How to determine if a variable is an empty object, empty array or empty string?

Simply use Object.keys()

if (Object.keys(something).length) console.log("Not empty");

This works for all of strings, arrays, and of course, objects.

You can check the Object.keys documentation for more details, specifically, the examples section and the non-object coercion section, which state:

// simple array const arr = ['a', 'b', 'c'];
console.log(Object.keys(arr)); // console: ['0', '1', '2']

// array-like object const obj = { 0: 'a', 1: 'b', 2: 'c' };
console.log(Object.keys(obj)); // console: ['0', '1', '2']

and

In ES5, if the argument to this method is not an object (a primitive), then it will cause a TypeError.

From ES2015 onwards, a non-object argument will be coerced to an object.

// In ES5 Object.keys('foo');  // TypeError: "foo" is not an object

// In ES2015+ Object.keys('foo');  // ["0", "1", "2"]

For empty strings simple compare the property to "". comparing for empty objects and empty arrays can be done using comparing the Object.keys of the property length compared to 0.

const obj = {
  key1: 'asdf',
  key2: {},
  key3: ['a','b','c'],
  key4: [],
  key5: '',
}

function filterObject(obj) {
    for (var propName in obj) {
        if (obj[propName] === "" || Object.keys(obj[propName]).length === 0) {
            delete obj[propName];
        }
    }
    return obj;
}

console.log(filterObject(obj));

You can easily achieve the result using Object.keys and reduce

Loop over the keys of object obj using Object.keys and set the properties of new object only if it is not empty

You can check for the type of value using typeof operator

1) string: typeof will return string and check for the length property

2) array: You can use Array.isArray and it will return boolean value whether the object is an array or not

3) object: You can first get all keys using Object.keys and then check if there is any property exist on that object or not using length property

Since, arrays are object in JS, so you can directly use

(typeof value === "object" && Object.keys(value).length)

to check where it is an object and if it contain any property or not

You can create a filter as

if (value !== "" && Object.keys(value).length) acc[key] = value;

const obj = {
  key1: "asdf",
  key2: {},
  key3: ["a", "b", "c"],
  key4: [],
  key5: "",
};

const result = Object.keys(obj).reduce((acc, key) => {
  const value = obj[key];
  if (value !== "" && Object.keys(value).length) acc[key] = value;
  return acc;
}, {});

console.log(result);