Load classes stored in ObservableHq into nodejs

observable-to-nodejs.js

/*
Create empty directory

From command line
 * note to self: nvm use 10

npm init -y
npm i fs-extra --save
npm i node-fetch --save
npm i download-cli --save
node node_modules/download-cli/cli.js -e --out notebooks 'https://api.observablehq.com/@potofcoffee2go/01-data-managment-toolbox.tgz'
node node_modules/download-cli/cli.js --out ./ 'https://gist.github.com/PotOfCoffee2Go/7433f0b7b5479c57bd8a4f3688e6f8ce/raw/479f7bc87ed1983408d18483d59c8c96b3ef6a3b/observable-to-nodejs.js'


node observable-to-nodejs.js

*/

'use strict';

const fs = require('fs-extra'),
    fetch = require('node-fetch');

/* Load {notebook}.js into Nodejs (CJS)
 *  Note: requires nodejs >= v10 due to generator functions in notebook
 *  Changes line 'export default notebook;' to 'module.exports = notebook;'
 *   leaving the original unmodified so can still be used by the browser
 *  Returns loaded module
 */
function requireNotebook(notebookPath) {
    let es = fs.readFileSync(notebookPath, 'utf8');
    let cjs = es.replace(/\nexport default notebook;\n/, 'module.exports = notebook;');
    fs.writeFileSync('./notebooks/temp.js', cjs);

    let module = require('./notebooks/temp.js');
    fs.unlinkSync('./notebooks/temp.js');
    return module;
}

/* Get a cell from notebook */
function getCell(cellname, ...args) {
    return notebook.modules[0].variables
        .find(variable => variable.name === cellname)
        .value(...args);
}

const notebook = requireNotebook('./notebooks/01-data-managment-toolbox.js');

// A utilities class defined in notebook 'Util' cell - this cell requires no params
//  but... since do not have the browsers 'fetch()' need to use node-fetch
const Util = getCell('Util');
const util = new Util; // instantiate
util.getJson = async(url) => { return await (await fetch(url)).json() }

// Classes in other cells use an object 'cf' as a lookup table
let cf = getCell('cf');
// or could just do a :
//let cf = {};

// Clerk is a base class to the other classes
const Clerk = getCell('Clerk', cf, util);

// Classes that will be instantiated later by our data surrogate class
const
    Meta = getCell('Meta', cf, Clerk, util),
    Edge = getCell('Edge', cf, Clerk, util),
    Feed = getCell('Feed', cf, Clerk, util),
    Eyes = getCell('Eyes', cf, Clerk, util);

// Instantiates and use all the classes above
const Surrogate = getCell('Surrogate', cf, Meta, Edge, Feed, Eyes, Clerk);
const surrogate = new Surrogate({ [cf.data]: 'A new surrogate' });

// Watch for when surrogate data updates
async function watchUpdates() {
    console.log(`At ${surrogate[cf.meta].updated} ` +
        `got ${JSON.stringify({data: surrogate[cf.data]},null,2)}\n`);
    await surrogate[cf.eyes].watch(); // Eyes promise resolves when data updated
    watchUpdates();
}
watchUpdates();

// Watch for feed href changes
async function watchFeedHref() {
    await surrogate[cf.feed].watch(); // Feed promise resolves when href updated
    console.log(`--> Feed url changed: ${surrogate[cf.feed].href}\n`);
    watchFeedHref();
}
watchFeedHref();


// Go out and get some data off the web
let counter = 0;
setInterval(() => {
    surrogate[cf.feed].href = `https://jsonplaceholder.typicode.com/todos/${++counter}`;
    surrogate[cf.feed].fetch();
}, 3000);