CODEWITHSUNDEEP
javascript
arachide
arachide

Reputation: 8066

extends javascript function inherits array

I have some javascript code shown below

function extends(Child, Parent) {
    var F = function() {
    };
    F.prototype = Parent.prototype;
    Child.prototype = new F();
    Child.prototype.constructor = Child;
}

function Parent() {
    this.cardArray = [];
}

function Child() {

}

then I call 

extends(Child , Parent);  
var a=new Child();

it reports

 a.cardArray is undefined

Your comment welcome

Upvotes: 1

Views: 124

Answers (1)

T.J. Crowder
T.J. Crowder

Reputation: 1074959

Two issues there:

First, you can't use extends as a function name (unless you use strict mode and only run the code in environments that support strict mode). It's a reserved word in loose mode. (It's not currently used and isn't likely to be, but it's reserved.)

The second, and more sigificant, is that you haven't called Parent anywhere, and so naturally that property has never been added to the object. You need to call Parent from within Child to get the things it sets up, and you need to do it such that this is correct within the call to Parent. We can do that via Function#call, which lets us call a function specifying what this should be (in our case, we want it to be the same as this within the call to Child):

function Child (){

    Parent.call(this);
}

So in total, and with the incorrect (but harmless) semicolons removed, and with the extends changed to something that isn't reserved, and with the indentation made consistent, we get:

Live Copy | Live Source

function extend(Child, Parent) {

    var F = function(){};
    F.prototype = Parent.prototype;
    Child.prototype = new F();
    Child.prototype.constructor = Child;
}

function Parent (){

    this.cardArray=[]; 
}

function Child (){

    Parent.call(this);
}

extend(Child, Parent);

var a = new Child();

console.log("typeof a.cardArray = " + typeof a.cardArray);

...which shows "typeof a.cardArray = object", which is correct.

Note that truly effective JavaScript inheritance requires (for now) a fair bit of plumbing. You have a lot of it there, but not all of it. (Calls to parent methods are awkward, for instance.) I've done a very small library called Lineage that does all the plumbing for you, FWIW.

Upvotes: 3

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