This is a copy of the Github readme.
Find the original on

SEY currency converter pwa

SEY currency converter Progressive Web App
version: 2022.511.1507 date: 2022-05-11 author: repository: Github

Lines in Rust code Lines in Doc comments Lines in Comments Lines in examples Lines in tests

License Rust Hits

Hashtags: #rustlang #tutorial #pwa #wasm #webassembly
My projects on Github are more like a tutorial than a finished product: bestia-dev tutorials.

SEY - $€¥

"A word that is used in time when one is extremely happy, or likes something."

screenshot icon on android

Try it:


PWA stands for Progressive Web Application.
It is a 2015 web browser standard that allows web pages to be used like native applications.
The major benefits are:

Currency exchange rates

I will get the daily exchange rate in json format from:
and fill it into the local browser indexeddb.
It will be possible for the user to modify the exchange rate to calculate exactly what he needs.
The exchange rates are downloaded every time the base currency is changed or a refresh is executed.


In Visual Studio Code press ctrl+j to open the Terminal window.
Use cargo make to see the prepared steps for deployment like:
$ cargo make release
And then follow the instructions on the screen like:

Oh, today I did everything right, but the browser said "This site can't be reached". After many attempts I discovered, that WSL2 localhost or connection to Win10 is broken after putting the laptop to sleep. I have to restart the WSL in PowerShell Run as administrator with
Get-Service LxssManager | Restart-Service.
Not nice and very difficult to discover because WSL2 is running just fine, except this.

In the browser (chrome, edge, firefox) use F12 developer tools to easily see the content of indexeddb in F12. Very convenient.
Some tips for PWA:


Indexeddb is the standard database storage inside the browser. It is not Sql. It is a document database.
It is more or less a key-value storage, but the value can be a javascript object and that can be complex.
The api is in javascript, uses old fashioned callbacks and events, completely async, without async/await or Promises.
I experimented with indexeddb in my project
Now it is time to make it work. Still I don't know about the performance because Rust and Javascript don't really use the same objects.

Typescript adventure

For my only module in javascript idb_export, I will rather use Typescript. I will write some typescript code, transpile it to javascript and invoke that from rust.
In runtime my code will use only the javascript file. Typescript is used only in development.
The Typescript compiler must be installed with npm that is a part of nodejs. I must first install nodejs.
On Debian the package sudo apt install nodejs is old version 10. The recommended version is 14, but it is from another package source. is providing a script to add the new package source and install nodejs.
This is the commands:

cd ~
curl -sL -o
sudo bash
sudo apt install nodejs
node -v
npm -v
sudo apt install build-essential

Now I can install Typescript:

npm install -g typescript
tsc --version
tsc --help

In the terminal I just use tsc to transpile my source code with settings from tsconfig.json.
I added this to my cargo make for easy developing.
The typescript file is inside the src folder like rust source code files.
The resulting javascript file is stored in the js folder of the web app folder.

typescript/javascript imports

I had major problems with import statements.
I tried first with npm install --save idb. It saves the files in a separate node_modules folder. That didn't work nice with my import statements. I don't know why.
At last I decided to create idb as a separate folder and copy the node_modules/idb/build/esm.
I needed to play with tsconfig.json to make it work.
I added the keys: baseUrl, rootDir, outDir, esModuleInterop and most important path.
From one side this import paths are just like folder structure, from the other side they are like url paths.
Confusing. But after a long experimentation I made it work. I hope I don't need ever to change this settings.

code flow

The browser opens index.html.
There it runs import init from "./pkg/sey_currency_converter_pwa.js";
and init("./pkg/sey_currency_converter_pwa_bg.wasm");
This is the wasm code compiled from and wasm-bindgen creates the magic to start the designated function.
The idb_exports.js is the result of typescript transpilation of idb_exports.ts, my only typescript module. Inside that module I need to import the idb module with:
import * as idb from '/sey_currency_converter_pwa/idb/index.js'; Then Rust code imports the idb_exports.js javascript module and functions.
From here on we are now in pure (more or less) rust code.

missing unsafe

When importing javascript functions with #[wasm_bindgen] and extern "C", the rust-analyzer shows a warning about missing unsafe. This is not correct, the rustc compiler compile it just fine. It is because the attribute macro wasm_bindgen uses magic and makes it safe. But rust-analyzer (for now) cannot understand attribute macros.
For those looking to disable the missing-unsafe rule until it's fixed and are using VS Code, adding the following to your settings.json and reloading your editor will suppress these errors:

"rust-analyzer.diagnostics.disabled": [

extern "C" - importing javascript functions

Javascript functions are imported using the extern "C" block.
For now rustfmt has a bug that removes the word async, because here we have javascript functions and not C functions.
The workaround is to add rustfmt::skip:

#[wasm_bindgen(raw_module = "/sey_currency_converter_pwa/js/idb_exports.js")]
extern "C" {
    fn check_browser_capability();
    fn init_db() -> Result<(), JsValue>;
    fn put_key_value(store: String, key: String, value: String) -> Result<(), JsValue>;

For a javascript function with no return value is simple:
pub(crate) fn check_browser_capability();
A javascript async function can return one JSValue.
pub(crate) async fn get_key_value(key: String, ) -> JsValue;
If we want to catch errors in the Promise, add attribute wasm_bindgen(catch), then the functions returns Result<JsValue, JsValue>:

pub(crate) async fn init_db() -> Result<JsValue, JsValue>;

The imported async fn needs to be await just like rust functions. The macro wasm_bindgen makes some magic to transform Promises to futures on import:
let currdb = open_db().await.unwrap();
Some of the functions are async and others are not. It can lead to strange problems if an async function is used as a normal function. This is a thing to be careful about. Rust will hopefully show a warning, but javascript will not.


This PWA will have more pages. Pages are complete static html files inside tha /pages/ folder. They use the same css as index.html.
It is easy to edit and preview pages because they are complete.
The rust code will fetch the html, extract only the body content and set_inner_html to div_for_wasm_html_injecting.
A page is a template, and some placeholders will be replaced with data.
For modal pages I prepared a separate div_for_modal.
I will not use a local router and will not change the URL address of this single page web app. I don't see a need for this.


The indexeddb is key-value. Key is a javascript string and value is any javascript object.
That is really practical for javascript, but not so for rust.
I will use serde-wasm-bindgen to work directly with javascript values from rust, because indexeddb stores javascript objects.
From Rust to javascript:
From javascript to rust:
let value: SomeSupportedRustType = serde_wasm_bindgen::from_value(value)?;


IndexedDB can have more than one database at once. In this project I use only one: Currdb.
My module idbr_mod contains all the necessary functions to operate with indexeddb. But it is all too generic. I don't want everywhere in my code to call a generic library. I want to isolate it.
For that I created the module currdb_mod and encapsulated all the necessary functions for this database. I also re-exported some structs from the idbr_mod.
My code must use only this module for any database need.
For every ObjectStore I created its own module to make things more easy to separate and encapsulate.

[currdb_manual_rates_mod] ..> [currdb_mod]
[currdb_currency_used_mod] ..> [currdb_mod]
[currdb_currency_mod] ..> [currdb_mod]
[currdb_config_mod] ..> [currdb_mod]
[currdb_mod] ..> [idbr_mod]

note right of (idbr_mod): I want to isolate this generic library
note right of (currdb_mod): encapsulates all functions for db work


A lot of functions are just public functions in a module. But for some I created objects with methods. It is a mix depending what is easier to write and maybe better for performance. I didn't use the Static methods (methods without self as opposed to instance methods) here because the module is already a good container for functions encapsulation.


The Swipe gesture is not a native html5 event. It is pretty complicated to recognize this gesture. Even more on different browsers (Safari).
Therefore I use this javascript library:
I added this library also to the static html page, for easy of development. But the code in rust uses it in a different manner.
For now I need to replace the row counter. I plan to use also a callback from javascript back to rust to execute the remove and reopen the same page.


I created a simple icon 512x512 in with the filename icon.pdn. The 2 layers are Background and Layer1. The background is a gradient of color. The Layer1 is the text in Bradley hand ITC font. I exported it as icon_original.png.
Then I used my web app to create all the pngs needed for pwa: TODO: add generation of favicon.ico.
The favicon.ico I create using


I decided to use the Roboto-Medium font that is the default android font. I copied the woff2 file in the css/ folder. For some icons I use the Font Awesome 5. The solid font is mostly free:
I downloaded the zip with a bunch of files. I copied only the fa-solid-90.woff2 and the fontawesome.css to the css/ folder. In my css file I added a couple of lines copied from solid.css. I will use only the woff2 font for now.

snackbar or toast

Snackbar or toast is a modal window that show some information to the user and automatically closes in a short time. It is nice for short and unobtrusive notifications

screenshot img in Github markdown

First I tried with a jpg img, but it becomes width=100% and that is not what ai want.
If I change it to png img, it looks good. Strange tricks:
![screenshot]( "screenshot")

Open-source and free as a beer

My open-source projects are free as a beer (MIT license).
I just love programming.
But I need also to drink. If you find my projects and tutorials helpful, please buy me a beer by donating to my PayPal.
You know the price of a beer in your local bar ;-)
So I can drink a free beer for your health :-)
Na zdravje! Alla salute! Prost! Nazdravlje! 🍻