Javascript中未定义函数的怪异错误

Question

I defined a base and derived class, both defined a function called "getYear":

function Base() {}
Base.prototype.getYear = function() {
    return 2015;
}

function Derived() {
    year = 2016;
}
Derived.prototype = new Base();
Derived.prototype.getYear = function() {
    return 2017;
}

var ins = new Derived();
console.log(ins.getYear());
console.log(ins.prototype.getYear());

The last statement will trigger a runtime error saying

 Cannot read property 'getYear' of undefined

Would you help to explain the reason? I think I have defined this function in both base/derived function.

Answer 1

The prototypes (instance methods) are declared on the Constructor , and only used on the instance . If you wanted to use the original prototype method based on the instance, you could do something like:

var ins = new Derived();
ins.constructor.prototype.getYear();

where you get the getYear prototype from the original constructor . However, this kind of defeats the purpose of using the prototypes in the instance sense.

Here is your example, reworked to do what you were trying to:

 function Base() {} Base.prototype.getYear = function() { return 2015; } function Derived() { year = 2016; } Derived.prototype = new Base(); Derived.prototype.getYear = function() { return 2017; } var ins = new Derived(); console.log(ins.getYear()); console.log(ins.constructor.prototype.getYear());