ES6 modules—formally, ECMAScript modules (ESM)—are JavaScript’s standard way to split code into reusable files. A module makes selected values available with export; another module uses import to access them. For example:
// math.js
export const pi = 3.14159;
export function area(r) {
return pi * r * r;
}
// app.js
import { pi, area as circleArea } from './math.js';
console.log(circleArea(2));That syntax is shared by modern browsers and Node.js, but each host has its own rules for loading files and resolving module paths.
How do exports and imports work?
A module can export top-level declarations—such as functions, variables, or classes—and another module can import them. The export declaration defines the module’s public interface; code that is not exported remains private to that module.
MDN describes static imports as importing “read-only live bindings” from another module. The importer cannot assign a new value to an imported binding, but it can observe changes made to an exported binding by the module that owns it. MDN’s static import reference explains the syntax and behavior.
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What is the difference between named and default exports?
Choose an export style based on how you want consumers to see and use the module’s interface. Neither style is inherently better.
| Style | Export example | Import example | Useful when |
|---|---|---|---|
| Named | export function area(r) { ... } |
import { area } from './math.js'; |
The module exposes distinct, clearly named values. Importers can select specific exports or rename one with as. |
| Default | export default function format(value) { ... } |
import format from './formatter.js'; |
The module has one primary value that consumers should be able to import directly. |
A module can have named exports and one default export. Named imports must use the exported name unless aliased; a default import uses the local name chosen by the importer. To access a module’s named exports as properties on one object, use a namespace import such as import * as math from './math.js';.
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How do you load modules in a browser?
Use a module script as the browser entry point. Static import and export syntax is allowed in module scripts, not ordinary scripts:
<script type="module" src="./main.js"></script>Browser module specifiers are URL-like. For a nearby file, write an explicit relative path such as ./math.js; including the extension is normally necessary. A bare name such as square is not automatically resolved to a package in a browser. It needs an import map that maps the name to a URL; otherwise, the browser throws a TypeError for the unresolved specifier. Browsers also support inline module scripts and import maps. See MDN’s JavaScript modules guide for those browser behaviors.
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How do you enable ES modules in Node.js?
Node.js recognizes a file as ESM through one of three mechanisms: the .mjs extension, a nearest package.json with the top-level field "type": "module", or the --input-type=module option for input supplied to Node. In a package marked with "type": "module", its .js files are treated as ES modules.
For example, a project can opt in through its package file:
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{
"type": "module"
}Then a .js file in that package can use static imports. If Node reports Cannot use import statement outside a module, check whether the file is being treated as CommonJS and mark it as ESM using one of the supported mechanisms. Node’s documentation calls ECMAScript modules “the official standard format to package JavaScript code for reuse.” See Node.js ECMAScript modules and Node.js package documentation for the loading rules.
When should you use static import or dynamic import()?
Use static imports for dependencies a module needs as part of its normal startup. Static imports are top-level declarations with fixed string-literal specifiers. The host can resolve and link those dependencies before evaluating the module body, which supports features such as top-level await. Their declarations are processed before the rest of the module body; this is sometimes called hoisting, but it is not ordinary function hoisting.
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Use dynamic import() when code should be loaded conditionally or only when needed. It is an expression that returns a promise fulfilling with a module object:
button.addEventListener('click', async () => {
const { renderChart } = await import('./chart.js');
renderChart();
});Dynamic imports are asynchronous and can defer loading until a user action or other condition. Browser support depends on context: import() is permitted on the main thread and in shared or dedicated workers, but it throws in service workers and worklets. Consult MDN’s dynamic import reference for its promise behavior and modules guide for browser-context details.
Why do module paths behave differently in browsers and Node.js?
ECMAScript defines module syntax, but it does not make browser and Node.js path resolution identical. Browsers resolve URL-like specifiers, so local imports commonly use explicit relative paths and extensions; bare names require an import map. Node applies its own file and package rules, including its ESM markers and package resolution behavior. A specifier that works in one host may not work in the other without adjustment.
Node also supports interoperability between ESM and CommonJS. In particular, a CommonJS module can load an ES module with dynamic import(); static import declarations belong in ES modules. For package and interoperability details, refer to Node.js ESM documentation.
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What should you check when an import fails?
- Browser says it cannot use an import statement: Load the entry file with
<script type="module">, not a classic script. - A browser cannot resolve a bare name: Use an appropriate relative or absolute URL, or define an import map for the bare specifier.
- Node rejects static import: Confirm the file is ESM, using
.mjs, a nearest package’s"type": "module", or the appropriate input mode. - A path works in one environment but not another: Check the host’s resolution rules; browser URL resolution and Node package resolution are different.
- Static import is inside a function or condition: Move it to module top level, or use
import()for conditional loading. - Default and named imports do not match: Use braces for a named export, such as
import { area } from './math.js';, and no braces for a default export, such asimport format from './formatter.js';. - A circular dependency behaves unexpectedly: ESM supports cycles, but evaluation order can expose an imported binding before it has been initialized. Review initialization dependencies and test the cycle deliberately.
Which module approach fits your situation?
Compare the choices along four separate axes rather than treating them as alternatives in one category:
Quick Recap
- Export API: Named exports make available names explicit at the import site; a default export makes one primary value convenient to import.
- Loading time: Static imports declare startup dependencies; dynamic
import()allows conditional or on-demand loading. - Specifier resolution: Browsers use URL-like paths and import maps; Node follows its file and package rules.
- Interoperability: ESM uses static import declarations, while Node offers dynamic
import()for loading ESM from CommonJS.
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