A jsFuck transpiler written in 20 lines.

jsFuck.js

let dict = {};
const digit = (d) => '(+[])' + '+(+!![])'.repeat(d);
const number = (n) => `+(${n.toString().split('').map(d => `[${digit(+d)}]`).join('+')})`;
const fill = (s) => eval(s).split('').forEach((c, i) => (dict[c] ??= `(${s})[${number(i)}]`));
const word = (s) => s.split('').map(c => dict[c] ?? c.is.not.exist.in.dict).join('+');

// at -> constructor -> return btoa -> toString -> fromCharCode -> *
fill('[]+![]'); // "false"
fill('[]+!![]'); // "true"
fill(`[]+[][${word('at')}]`); // "function at() { [native code] }"
fill(`[]+(+[])[${word('constructor')}]`); // "function Number() { [native code] }"
fill(`[][${word('at')}][${word('constructor')}](${word('return btoa')})()(${word('  ')})`); // "ICA="
fill(`[]+([]+[])[${word('constructor')}]`); // "function String() { [native code] }"
fill(`(${number(17)})[${word('toString')}](${number(36)})`); // "h"

const char = (c) => (dict[c] ??= `([]+[])[${word('constructor')}][${word('fromCharCode')}](${number(c.charCodeAt(0))})`);
const string = (s) => s.split('').map(char).join('+');
const transpile = (code) => `[][${word('at')}][${word('constructor')}](${string(code)})()`;
const run = (code) => eval(transpile(code));

run("console.log(('b' + 'a' + + 'a' + 'a').toLowerCase());");

// process.stdin
//      .once('data', () => process.stdout.write(`[][${word('at')}][${word('constructor')}](`))
//      .on('data', (data) => process.stdout.write(`${string(data.toString())}+`))
//      .once('end', () => process.stdout.write(`${digit(0)})()`));