Senior LevelNode JS

How do you write custom Node.js ESM loaders (--import hook) to transpile TypeScript or mock modules on the fly?

Q: How do you write custom Node.js ESM loaders (--import hook) to transpile TypeScript or mock modules on the fly?

Node 20+ provides customizable **ESM Module Loaders** via 'module.register()': ### Loader Hooks: - **'resolve(specifier, context)'**: Intercepts and rewrites module path URLs. - **'load(url, context)'**: Transpiles file content (e.g. TypeScript to JS) in memory before handing code to V8! Key Takeaway: Use module.register() to build custom ESM loaders for on-the-fly TypeScript compilation and module mocking.

❓ Interview Prompt: Explain `node:module` `register()`, custom loader hooks (`resolve`, `load`), transpiling TypeScript in memory, and module mocking.

Node 20+ provides customizable ESM Module Loaders via 'module.register()':

Loader Hooks:

•

'resolve(specifier, context)': Intercepts and rewrites module path URLs.

•

'load(url, context)': Transpiles file content (e.g. TypeScript to JS) in memory before handing code to V8!

Code Implementation Example

javascript

import { register } from 'node:module';

// Register custom TypeScript ESM loader hook
register('./ts-loader-hook.mjs', import.meta.url);

💡Registering custom ESM module loader hooks in Node 20+.

🎯 Interview Key Takeaway

Use module.register() to build custom ESM loaders for on-the-fly TypeScript compilation and module mocking.

#ESM Loaders#register#TypeScript#Senior

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