Sets the specified field in the hash stored at key to value, only if field does not yet exist. If key does not exist, a new key holding a hash is created. If field already exists, this operation has no effect.
|key holding the hash to set the field of.
|field to set in the hash.
|value to set the field to.
|On success, the promise resolves with 1 if field is a new field in the hash and value was set, and with 0 if field already exists in the hash and no operation was performed.