Unaudited Condensed Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	3 Months Ended
Mar. 31, 2021 USD ($)
Series A Convertible Preferred Stock [Member]
Issuance costs	$ 278

The cash inflow from stock issuance costs of an equity security.

No definition available.

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