From a person that is unknown...

#!/usr/bin/node

const fs = require('fs');

const LED0 = "/sys/class/leds/beaglebone:green:usr0";

// initialize LED0
fs.writeFileSync(LED0+'/trigger', 'none');
fs.writeFileSync(LED0+'/brightness', '0');

let led0_is_on = false;  // initial state

function toggle_led0() {
    if( led0_is_on ) {
        led0_is_on = false;
        fs.writeFileSync(LED0+'/brightness', '0');
    } else {
        led0_is_on = true;
        fs.writeFileSync(LED0+'/brightness', '255');
    }
}

setInterval(toggle_led0, 1000);  // call toggle_led0() every second