Unwind a JavaScript object

Given an object with an array-valued property, you might want to unwind it into an array of objects. This is a common operation when working with data that has been aggregated or grouped.

For starters, you have to exclude the key-value pair for the specified key from the object. You can use object destructuring for this purpose. Then, you can use Array.prototype.map() for the values of the given key to create an array of objects. Each object contains the original object's values, except for key which is mapped to its individual values.

const unwind = (key, obj) => {
  const { [key]: _, ...rest } = obj;
  return obj[key].map(val => ({ ...rest, [key]: val }));
};

unwind('b', { a: true, b: [1, 2] });
// [{ a: true, b: 1 }, { a: true, b: 2 }]