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?
How this differs:
tsimp
is strict consistency
with the "official" tsc
program, and just using it is the
simplest way to do that.--import
and Module.register()
behavior
added in node v20.6, only falling back to warning-laden
experimental APIs when that's not available.tsc --noEmit
step after running tests, using a persistent
sock daemon and a
generous amount of caching to make it performant.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.
extends
OptionsIf 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.
"module"
, "moduleResolution"
, and other must-havesThe 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.
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