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https://github.com/leporello-js/leporello-js


https://github.com/leporello-js/leporello-js

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README 

        
# Leporello.js



Leporello.js is an interactive JavaScript environment with a time-travel debugger



[](https://vimeo.com/845773267)



## **[Website](https://leporello.tech)**

## **[Try online](https://app.leporello.tech)**



## Leporello.js is funded solely by your donations



Support us on [Github Sponsors](https://github.com/sponsors/leporello-js) and be the first to gain access to the Leporello.js Visual Studio Code plugin with TypeScript support.



## Features



### Going beyond the REPL

Your code is executed instantly as you type, with the results displayed next to it. No need to set breakpoints for debugging. Just move the cursor to any line and see what's happening.



### Next level debugging capabilities

Visualise and navigate a dynamic call graph of your program in a time-travel manner.



### Develop HTML5 apps interactively

Modify your code and instantly see the updated version without losing the application state. Interact with your app and debug it later, similar to using a time machine.



### Save time when working on IO-heavy programs

IO operations are traced and transparently replayed on subsequent program executions.



### Self-hosted

Leporello.js source code is developed within Leporello.js itself



## Supported javascript subset



Variables are declared using the `const` or 'let' declaration. The use of `var` is not supported.



Loops of any kind are not supported. Instead, consider using recursion or array functions as alternatives.



The `if` / `else` statements can only contain blocks of code and not single statements (TODO).



Both traditional functions and arrow functions, with block bodies and concise bodies, are supported. Method definitions, however, are not currently supported.



Classes are not supported at the moment. The `this` keyword is not currently supported. The `new` operator can be used for instantiating built-in classes or classes imported from [third-party](#importing-third-party-libs) libs.



`switch` statements will be supported in future updates.



Support for `try`, `catch`, and `finally` blocks is planned for future updates. However, the `throw` statement is currently supported.



ES6 modules are fully supported. Note that circular module dependencies are not currently supported and may lead to IDE crashes (with a TODO for improvement). Import/export aliases are not supported, and exporting `let` variables is also not supported. Additionally, `import.meta` is not currently supported.



`async` functions and `await` are fully supported.



Generators are not supported.



Destructuring is supported.



Some operators are not supported at the moment, including:

  - Unary plus

  - Bitwise operators

  - `in`

  - `void`

  - Comma operator

  - Increment and decrement

  - `delete`



## Importing third-party libs



To enable its comprehensive functionalities, Leporello.js parses and instruments your source code. Should you wish to import a module as a black box, preventing the ability to step into its functions, you can utilize the `external` pragma. For instance:



```

/* external */

import {Foo} from './path/to/foo.js';

```



`external` pragma is just a comment that contains only the literal string `external` (both styles for comments and extra whitespaces are allowed).



If a module path is a non-local path including a protocol and a host then it is always imported as an external module. Example:



![External import](docs/images/external_import.png)



Now the module is imported as a black box - you cannot debug `BigNumber` methods.



## IO



To enhance the interactive experience, Leporello.js traces the calls made to IO functions within your application. This trace can be replayed later, enabling you to program iteratively by making incremental changes to your code and promptly receiving feedback.



The current list of built-in functions for which calls are traced includes:

- `Date`

- `Math.random()`

- `fetch`

- `Response` methods:

    - `arrayBuffer`

    - `blob`

    - `formData`

    - `json`

    - `text`

- `setTimeout`

- `clearTimeout`



Leporello.js follows this process to manage IO calls:

- Initially, when the code is run, Leporello.js traces all IO calls, storing arguments and return values in an array as a trace.

- Whenever you edit your code, Leporello.js attempts to execute it, using the results of IO calls from the trace (replay).

- During replay, when an IO call is made, Leporello.js compares the current call to the traced call in the array. It checks if the function and arguments are the same. If they match, Leporello.js returns the result from the trace.

- To compare arguments for equality, Leporello.js uses deep equality comparison with `JSON.stringify`.

- If they do not match, the trace is discarded, and Leporello.js executes the code again, this time without the trace. This process populates a new trace array.



Additionally, there are options to manually discard the trace, including a button and a hotkey for this purpose.



## Hotkeys



See built-in Help



## Editing local files



Editing local files is possible via [File System Access API](https://developer.mozilla.org/en-US/docs/Web/API/File_System_Access_API). Click "Allow access to local project folder" to grant access to local directory.



## Selecting entrypoint module



After you granted local filesystem access you can select which javascript file

to run. See the following picture



![Entrypoint module](docs/images/entrypoint.png)



## Selecting html file



By default code in run in context of empty HTML file. If you want to use custom

HTML files with third party scripts or CSS stylesheets, you should choose HTML

file:



![HTML file](docs/images/html_file.png)



In typical HTML5 app you add to your html file a `script` element pointing to

your entry js module, like this:



```html



```



Because Leporello has built in bundler, you dont point to your entry module in

HTML file. Instead, you [select entrypoint module in

UI](#selecting-entrypoint-module).



If you want to use the same HTML file both for developing in Leporello.js and

in production, you can do it like this:



```html



  if(new URLSearchParams(window.location.search).get('leporello') == null) {

    import('./src/index.js');

  }



```



Leporello.js appends `?leporello` query parameter to your HTML file, so you can

test if HTML file is run in Leporello.js or in production.



You can add javascript libraries by including `script` tag to HTML file. If the library is exposing globals, they will be available in your javascript code after you select that HTML file as an entrypoint.



## Run and debug UI code in separate window



By default your code is run in invisible iframe. If you want to run and debug

UI code then you can open separate browser window. Click "(Re)open app window"

in statusbar or press corresponding hotkey. New browser window will be opened

and your code will be run in that window.



While you interacting with your app in separate browser tab, all function calls

are recorded. You can inspect and debug them.



To try live example, grant file system access to

[./docs/examples/preact](./docs/examples/preact) folder. Then select `index.js`

as an entrypoint and click "(Re)open app window". You will see the app where

you can calculate Fibonacci numbers:



![Entrypoint module](docs/images/fib_ui.png)



Try to click buttons and then get back to Leporello window. Now you can see

that all function calls have been recorded and you can inspect and debug

them:



![Deferred calls](docs/images/deferred_calls.png)



## Saving state between page reloads



Leporello.js allows preserving the state of the application between page reloads. To achieve this, Leporello.js provides a special API:



```javascript

window.leporello.storage.get(key: string)

window.leporello.storage.set(key: string, value: any)

```



Unlike localStorage and sessionStorage, these functions allow saving and retrieving non-serializable objects.



The storage can be cleared using the "(Re)open app window" button.



You can try the online demo [here](https://app.leporello.tech/?example=todos-preact). Create TODO items, then edit the code, and you will observe that your TODOs are preserved.



The code for interacting with the Leporello API is in the file `app.js`. When `app.js` module initializes, it checks whether Leporello.js API is present and loads app state:



```javascript

let state



if(globalThis.leporello) {

  // Get initial state from Leporello storage

  state = globalThis.leporello.storage.get('state')

}

```



Later, when state changes, it saves it back to the storage:



```javascript

// on state change

if(globalThis.leporello) {

  // Save state to Leporello storage to load it after page reload

  globalThis.leporello.storage.set('state', state)

}

```



## Run Leporello locally

To run it locally, you need to clone repo to local folder and serve it via HTTPS protocol (HTTPS is required by [File System Access API](https://developer.mozilla.org/en-US/docs/Web/API/File_System_Access_API)). See [How to use HTTPS for local development](https://web.dev/how-to-use-local-https/)



## Running test suite

run tests in node.js:



```

node test/run.js

```



run tests in leporello itself:



![Tests](docs/images/test.png)



- grant local folder access

- select `test/run.js` as entrypoint



## Roadmap



* Use production level JS parser, probably TypeScript parser

* Implement VSCode plugin