How to recursively search for a key?

The function below recursively searches for a key in a JsonObject. It allows you to find a key in nested objects.

JsonVariant findNestedKey(JsonObject obj, const char* key) {
    JsonVariant foundObject = obj[key];
    if (!foundObject.isNull())
        return foundObject;

    for (JsonPair pair : obj) {
        JsonVariant nestedObject = findNestedKey(pair.value(), key);
        if (!nestedObject.isNull())
            return nestedObject;
    }

    return JsonVariant();
}

For example, suppose you have this JSON document:

{
  "europe": {
    "france": {
      "paris": 2165423,
      "marseille": 870731,
    },
    "italy": {
      "rome": 2844395,
      "milan": 3368590,
    },
  }
}

You could use this function below to find the population of Paris, like so:

unsigned long parisPopulation = findNestedKey(doc.as<JsonObject>(), "paris");

In addition, if you want to support constant references, you need to use this overload:

JsonVariantConst findNestedKey(JsonObjectConst obj, const char* key) {
    JsonVariantConst foundObject = obj[key];
    if (!foundObject.isNull())
        return foundObject;

    for (JsonPairConst pair : obj) {
        JsonVariantConst nestedObject = findNestedKey(pair.value(), key);
        if (!nestedObject.isNull())
            return nestedObject;
    }

    return JsonVariantConst();
}

See also: