Convert gql-object back to string

Question

I have a user.gql - file with some queries, mutations, inputs etc. I get content of user.gql, using gql(), then filter an document.definition to find needed a query. After filter

document.definitions
    .filter(item => item.kind === 'OperationDefinition')
    .filter(item => item.operation === 'query')

, one request object remains. smth like

{
    kind: "OperationDefinition",
    name: {kind: 'Name', value: 'authenticate'},
    operation: "query",
    selectionSet: {...}
    variableDefinitions: [
        ...
    ]
}

How to convert it back to string? In the file this code is written like this

query authenticate($login: String!, $password: Password!) {
    user(login: $login, password: $password) {
        token,
        name,
        additional
    }
}