defaults

Assigns default values for all properties in an object that are undefined.

• Use Object.assign() to create a new empty object and copy the original one to maintain key order.
• Use Array.prototype.reverse() and the spread operator (...) to combine the default values from left to right.
• Finally, use obj again to overwrite properties that originally had a value.

const defaults = (obj, ...defs) =>
  Object.assign({}, obj, ...defs.reverse(), obj);

defaults({ a: 1 }, { b: 2 }, { b: 6 }, { a: 3 }); // { a: 1, b: 2 }