javascript util.inherits issue in calling derived class functions

Question

I have the following code where there are two functions a, c and c is inheriting from a.

function a(){
    console.log("constructor-a");
}

a.prototype.fun1 = function(){
    console.log("a-fun1");
}
a.prototype.fun2 = function(){
    console.log("a-fun2");
}


function c(){
    c.super_.call(this)
    console.log("constructor-c");
}

c.prototype.fun5 = function(){
    console.log("c-fun1");
}
c.prototype.fun6 = function(){
    console.log("c-fun2");
}
util.inherits(c,a);

var X = new c();
console.log("X instanceof a", X instanceof a);
console.log("X instanceof a", X instanceof c);

Since c is inheriting from a, the functions of both c and a should be executable from an object of C, however, in this case the outputs are as follows:

X.fun1() --> a-fun1
X.fun2() --> a-fun2
X.fun5() --> TypeError: X.fun5 is not a function
X.fun6() --> TypeError: X.fun6 is not a function

util.inherits needs to be called before assignments to the prototype. — Bergi
– Bergi, Commented Feb 16, 2017 at 5:08

RaR · Accepted Answer · 2017-02-16 04:22:19Z

1

Do util.inherits(c,a) first and then create fun5 and fun6.

function c(){
    c.super_.call(this)
    console.log("constructor-c");
}

util.inherits(c,a);

c.prototype.fun5 = function(){
    console.log("c-fun1");
}
c.prototype.fun6 = function(){
    console.log("c-fun2");
}

This is because of the very implementation of util.inherits. That function will re-assign prototype of c with the parent's one instead of merging. So let util.inherits assign b's prototype to c's first and then add extra functions you want.

answered Feb 16, 2017 at 4:22

RaR

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