手写 instanceof

[habc0807]

[habc0807]

function instanceof(L, R) { 
    L = L.__proto__
    const O = R.prototype
    
    while(true) {
        if (L === null)  return false
        if (L === O)  return true
        L = L.__proto__
    }
}

左的__proto__存在，是遍历条件，L=== R.prototype 和 L===null 是终止条件

[kayac-chang]

function instance_of(A, B) {
  const current = Object.getPrototypeOf(A);
  if (current === null) return false;
  if (current === B.prototype) return true;
  return instance_of(current, B);
}