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JavaScript Modules


Modules
JavaScript modules allow you to break up your code into separate files.
This makes it easier to maintain a code-base.
Modules are imported from external files with the import statement.
Modules also rely on type="module" in the <script> tag.

Example

<script type="module">
import message from "./message.js";
</script>


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Export
Modules with functions or variables can be stored in any external file.
There are two types of exports: Named Exports and Default Exports.

Named Exports
Let us create a file named person.js, and
fill it with the things we want to export.
You can create named exports two ways. In-line individually, or all at once at the bottom.

In-line individually:
person.js

export const name = "Jesse";
export const age = 40;



All at once at the bottom:
person.js

const name = "Jesse";
const age = 40;

export {name, age};



Default Exports
Let us create another file, named message.js, and
use it for demonstrating default export.
You can only have one default export in a file.

Example
message.js

const message = () => {
const name = "Jesse";
const age = 40;
return name + ' is ' + age + 'years old.';
};

export default message;









Import
You can import modules into a file in two ways, based on if they are named
exports or default exports.
Named exports are constructed using curly braces. Default exports are not.

Import from named exports
Import named exports from the file person.js:

import { name, age } from "./person.js";


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Import from default exports
Import a default export from the file message.js:

import message from "./message.js";


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Note
Modules only work with the HTTP(s) protocol.
A web-page opened via the file:// protocol cannot use import / export.













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Reference: https://www.w3schools.com/js/js_modules.asp