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JavaScript Modules - Transitively Re-Exporting Modules

[Updated: Nov 30, 2018, Created: Nov 30, 2018]

ES6 specification allows a module to re-export the imported features from other module.

Re-exporting everything from otherModule's exports:

import * as otherModuleStuff from "otherModule";
export * from "otherModule";
.....

Re-exporting selectively:

import * as otherModuleStuff from "otherModule";
export {func1, func2 as otherFunction} from "otherModule";
.....

Exporting default:

import * as otherModuleStuff from "otherModule";
export {default} from "otherModule";

Exporting a feature of otherStuff as current module's default:

import * as otherModuleStuff from "otherModule";
export {func1 as default} from "otherModule";

The above two default exporting cannot be used at a time.

Example

js/lib2.js

export default function help() {
    console.log("available function from lib2: getCubicRoot(x)");
}

export function getCubicRoot(x) {
    return Math.cbrt(x);
}

export const xyz = 123;
export const desc = "lib2 description";

js/lib1.js

import * as mathUtil from "./lib2.js";

export {default} from "./lib2.js"; //exporting default from lib2
/*following is commented because there should be only one default*/
//export {desc as default} from "./lib2.js"; //exporting desc from module lib2 as default
export {xyz as constNum, getCubicRoot} from "./lib2.js"; //exporting other stuff from lib2

export function getCubicRootSum(x, y) {
    return mathUtil.getCubicRoot(x) + mathUtil.getCubicRoot(y);
}

//following is not possible as there should be only one default
/*export default function(){
    console.log("this is ./lib2's default function");
}*/


js/app.js

import * as util from "./lib1.js";
let sum = util.getCubicRootSum(2,3);
console.log(sum);
let cubicRoot = util.getCubicRoot(27);
console.log(cubicRoot);
console.log(util.constNum);
util.default(); // lib2#help()
//console.log(util.default); lib2#desc

index.html

<html>
<script type="module" src="./js/app.js"></script>
<body>

</body>
</html>

Output

To try examples, run http-server from project root:

$ http-server

Loading index.html in browser will print following in console:

2.7021706202022813
3
123
available function from lib2: getCubicRoot(x)

Example Project

Dependencies and Technologies Used:

  • Node.js v8.11.3
  • Google Chrome Browser Version 70.0.3538.102 (Official Build) (64-bit)
JavaScript Modules - Transitively Re-Exporting Modules Select All Download
  • javascript-transitive-export
    • js
      • lib1.js

    See Also