This content originally appeared on DEV Community 👩‍💻👨‍💻 and was authored by gao-sun
Intro
After the third try, we successfully migrated all existing Node.js code from CJS to native ESM, and the CI time of unit testing was significantly reduced.
Pull requests refactor: use ESM test: use native ESM
Before we start, I'd like to demonstrate the status quo ante for a better context. You may have different choices on the repo setting or toolchain, but the core steps and concepts should be the same:
- A TypeScript monorepo that includes both frontend and backend projects / packages.
- Total TypeScript code ~60k LOC (including frontend).
- Use
importin TypeScript. - Use PNPM for workspace management.
- Use
tscto compile Node.js, and Parcel to bundle frontend projects. - Use Jest +
ts-jestfor unit testing. - Use package
module-aliasfor internal path aliases.
BTW, our project Logto is an open-source solution for auth.
Why ESM?
When we noticed more and more NPM packages are "ESM-only", and we closed tons of PR because of it (except Parcel / Jest ones):
Disregard the war of ESM v.s. CJS, we found ESM does have several advantages:
No another-language-like code transpilation
Especially for TypeScript: Comparing the original code to the compiled version, ESM is much easier to read, edit, and debug.
Given a simple TypeScript snippet:
import path from 'path';
const replaceFile = (filePath: string, filename: string) =>
path.resolve(path.dirname(filePath), filename);
Results after tsc:
// CJS
"use strict";
var __importDefault = (this && this.__importDefault) || function (mod) {
return (mod && mod.__esModule) ? mod : { "default": mod };
};
Object.defineProperty(exports, "__esModule", { value: true });
const path_1 = __importDefault(require("path"));
const replaceFile = (filePath, filename) => path_1.default.resolve(path_1.default.dirname(filePath), filename);
// ESM
import path from 'path';
const replaceFile = (filePath, filename) => path.resolve(path.dirname(filePath), filename);
Top-level await
This is one of our favorites. Finally no need for wrapping top-level async expressions into a function and using void to execute it.
// CJS
(async () => {
await doSomething();
})();
// ESM
await doSomething();
Compatibility
- While ESM can easily load CJS modules, it'll be harder for CJS to load ESM. The primary reason is CJS
require()is synchronous, while ESMimportis asynchronous. You must useawait import()in CJS which is painful w/o top-level await. - Plus, CJS is Node-only, which means a universal package needs to compile another version for browser users. (We know there are transpilers, but, huh)
Stick with the standard
ESM is the module standard of JavaScript. TypeScript also uses ESM syntax by default.
Immutable
This is more like a double-edged sword. ESM can significantly improve module security by its immutable design (see this article), but it also brings a little inconvenience for test mocking.
Dramatic testing time reduction
Yes, we're talking about Jest. While Jest is sticking with CJS and only has experimental ESM support, we've been using ts-jest for a while, but obviously, it's a challenging task even for an M1 Pro MacBook. The fan spins with a noticeable sound, and the core temperature keeps going up while running unit tests.
After migrating all unit tests to ESM as well, my MacBook became silent and pro again! Here's the CI time comparison (with default machine spec. in GitHub Actions):
Execution time are not that stable in GitHub Actions, on average it shows 3x - 4x faster.
Code migration
For official docs, you may find Modules: ECMAScript modules and ECMAScript Modules in Node.js helpful.
Basic configuration
Let's start with tsconfig.json. Two key points:
- Set
compilerOptions.moduleResolutiontonodenextin order to tell TSC to use the "cutting-edge" Node.js module resolution strategy. - Set
compilerOptions.moduletoesnextto ensure the output also keeps ESM.
For Node.js, add "type": "module" to your package.json to let it treats the package as ESM.
Path aliases
We were mapping @/ to ./src/ using module-alias alias but it doesn't work in ESM. The good news is Node.js provides a native support called Subpath imports by defining the imports field in package.json:
{
"imports": {
"#src/*": "./build/*"
}
}
Note imports can only starts with # and must have a name. So we use #src/ to replace the original @/. Also update your tsconfig.json accordingly:
{
"compilerOptions": {
"paths": {
"#src/*": ["src/*"]
}
}
}
We also need to replace all @/ with #src/ in source code.
File extension
At this point, both TSC and Node.js start to work in ESM mode, but most likely some "Cannot find module..." errors float. Because the existing convention of importing is minimalistic and elegant:
import x from './foo';
// It can be
import x from './foo.js';
// Or
import x from './foo/index.js';
And the .js extension can be replaced with .jsx, .ts, and .tsx, etc.
However, this becomes unacceptable in native ESM. You must explicitly write the full path with the extension, e.g. import x from './foo/index.js';.
So how it should be in TypeScript? Our first idea is changing the file extension to .ts, which turns the path to './foo/index.ts', since that's the file we can find in the source directory, right?
Unfortunately, the TypeScript team has the principles like "TS is the superset of JS" and "TS doesn't rewrite paths". (You can see #13422 #16577 #42151 since 2017) So .ts doesn't work here, and it led to the result: use .js. :-)
It doe works, and I think I'm not qualified to judge the solution. So let's move to the actions we took to add extensions:
Since most packages in
node_modulesare not affected by this (at least for the main entry), we can omit them during the process.
- Replace all
from '\.'(RegExp) withfrom './index'. - Replace all
from '\./(.*)'(RegExp) withfrom './$1.js'. - If you have path alias, use the similar technique in step 2 to add extensions to them.
- Try to compile the project. It may show some errors for those paths with omitted
/index, e.g. a./foowhich actually points to./foo/index.js. They are updated to wrong paths like./foo.jsin step 2. - Try to compile again and it should show no error this time.
Misc.
It's exciting to see the output files are almost the same as the input, but when you run the project, Node.js may complain about some special variables, for example:
ReferenceError: __dirname is not defined in ES module scope
Don't give up, we're almost there! Read the Node.js official doc Differences between ES modules and CommonJS to handle them, then you're good to go.
Recap
We successfully migrated our Node.js packages from CJS to ESM. Now the entry-point file is runnable after tsc.
However, some issues still remain in unit tests:
- As of today (12/26/22), Jest only has experimental support for ESM.
- ESM is immutable, thus
jest.mock()will not work andjest.spyOn()also doesn't work on first-level variables (export const ...). This also applies to other test libraries like Sinon. - You may find some libraries for mocking ESM, but almost all of them are creating "a new copy" of the original module, which means if you want to import module A that depends on module B, you must import A AFTER B is mocked to get it to work.
The solution is the key to boosting the CI time. Since this article already pulled out a lot of things to digest, we'll cover them in the next chapter and provide a minimal workable repo then.
P.S. The first PR in the beginning only migrated code transpilation and Node.js config to ESM but left Jest in CJS mode. You may find the second PR test: use native ESM helpful before we publish the next chapter.
This content originally appeared on DEV Community 👩‍💻👨‍💻 and was authored by gao-sun
gao-sun | Sciencx (2022-12-25T16:57:52+00:00) Migrate a 60k LOC TypeScript (NodeJS) repo to ESM and testing become 4x faster (1/2). Retrieved from https://www.scien.cx/2022/12/25/migrate-a-60k-loc-typescript-nodejs-repo-to-esm-and-testing-become-4x-faster-1-2/
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