Deep Equality Check for JSON Documents

Deep equality means two JSON documents represent the exact same data structure with the exact same values at every level of nesting. A deep equality check is the foundation of JSON diffing: if two documents are deeply equal, the diff is empty.

What deep equality ignores:

1. Key order: {"a":1,"b":2} and {"b":2,"a":1} are deeply equal because JSON objects are unordered.
2. Whitespace and formatting: Minified and pretty-printed versions of the same data are deeply equal.
3. Trailing commas / comments: These are not valid JSON, but if a parser strips them before comparison, the resulting structures may be deeply equal.

What deep equality checks:

1. Types must match: "42" (string) and 42 (number) are NOT deeply equal.
2. Values must match exactly: 3.14 and 3.140 are deeply equal (same IEEE 754 value), but 3.14 and 3.14159 are not.
3. Array order matters: [1, 2, 3] and [3, 2, 1] are NOT deeply equal because arrays are ordered.
4. Null vs. absent: { "a": null } and {} are NOT deeply equal.

// Document A
{
  "users": [
    { "id": 1, "name": "Alice" },
    { "id": 2, "name": "Bob" }
  ],
  "count": 2
}

// Document B
{
  "count": 2,
  "users": [
    { "id": 1, "name": "Alice" },
    { "id": 2, "name": "Bob" }
  ]
}

These two documents are deeply equal despite different key order at the root level. The array contents are in the same order, so they are also equal.

Implementation in JavaScript:

JavaScript's === operator compares object references, not values, so {a:1} === {a:1} is false. Deep equality requires a recursive comparison function. Libraries like Lodash (_.isEqual) or custom implementations are commonly used. The JSON.stringify() trick (stringifying both and comparing strings) works only if key order is guaranteed to be the same.

Performance:

Deep equality checking is O(n) where n is the total number of nodes in the document. For documents with millions of keys, consider hashing subtrees for quick inequality detection before performing full comparison.