JavaScript, transform object into array

The obvious way would be to do a for-in loop, as @quixoto suggests, but just for the record, and since you are looking for a built-in way, you could pair the new ECMAScript 5 methods Object.keys and Array.prototype.map, available on latest browsers:

function valuesToArray(obj) {
  return Object.keys(obj).map(function (key) { return obj[key]; });
}

UPDATE: ES2017 introduced the Object.values method, which does exactly what you want.

Additionally, ES2017 adds another often useful method, Object.entries. This method returns an array of key-value pairs.

const obj = {
    "Mike": 24,
    "Peter": 23,
    "Simon": 33,
    "Tom": 12,
    "Frank": 31
};

const values = Object.values(obj);
const entries = Object.entries(obj);
console.log('values:', values);
console.log('entries:', entries);