if the first term is undefined, isDone will be undefined, not false. At least in Javascript. && returns the falsy value on the left side of the expression if i

if the first term is undefined, isDone will be undefined, not false. At least in Javascript. && returns the falsy value on the left side of the expression if it evaluates to a falsy value (undefined && true) evaluates to undefined correspondingly || returns the truthy value (which might be an object) on the left hand side if evaluates truthy. That's useful for expressions like this: const a = b || 'default value'; !! turns truthy/falsy values into booleans.