JSON Diff compares two JSON documents structurally. Objects are compared key by key and arrays by index, and every difference is listed with its path: plus for added, minus for removed, tilde for changed.
Both documents stay in the page. Nothing is uploaded.
By index. Element 0 is compared to element 0, and so on; extra elements on either side are reported as added or removed.
A change from, say, an object to an array is reported as a single change, not a deep diff.
No. Objects are compared by key regardless of order.
No. The comparison runs in JavaScript in the open tab.