Json Schema validation: do not allow fields other than those declared in schema [duplicate]
You can create a json-schema and use the option:
additionalProperties = false
That way you only allow the attributes defined in properties. In your case:
{
"properties": {
"fname": {"type": "string"},
"lname": {"type": "string"},
"age": {"type": "string"}
},
"additionalProperties": false
}