How can I get a collection of keys in a JavaScript dictionary?

Use Object.keys() or shim it in older browsers...

const keys = Object.keys(driversCounter);

If you wanted values, there is Object.values() and if you want key and value, you can use Object.entries(), often paired with Array.prototype.forEach() like this...

Object.entries(driversCounter).forEach(([key, value]) => {
   console.log(key, value);
});

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Alternatively, considering your use case, maybe this will do it...

var selectBox, option, prop;

selectBox = document.getElementById("drivers");

for (prop in driversCounter) {
   option = document.createElement("option");
   option.textContent = prop;
   option.value = driversCounter[prop];
   selectBox.add(option);
}

One option is using Object.keys():

Object.keys(driversCounter)

It works fine for modern browsers (however, Internet Explorer supports it starting from version 9 only).

To add compatible support you can copy the code snippet provided in MDN.