Outline
- Introduction
- Import a Module from the File System
- Import a Module Dynamically
- Export Lists
- Alias or Rename Imports and Exports
Introduction
ES Modules (JavaScript Modules, if you like) help us structure, organize and isolate code. These examples focus on the ESM features available in a Node environment such as dynamic modules, export lists, and module aliasing.
Most JS apps in early 2021 are still bundled before being sent to the browser. Features covered here are common to the latest LTS Node environments for most popular Node bundling tools like Webpack, Rollup, and esbuild.
Index Main File
- This is where all exercises are run
- Examples in
readme.md
can be copy/pasted intoindex.js
Prepared Module Files
- The names relate to the type of content inside
- Various examples might use these prepared modules to explore a concept
Assignments
-
./modules/assignment.mjs
is an empty module.
Import a Module from the File System
Modules are imported using the import
keyword and a string path to that module. This is predominantly done at the opening of a file. We can import any module that exists on the file system. Import the assignment.mjs
module using the import
keyword and file path:
// index.js
import "./modules/assignment.mjs"
Leaky Modules
The first thing to know about modules is that they leak. They don't perfectly encapsulate all code. Global code is global code, even if in a module. This might sounds bad (and it can be) but it's an important feature.
Add Globally Executing Code to a Module
Globally executing code can be console.log("booo!!")
or an assignment like global.leak = "oh no!"
.
// modules/assignment.mjs
console.log("sah dude")
global.leak = "oops"
// index.js
import "./modules/assignment.mjs"
console.log(global.leak)
sah dude
oops
Import a Module Dynamically
The import
keyword has two variants: static and dynamic. You can change a static import to a dynamic import by adding parenthesis around the path string.
import "./modules/assignment.mjs" // static
import("./modules/assignment.mjs") // dynamic
Change the Static Import to a Dynamic Import
A dynamic import
returns a Promise
, so try handling that promise using await
.
// index.js
await import("./modules/assignment.mjs")
console.log(global.leak)
Or use .then()
to resolve the Promise
.
// index.js
import("./modules/assignment.mjs")
.then(() => console.log(global.leak))
sah dude
oops
Export Lists
Global code is executed at import
but variables and function declarations are not. Even though a function or variable might exist in an imported module, it can't be accessed outside of that module.
Add a Variable and Function Declaration to the Module
// modules/assignment.mjs
let fallbackName = "there"
function greet(name = fallbackName) {
return `Hey, ${name}!`
}
Use an Export List to Export a Function
We can export anything defined in our module by adding it to the comma separated export
list.
// modules/assignment.mjs
let fallbackName = "there"
function greet(name = fallbackName) {
return `Hey, ${name}!`
}
export { fallbackName, greet }
Anything exported can also use the same list syntax, {}
, for import
. Importing specific imports from a module requires the from
keyword before the path string.
// index.js
import { greet } from "./modules/assignment.mjs"
console.log(greet())
Hey, there!
export
list is commonly at the end of a file to guarantee that everything exported — or referenced — already exists. Now with an export
list, export and import just the greet
function.
// modules/assignment.mjs
let fallbackName = "dude"
function greet(name = fallbackName) {
return `Sah, ${name}`
}
export { greet }
greet
still has access to fallbackName
, even though fallbackName
isn't exported.
// index.js
import { greet } from "./modules/assignment.mjs"
console.log(greet())
Sah, dude
Alias or Rename Imports and Exports
Modules might not share object syntax with modules but they still allow for aliasing (or renaming) of variables and functions using the as
keyword.
// modules/assignment.mjs
export { aGoodLocalName as aBetterExportedName }
It works identically on both the import
and export
sides.
// index.js
import {
aBetterExportedName as anEvenBetterContextualName
} from "./modules/assignment"
Rename a Function while Exporting
At import, use the new function name you've exported and then rename it back to greet
at import
.
// modules/assignment.mjs
let fallbackName = "dude";
function greet(name = fallbackName) {
return `Sah, ${name}`
}
export { greet as sahDude }
// index.js
import { sahDude as greet } from "./modules/assignment.mjs"
console.log(greet())
Sah, dude
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