DynamicJsonDocument is a JsonDocument that allocates its memory pool in the heap.

Because it calls malloc() and free(), DynamicJsonDocument is slightly slower than StaticJsonDocument.

DynamicJsonDocument allows storing much larger documents than StaticJsonDocument because it is not limited by the size of the stack.

Custom allocator

DynamicJsonDocument uses a default memory allocator that calls malloc() and free(). You can use other functions by passing a custom allocator class to BasicJsonDocument<T>. As an example, here is how DynamicJsonDocument is defined:

struct DefaultAllocator {
  void* allocate(size_t n) {
    return malloc(n);

  void deallocate(void* p) {

typedef BasicJsonDocument<DefaultAllocator> DynamicJsonDocument;

See BasicJsonDocument<T> for more information.

Member functions

  • as<T>() casts the root to the specified type (e.g. JsonArray or JsonObject)
  • add() adds elements to the root array
  • capacity() returns the capacity of the memory pool
  • clear() empties the document and resets the memory pool
  • containsKey() tests if the root object contains the specified key
  • createNestedArray() creates a nested array attached to the root
  • createNestedObject() create a nested object attached to the root
  • garbageCollect() reclaims leaked memory blocks
  • operator[] gets or sets values in the document
  • overflowed() tells if the memory pool was large enough 🆕
  • is<T>() tests the type of the root
  • isNull() tells if the document is null or empty
  • memoryUsage() tells how many bytes are used in the memory pool
  • nesting() returns the number of nesting layers in the document
  • remove() removes an element (or member) at the specified index (or key)
  • set() replaces the root with the specified value
  • shrinkToFit() reduces the capacity of the memory pool to match the current usage* size() returns the number of elements (or members) that the root array (or object) contains
  • to<T>() clears the document and converts it to the specified type (e.g. JsonArray or JsonObject)


Here is a program that deserializes a JSON document using a DynamicJsonDocument:

DynamicJsonDocument doc(2048);

char json[] = "{\"hello\":\"world\"}";
deserializeJson(doc, json);

const char* world = doc["hello"];


In older versions, DynamicJsonDocument was able to grow if needed. Starting with version 6.7.0, DynamicJsonDocument has a fixed capacity, just like StaticJsonDocument. This change allows better performance, smaller code, and no heap fragmentation.

Arduino 6.6.0 contained a full-blown allocator (i.e., non-monotonic) and was able to compact the memory inside the JsonDocument. This feature was reverted in version 6.7.0 because the overhead was unacceptable.

See also