir is hard-coded to .tsimp-compiled, but this is never used.
• sourceMap This is always set to undefined, because:
• inlineSourceMap is always set to true. It's just much simpler and faster to have the sourcemap inline with the generated JavaScript output.
• inlineSources is always set to false. There is no need to bloat the output, when the input is definitely present on disk.
• declarationMap and declaration are always set to false, because type declarations are not relevant.
• noEmit is always set false, because the entire point is to get the JavaScript code for Node to run. That said, the "emit" is fully virtual, and nothing is written to disk (except to avoid compiling the same code multiple times).

The same rules for file extensions, module resolution, and everything else apply when using tsimp as when using tsc.

That means: if you're running in ESM mode, you need to write your imports ending in .js even though the actual file on disk is .ts, because that's how TS does it when module is set to "NodeNext" and the target dialect is ESM.

Set the TSIMP_DIAG environment variable to control what happens when there are compilation diagnostics.

• TSIMP_DIAG=warn (default) Print diagnostics to stderr, but still transpile the code if possible.
• TSIMP_DIAG=error Print diagnostics to stderr, and fail if there are any diagnostics.
• TSIMP_DIAG=ignore Just transpile the code, ignoring all diagnostics. (Similar to ts-node's TS_NODE_TRANSPILE_ONLY=1 option.)

If the daemon is running, it's very fast, even if type checking is enabled. If the daemon is running and its previously compiled the file you're running, it's zomg extremely fast, like "so fast you'll think it's broken" fast, outperforming TypeScript compilers written in Rust and Go, since it literally doesn't have to do anything except check some file stats and then hand the cached results to Node. (In fact, since it caches in memory as well as to disk, it might even be faster in many cases than running plain old JavaScript, if the program is large.)

And, this is with full type checking, which is sort of the point of using TypeScript. No matter how fast your compiler is, if you're then running tsc --noEmit to check your types, then it's not actually gaining much.

If the daemon is not running, and it's a cold start with no cache, it's pretty slow, comparable with ts-node, especially if type checking is enabled.

An exceptionally not scientific example comparison:

$ time node --loader @swc-node/register/esm hello.ts
(node:89220) ExperimentalWarning: `--experimental-loader` may be removed in the future; instead use `register()`:
--import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register("%40swc-node/register/esm", pathToFileURL("./"));'
(Use `node --trace-warnings ...` to show where the warning was created)
hello, world

real	0m0.268s
user	0m0.255s
sys	0m0.033s

$ time node --import=tsx hello.ts
hello, world

real	0m0.135s
user	0m0.126s
sys	0m0.020s

$ time node --import=./dist/esm/hooks/import.mjs hello.ts
hello.ts:2:18 - error TS2322: Type 'string' is not assignable to type 'boolean'.

2 const f: Foo = { bar: 'hello' }
                   ~~~

  hello.ts:1:14
    1 type Foo = { bar: boolean }
                   ~~~
    The expected type comes from property 'bar' which is declared here on type 'Foo'

hello, world

real	0m0.126s
user	0m0.110s
sys	0m0.022s

Basic caching and work skipping.