Predict the output: does && return a boolean?

Not always. The && operator returns the first falsy operand, or the last operand when all the earlier ones are truthy. It uses short-circuit evaluation.

let a = 0;
let b = 0;
function run() {
  b++;
  console.log(a && b);
}
run(); // 0
console.log('ok' && 42); // 42
console.log(Boolean('ok' && 42)); // true

Pitfall in React: count && <Badge /> can render 0. If the condition should be a boolean, I write count > 0 && <Badge />.

The code adapts a predict-the-output question, with an explicit function call.