ES modules preview

Last reviewed May 28, 2026 Content v20260528

Track mode

client_javascript

Means

In-browser JS

Reading

~1 min

Level

intermediate

This lesson

This lesson teaches ES modules preview—the ideas, syntax, and habits you need before moving on in JavaScript.

Without a solid grasp of ES modules preview, you will repeat mistakes in JavaScript exercises and on real pages or scripts.

You will apply ES modules preview in contexts like: Browsers, Node.js services, edge workers, and tooling ecosystems (bundlers, test runners).

Run JavaScript in the in-browser sandbox, use the terminal output panel, and verify with MCQs.

Toward the end of the track—use it to consolidate patterns before the capstone or summary lessons.

ES modules (import / export) are the standard in browsers and Node. One default export per file is common; named exports scale better.

Browser

<script type="module"> — deferred by default, strict mode, CORS for cross-origin.

package.json "type": "module" or .mjs — covered on Node modules lesson.

Q: default vs named export?
A: Import syntax differs; named enables tree-shaking clarity.
Q: Circular imports?
A: Possible but design smell—extract shared module.

Tip: Re-run the playground code for modules-esm-preview and tweak one line before the MCQs.

Playground

Runs in your browser in a sandboxed frame. Backend runners appear when this track’s profile allows them.

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