const add = (...nums) => { if (nums.length !== 2) { throw 'add() requires two arguments' } return nums[0] + nums[1] } if (add(1, 1) === 2) { document.write('pass') } else { document.write('fail') } try { document.write(add(1)) document.write('fail: no error thrown') } catch (e) { if (e === 'add() requires two arguments') { document.write('pass') } else { document.write('fail: wrong error') } }