Nodejs `+=` Race Condition

raceist.js

/*
 * Using node v12.19.0 This demonstrates the potential dangers of sync/async. 
 * This likely will not print 20 ((0 + 1) + (1 + 1) ... + (4 + 1)) = 1 + 2 + 3 + 4 + 5 = 15
 */

/**
 * @param arg {number}
 * @return {Promise<number>} returns the arg provided after a timeout
 */
function sleep(arg) {
    return new Promise((resolve, reject) => {
        setTimeout(resolve, 1000 * Math.random(), arg);
    });
}

let globalVar = 0;

// swap the comments in order to get expected behavior.
async function thred(args) {
    //let add = await sleep(args + 1);
    //globalVar += add;
    globalVar += await sleep(args + 1);
}

// spawn threads
for (let i = 0; i < 5; i++) {
    thred(i);
}

//after everything has resolved, print the var
process.on('exit', () => {
    console.log(globalVar);
});