Mountebank - How do I assert message body is JSON format in a POST request in mountebank - mockserver

Question

I want to stub and check the message body in POST request in mountebank,
{
"port": "22001",
"protocol": "http",
"name": "login_user",
"stubs": [
{
"responses": [
{
"is": {
"statusCode": 201,
"headers": {
"Content-Type": "application/json"
},
"body": {}
},
"_behaviors": {
"wait": 100
}
}
],
"predicates": [
{
"equals": {
"path": "/login_user",
"method": "POST",
"headers": {
"Content-Type": "application/json"
},
"body": {
"name": "Tony",
"age": "20"
}
}
}
]
}
]
}
if a message body in JSON format. expected response status Code 200.
for Example
{
"body": {
"name": "Tony",
"age": "20"
}
}
if a message body in JSON format but JSON string. expected response status Code 400.
for Example
{
"body": "{\"name\": \"Tony\", \"age\": \"20\"}"
}

Answer 1

You could achieve this with a 'matches' predicate containing a (pretty crude in this example) regex to capture any string input and return a 400:
"stubs": [
{
"responses": [
{
"is": {
"statusCode": 400,
"headers": {
"Content-Type": "application/json"
},
"body": {}
},
"_behaviors": {
"wait": 100
}
}
],
"predicates": [
{
"matches": {
"path": "/login_user",
"method": "POST",
"body": ".*\\\\\\\"name.*"
}
}
]
},
{
"responses": [
{
"is": {
"statusCode": 201,
"headers": {
"Content-Type": "application/json"
},
"body": {}
},
"_behaviors": {
"wait": 100
}
}
],
"predicates": [
{
"equals": {
"path": "/login_user",
"method": "POST",
"headers": {
"Content-Type": "application/json"
},
"body": {
"name": "Tony",
"age": "20"
}
}
}
]
}
]
Alternatively, you can also specify a default response code of 400 in the imposter declaration, so that anything that doesn't match a specific predicate will return a 400 response by default:
{
"port": "22001",
"protocol": "http",
"name": "login_user",
"defaultResponse": {
"statusCode": 400
},
"stubs": [
{
.... snip ....