Several issues:

• forEach always returns undefined. You need to use map instead.
• (key, value["Penetration_2018"]) is not a tuple, but a comma operator expression and results in the value of value["Penetration_2018"]. Use square bracket notation instead.
• A Map has no useful representation when cast to string. You need to turn it to an array or something else to get a more useful string representation:

Code:

const mapTest = new Map(
    Object.entries(filterCounties).map(
        ([key, value]) => [key, value["Penetration_2018"]]
    )
);
console.log(`This is mapTest ${JSON.stringify([...mapTest])}`);

With arrays it is more conventional to just apply map directly on the array, and it is a mystery why you would need a Map anyway. If you want the unique values, then use a Set:

const uniques = [...new Set(filterCounties.map(value => value["Penetration_2018"]))];
console.log(`This is uniques: ${uniques}`);

As stated in the comments, objects are not required to iterate properties in insertion order. If you require the iteration order to match the insertion order, use a Map instead:

const map1 = new Map([
  ['01', "val1"],
  ["06", "val6"],
  ["13", "val13"],
  ["88", "val88"],
  ["100", "val100"]
]);

for (const [k, v] of map1) {
  console.log(k + "-" + v);
}

The Docs say that Object.entries returns an array of given objects enumerable property [key,value] pairs . So yes its confusing if you look at this statement

const object3 = { 100: 'a', 2: 'b', 7: 'c' };

and end up getting ["2", "b"] when you call Object.entries(object3)[0].

When you are doing this Object.entries(object3)[0] , you are accessing a pair at the index of 0 returned by this function Object.entries(object) . The order of this array has nothing to do with how you defined the object3 in the first place. The order according to the doc is the same as the provided by a for...in loop. I ran the for...in loop on the object and this is what i got as the order.

2,7,100.

This is why you are getting ["2", "b"] instead of ["100", "a"]. As others have mentioned here , the order seems to be that way because 2<7<100.

You can use the for-in loop as shown by others. However, you also have to make sure that the key you get is an actual property of an object, and doesn't come from the prototype.

Here is the snippet:

var p = {
    "p1": "value1",
    "p2": "value2",
    "p3": "value3"
};

for (var key in p) {
    if (p.hasOwnProperty(key)) {
        console.log(key + " -> " + p[key]);
    }
}

For-of with Object.keys() alternative:

var p = {
    0: "value1",
    "b": "value2",
    key: "value3"
};

for (var key of Object.keys(p)) {
    console.log(key + " -> " + p[key])
}

Notice the use of for-of instead of for-in, if not used it will return undefined on named properties, and Object.keys() ensures the use of only the object's own properties without the whole prototype-chain properties

Using the new Object.entries() method:

Note: This method is not supported natively by Internet Explorer. You may consider using a Polyfill for older browsers.

const p = {
    "p1": "value1",
    "p2": "value2",
    "p3": "value3"
};

for (const [key, value] of Object.entries(p)) {
  console.log(`${key}: ${value}`);
}