Firebase未捕获的TypeError：无法读取未定义的属性'forEach'

Question

I'm trying to count the number of misbehavior on the two routes I've made in my database. Below are the structure of my firebase database under drivers and reports database respectively:

[drivers database] - i.stack.imgur.com/Q6GKs.png

[reports database] - i.stack.imgur.com/ALWPu.png

Here's my counter for counting the number of misbehavior:

        <script>
        var route1Count = 0;
        var route2Count = 0;
        var drivers;
        var reports;

        var driversRef = firebase.database().ref('drivers/');
        var reportsRef = firebase.database().ref('reports/');

        driversRef.once('value', (snapshot) => {
            drivers = snapshot;
        });

        reportsRef.once('value', (snapshot) => {
            reports = snapshot;
        });

        drivers.forEach((driver) => {

            var violationCount = reports.filter((report) => report.val().plateNumber === driver.key).length;

            if(driver.val().route === "Fairview - Quiapo"){
                route1Count += violationCount;
            }else if(driver.val().route === "Quiapo - Fairview"){
                route2Count += violationCount;
            }
        });

        document.getElementById("demo").innerHTML = "route1: " + route1Count + "route2: " + route2Count;

    </script>

I get this error message: Uncaught TypeError: Cannot read property 'forEach' of undefined at drivers.forEach, all inputs will be greatly appreciated! Thanks!

Error Message :

Answer 1

you could nest them, or if you run this in an environment that supports es6's Promise object (which your code suggests), you could use the once() returning a promise and more elegantly do:

Promise.all([driversRef.once('value'), reportsRef.once('value')])
  .then(([driversSnapshot, reportsSnapshot]) => {
    // ...
  })