Get array of object's keys

You can use jQuery's $.map.

var foo = { 'alpha' : 'puffin', 'beta' : 'beagle' },
keys = $.map(foo, function(v, i){
  return i;
});

Of course, Object.keys() is the best way to get an Object's keys. If it's not available in your environment, it can be trivially shimmed using code such as in your example (except you'd need to take into account your loop will iterate over all properties up the prototype chain, unlike Object.keys()'s behaviour).

However, your example code...

var foo = { 'alpha' : 'puffin', 'beta' : 'beagle' };
var keys = [];
for (var key in foo) {
    keys.push(key);
}

jsFiddle.

...could be modified. You can do the assignment right in the variable part.

var foo = { 'alpha' : 'puffin', 'beta' : 'beagle' };
var keys = [], i = 0;
for (keys[i++] in foo) {}

jsFiddle.

Of course, this behaviour is different to what Object.keys() actually does (jsFiddle). You could simply use the shim on the MDN documentation.

In case you're here looking for something to list the keys of an n-depth nested object as a flat array:

const getObjectKeys = (obj, prefix = '') => {
  return Object.entries(obj).reduce((collector, [key, val]) => {
    const newKeys = [ ...collector, prefix ? `${prefix}.${key}` : key ]
    if (Object.prototype.toString.call(val) === '[object Object]') {
      const newPrefix = prefix ? `${prefix}.${key}` : key
      const otherKeys = getObjectKeys(val, newPrefix)
      return [ ...newKeys, ...otherKeys ]
    }
    return newKeys
  }, [])
}

console.log(getObjectKeys({a: 1, b: 2, c: { d: 3, e: { f: 4 }}}))

I don't know about less verbose but I was inspired to coerce the following onto one line by the one-liner request, don't know how Pythonic it is though ;)

var keys = (function(o){var ks=[]; for(var k in o) ks.push(k); return ks})(foo);

Summary

For getting all of the keys of an Object you can use Object.keys(). Object.keys() takes an object as an argument and returns an array of all the keys.

Example:

const object = {
  a: 'string1',
  b: 42,
  c: 34
};

const keys = Object.keys(object)

console.log(keys);

console.log(keys.length) // we can easily access the total amount of properties the object has

In the above example we store an array of keys in the keys const. We then can easily access the amount of properties on the object by checking the length of the keys array.

Getting the values with: Object.values()

The complementary function of Object.keys() is Object.values(). This function takes an object as an argument and returns an array of values. For example:

const object = {
  a: 'random',
  b: 22,
  c: true
};


console.log(Object.values(object));

Year 2022 and JavaScript still does not have a sound way to work with hashes?

This issues a warning but works:

Object.prototype.keys = function() { return Object.keys(this) }
console.log("Keys of an object: ", { a:1, b:2 }.keys() )

// Keys of an object:  Array [ "a", "b" ]
// WARN: Line 8:1:  Object prototype is read only, properties should not be added  no-extend-native

That said, Extending Built-in Objects is Controversial.

If you decide to use Underscore.js you better do

var foo = { 'alpha' : 'puffin', 'beta' : 'beagle' };
var keys = [];
_.each( foo, function( val, key ) {
    keys.push(key);
});
console.log(keys);