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Showing content from https://arduinojson.org/v6/api/staticjsondocument/ below:

StaticJsonDocument | ArduinoJson 6

Description

StaticJsonDocument is a JsonDocument that allocates its memory pool in-place, so it doesn’t rely on dynamic memory allocation.

Because it doesn’t call malloc() and free(), StaticJsonDocument is slightly faster than DynamicJsonDocument.

If you declare a local variable of type StaticJsonDocument, it allocates the memory pool in the stack memory. Beware not to allocate a memory pool too large in the stack because it would cause a stack overflow. Use StaticJsonDocument for small documents (below 1KB) and switch to a DynamicJsonDocument if it’s too large to fit in the stack memory.

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
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)

Example

Here is a program that deserializes a JSON document using a StaticJsonDocument

StaticJsonDocument<200> doc; // <- a little more than 200 bytes in the stack

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

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