How module works in DenoJS

Overview

Deno supports es6 module system that means each module will have separate file and JavaScript's import and export statement would work here.

Unlike NodeJS, we'll have to explicitly mention file name extension while import.

//Main.js
import { squareRoot } from "./module.js";
console.log(squareRoot(Deno.args[0]));

//Module.js
export const squareRoot = (number)=>{
    return number * number;
}

//Output:
//$ deno run main.js 5
// 25

NOTE: deno info main.js, info parameter shows module dependency chain and other module related information.

deno info main.js

local: /home/madhukar/Documents/code/deno/deno-modules/main.js
type: JavaScript
dependencies: 1 unique (total 148B)

file:///home/madhukar/Documents/code/deno/deno-modules/main.js (80B)
└── file:///home/madhukar/Documents/code/deno/deno-modules/module.js (68B)

External modules

• deno has a concept of url modules similar to golang
• deno doesn't have package manager like npm, instead it has set of standard(created by deno team) and external libraries(open source community), that can be used:
  • Standard library: https://deno.land/std
  • Third party modules: https://deno.land/x
• We can download a stardard or third party module by:

deno info https://deno.land/std@0.136.0/datetime/mod.ts

//output
Download https://deno.land/std@0.136.0/datetime/mod.ts
Download https://deno.land/std@0.136.0/datetime/formatter.ts
Download https://deno.land/std@0.136.0/datetime/tokenizer.ts
local: /home/madhukar/.cache/deno/deps/https/deno.land/6afa2c7cc88568787a9ac4e0288aa2e477a8d6e0f2b92617a5575bad71e9032e
type: TypeScript
dependencies: 2 unique (total 24.62KB)

https://deno.land/std@0.136.0/datetime/mod.ts (6.87KB)
└─┬ https://deno.land/std@0.136.0/datetime/formatter.ts (15.98KB)
  └── https://deno.land/std@0.136.0/datetime/tokenizer.ts (1.77KB)

• If you have imported external library in your source file then deno run command will download and caches it locally in your system.

    deno run main.js 

    // --reload will reload cache from remote/external repository
    deno run --reload main.js

• Deno caches remote imports in a special directory specified by $DENO_DIR

Create remote import's cache in local project folder

• Create deno_dir in your local project directory.
• set DENO_DIR ./deno_dir or on bash shell export DENO_DIR=./deno_dir
• deno cache main.ts

Deps.ts, handling dependencies

Handling dependencies from a central deno module is a best practice in Deno:

• Create a deps.ts file and export all the dependencies from that file.
• import all dependencies from deps.ts file instead of direct url.
• This approach is very similar to NodeJs's package-lock.json concepts.

//deps.ts
export { parse } from "https://deno.land/std@0.136.0/datetime/mod.ts";

//main.ts
import { parse } from "./deps.js";
import { squareRoot } from "./module.js";
console.log(squareRoot(Deno.args[0]));
console.log(parse("20-01-2019", "dd-MM-yyyy"));

Version locking

If you wants to work or have dependencies on a particular version of any library, deno has an inbuilt method to handle this.

• create a lock.json file

deno cache --lock=lock.json --lock-write ./deps.ts

NOTE: Upgrading deno to latest version is super simple deno upgrade

I hope you find this small introduction to deno module helpful, thanks for reading.