你是说实例化一个类吧，对于用户自定义的类，

 new f(a, b, c)

相当于

 // Create a new instance using f's prototype.
var newInstance = Object.create(f.prototype), result;

// Call the function
result = f.call(newInstance, a, b, c),

// If the result is a non-null object, use it, otherwise use the new instance.
result && typeof result === 'object' ? result : newInstance

答案摘自这里