Khan/khan-exercises — comment on commit daf6396a881d5ae9dc0730d7235f66f74bbe7567
File: khan-exercise.js line 39
By divad12:

I think

testMode = userExercise === undefined,

would suffice here, since `userExercise` _has_ been declared (no risk of ReferenceError), and according to @spicyj we'll never let anyone redefine `undefined`. :)