You will just lose that week's benefit assuming you earn more than the weekly benefit figure of around £72. (If you earn less than that they'll just deduct what you earn from your weekly JSA payment). This is because you were only unavailable for jobseeking for one week.
But if the work is for more than 16 hours you need to let them know before or on the day you start and sign off from JSA. You should do this over the phone. Then when the work finishes you have to submit a rapid-reclaim.
The Job Centre has a leaflet on this, or go on the JCP/Gov.uk Website