Unaudited Condensed Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020
Statement of Cash Flows [Abstract]
Prepaid expenses and other current assets includes related party amounts	$ 14	$ 0
Accounts payable includes related party amounts	0	50
Accrued expenses includes related party amounts	$ (544)	$ 195

Prepaid expenses and other current assets includes related party amounts.

No definition available.

Name:	janx_PrepaidExpensesAndOtherCurrentAssetsIncludesRelatedPartyAmounts
Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration