ESM Loaders: Enhancing Module Loading in Node.js

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Native ESM support for Node.js was a chance for the Node.js project to release official support for enhancing the module loading experience, to enable use cases such as on the fly transpilation, module stubbing, support for loading modules from HTTP, and monitoring.


While CommonJS has support for all this, it was never officially supported and was done by hacking into the Node.js runtime code. ESM has fixed all this. We will look at the architecture of ESM loading in Node.js, and discuss the loader API that supports enhancing it. We will also look into advanced features such as loader chaining and off thread execution.

Gil Tayar
Gil Tayar
22 min
05 Jun, 2023

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Video Summary and Transcription

ESM Loaders enhance module loading in Node.js by resolving URLs and reading files from the disk. Module loaders can override modules and change how they are found. Enhancing the loading phase involves loading directly from HTTP and loading TypeScript code without building it. The loader in the module URL handles URL resolution and uses fetch to fetch the source code. Loaders can be chained together to load from different sources, transform source code, and resolve URLs differently. The future of module loading enhancements is promising and simple to use.

Available in Español

1. Introduction to ESM and Module Loading in Node.js

Short description:

ESM Loaders, Enhancing Module Loading in Node.js. ESM is the standard JavaScript module system native to Node.js. Module loading in Node.js involves executing the code inside a module. TypeScript users need to transpile to common.js. Module execution in Node.js involves resolving URLs.

♪♪ ESM Loaders, Enhancing Module Loading in Node.js. Hi, I'm Gil Tayar. I'm pretty old. We go all the way back to the 80s. I always was a developer. I did lots of stuff, but basically that is what I love to do. Another thing I love to do is use NPM and ESM and all that to enhance my code using extreme modularity and testing code. I'm currently a software engineer at Microsoft and working on cool stuff like the Azure Data Explorer, which is a real-time analytics database engine. Nothing to do with ESM.

Okay, let's get a few things out of the way before we talk about enhancing module loading. Let's talk about what ESM is. ESM is the standard JavaScript module system and it is now native to Node.js. So if previously we had common.js module.export to export something and require to import it, in ESM, you use standard syntax export and import to do module export and import. Native syntax works in Node.js, most people use today still use common.js but ESModule is getting more and more frequently used.

What is module loading in Node.js? Let's talk a little bit about that module loading. First of all, what is a module? A module is basically a glorified file, it's just called a module. So when I run a module or import a module, I'm basically executing the code inside the module, it's glorified execution module loading. So even when I'm importing module.js on the top, it's executing module.js at the bottom and then continuing executing main.js. It's all about glorified execution. First of all, a note to TypeScript users, if you're using TypeScript in Node.js, you're using the import-export syntax but you're probably transpiling that to common.js because ESM is pretty new to the game. So TypeScript usually, by default, transpiles to common.js. So if you're trying all the things I'm doing here with ASM loaders on your TypeScript code, it won't work. You have to not translate the ESM using module.node.next. And you have an example in the GitHub repo, don't worry, at the end, you'll have a link to the GitHub repo with a nice QR code and a link to this presentation.

What are the phases of module execution? How does Node.js execute a module? It's not simple. First of all, we resolve the URL. So if we're doing import.slash module.js, we have to resolve it to the absolute path on the disk. And ESM doesn't talk about paths. It talks about URLs.

2. ESM Module Loading in Node.js

Short description:

ESM module loading in Node.js involves resolving URLs and reading files from the disk. It recursively loads all the modules in the file without executing them. Once all the files are loaded, it executes the code of the modules in the right order.

It talks about URLs. Usually the URLs are file URLs, but they're URLs nonetheless. Once they resolve the URL, once Node.js resolves the URL, it reads the file from the disk. And then this is different from CommonJS, but let's not talk about what, it recursively loads all the modules in the file without executing them.

Notice that we haven't executed the ESM module above. Once it recursively loaded all the files, it executes the code of all the modules bottom up in the right order. These are the phases of module execution in Node.js. And basically in the browsers too, anywhere ESM is implemented, these are the phases. And this is different from CommonJS, we don't get into where.

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