Deep equal

Implement deepEqual(a, b). It returns true when the two values have the same structure and contents, and false otherwise.

• Primitives are compared with Object.is: NaN equals NaN, and 0 does not equal -0.
• Arrays are equal when they have the same length and their items are deeply equal position by position.
• Plain objects are equal when they have the same set of own keys (from Object.keys), in any order, and the values under each key are deeply equal. A key set to undefined is still a key, so { a: undefined } does not equal {}.
• Dates are equal when they hold the same time.
• An array never equals a plain object, and a date never equals a non-date.

Values are only primitives, arrays, plain objects and dates. There are no cycles. Don't use JSON.stringify.