Using console.log can change program state.

console-state.js

/* To run:
 *   mkfifo fifo
 *   node console-state.js | tee fifo
 * Wait 2 seconds, then look for state.json in the same directory.
 */

const fs = require('fs');

const state = [];

const stream = fs.createReadStream('fifo');
//const stream = process.stdin;
stream.on('data', d => {
    state.push(d.toString());
});
stream.on('error', e => {
    console.log(e);
});

setTimeout(() => {
    // If console.log is pure, state.json should read '[]'.
    console.log('a');
    console.log('b');
    console.log('c');
    console.log('d');
}, 1000);

setTimeout(() => {
    fs.writeFileSync('state.json', JSON.stringify(state));
}, 2000);