Custom NodeJs REPL

README.md

Custom NodeJs REPLs

A way to create read-eval-print-loops for your own projects, with a per-instance history, rather than a global history.

Sample Usage

const customRepl = require('./custom-repl.js');

/**
 * Add context to the REPL: 
 * What things are accessible from the prompt?
 */

const foobarRepl = customRepl('foobar');
  
function syncThing (x) {
    return `sync ${x}`;    
}

function asyncThing (x) {
    return `async ${x}`;    
}

// add synchronous context
fooBarRepl.addContext(syncThing);

// add asynchronous context
setTimeout(() => {
    foobarRepl.addContext(asyncThing);
}, 5000);

• You may run syncThing() immediately
• You need to wait five seconds before you may run asyncThing()
  • IRL, you would do this to wait for something that is asynchronous to finish loading, or similar

custom-repl.js

const fs = require('fs');
const repl = require('repl');

module.exports = customRepl;

function customRepl (name, options = {}) {

    const historyFilePath = 
        options.historyFilePath || 
        `${process.env.HOME}/.custom_repl/${name}_repl_history`;
    
    let historyFileExists = false;
    try {
        // ensure history file exists
        fs.statSync(historyFilePath);
        historyFileExists = true;
    } catch (ex) {
        // Do nothing
    }
    let historyLines;
    if (historyFileExists) {
        // load command history
        console.log(`${name}: Loading your history...`);
        historyLines = fs.readFileSync(historyFilePath)
            .toString()
            .split('\n')
            .reverse()
            // Discard empty lines
            .filter((line) => (line.trim().length > 0));
    } else {
        historyLines = [];
    }
    const replServer = repl.start({
        prompt: `${name}> `,
    });
    historyLines.forEach((line) => replServer.history.push(line));
    
    replServer.on('exit', () => {
        console.log(`${name}: Saving your history...`);
        const lines = replServer.lines;
        fs.appendFileSync(
            historyFilePath,
            `${lines.join('\n')}\n`);
        console.log(`${name}: Bye!`);
        process.exit();
      });

    function addContext (property, value) {
        if (typeof property === 'function') {
            value = property;
            property = value.name;
        }
        if (typeof replServer.context[property] !== 'undefined') {
            throw `context already exists: ${property}`;
        } 
        replServer.context[property] = value;
    }

    return {
        addContext,
    };
}