The numberOfOutputs property of the AudioNode interface returns the number of outputs coming out of the node. Destination nodes — like AudioDestinationNode — have a value of 0 for this attribute.
An integer ≥ 0.
const audioCtx = new AudioContext();
const oscillator = audioCtx.createOscillator();
const gainNode = audioCtx.createGain();
oscillator.connect(gainNode).connect(audioCtx.destination);
console.log(oscillator.numberOfOutputs); // 1
console.log(gainNode.numberOfOutputs); // 1
console.log(audioCtx.destination.numberOfOutputs); // 0