#611 Deep Equal

=== only tells you whether two values are the same reference. Two objects that look identical, like the props of a component before and after a re-render, or a form's saved state and its current state, are still "not equal" to it. To know whether anything actually changed, you need to compare the values themselves, all the way down.

Write a function deepEqual(a, b) that returns true when a and b hold the same data and false otherwise.

deepEqual({ id: 1, tags: ["a", "b"] }, { tags: ["a", "b"], id: 1 }); // true
deepEqual([1, [2, 3]], [1, [2, 4]]); // false
deepEqual(new Date(2024, 0, 1), new Date(2024, 0, 1)); // true
deepEqual({ a: undefined }, {}); // false

Rules

• Primitives are equal when they are strictly equal, with two exceptions: NaN equals NaN, and 0 equals -0.
• Plain objects are equal when they have the same own enumerable keys and each key's values are deeply equal. Key order doesn't matter, but a key that is present with the value undefined is not the same as a missing key.
• Arrays are equal when they have the same length and their items are deeply equal position by position. An array never equals a plain object, even one with keys 0, 1, ...
• Dates are equal when they represent the same time. RegExps are equal when their source and flags match.
• Maps are equal when they have the same size, the same keys, and deeply equal values for each key.
• Sets are equal when they have the same size and every member of one is a member of the other (as Set.prototype.has sees it).
• Two objects with different prototypes are never equal, so new Point(1, 2) does not equal { x: 1, y: 2 }. Instances of the same class are compared like plain objects.
• Functions are only equal to themselves.
• The inputs may contain circular references. Your function must not overflow the stack on them.