Elements in the XML DOM can easily be changed: just do a insertBefore. Or to change some text, just write to the data property of a text node. These elements can be retrieved using XPath (shameless plug for FontoXPath) and bob's your uncle!
Not with JSON though. XPath 3.1 gives wonderful JSON queries like ?a?b?c or ?1?2?3 which are very cool, but good luck trying to change the values of whatever you just got back from XPath: if you're in luck it's just an object/array that you can mutate (but don't let the functional people see it). If it's a boolean/string/number, you're out of luck.
This is where xpath-lenses comes in🎉
By using a lens to get a setter to whatever the XPath returned, you can just set it!
const input = { a: 1, b: [1, 2, 3, { c: 4, d: 5, e: [6] }] };
const result = createSetter("?b?4?e?1")
console.assert(result.isSuccessful === true);
const setter = result.lens(input);
const resultObject = setter(7);
console.assert(resultObject.b[3].e === 7, "Did set the result correctly");