A simple demonstration of how race conditions can occur in nodeJs and also how to tackle that

NodejsRaceAndLockDemo.js

let _arr = [];
let count = 0;

class Lock {
    constructor() {
        this._locked = false;
        this._waiting = [];
    }
    lock() {
        var unlock = () => {
            let nextResolve;
            if (this._waiting.length) {
                nextResolve = this._waiting.pop(0);
                nextResolve(unlock);
            } else {
                this._locked = false;
            }
        }
        if (this._locked) {
            return new Promise((res, rej) => {
                this._waiting.push(res);
            })
        } else {
            this._locked = true;
            return new Promise((resolve) => {
                resolve(unlock);
            });
        }

    }
}

let lock = new Lock();

async function addToArray(item) {
    count++;
    let unlock = await lock.lock();
    _arr.push(item);
    if (_arr.length == 1000000) {
        await backupAndEmptyArray();
    }
    await unlock();
}

async function backupAndEmptyArray() {
    const backup = [];
    for (let i = 0; i < 1000000; i++) {
        backup.push(_arr[i]);
    }
    await delay(1);
    _arr = [];
}

async function delay(t) {
    return new Promise((res, rej) => {
        setTimeout(res, t);
    });
}
async function add10000000ElementsToArray(k) {
    let i = 10000000;
    while (i--) {
        if (i % 5996 == 0) {
            await delay(1);
        }
        await addToArray({
            key: "Value"
        });
    }
}
async function main() {
    let promise1 = add10000000ElementsToArray(1);
    let promise2 = add10000000ElementsToArray(2);
    let promise3 = add10000000ElementsToArray(3);
    let promise4 = add10000000ElementsToArray(4);

    await promise1;
    await promise2;
    await promise3;
    await promise4;

    console.log(count);
    console.log(_arr.length);
}

main().then(() => {
    console.log("DONE");
});