How to create Abstract base class in JavaScript that can't be Instantiated

Question

I have a class

function Node() {     //implementation } 

and another class

function AttributionalNode() {     this.prototype.setAttr = function (attr) {         this.atText = attr;     }; }  AttributionalNode.prototype = new Node(); AttributionalNode.prototype.constructor = AttributionalNode; 

How to make class Node() so it can't be instantiated? e.g when I try

var node = new Node(); 

So it throws an Exception?

Answer 1

In JavaScript engines that support ECMAScript 2015 (aka ES6) class syntax, this can be accomplished using the new.target meta-property:

function Node() {    if (new.target === Node) throw TypeError("new of abstract class Node"); } 

or using class syntax:

class Node {    constructor () {       if (new.target === Node) throw TypeError("new of abstract class Node");    } } 

in either case, just define AttributionalNode as:

class AttributionalNode extends Node {    constructor () {       super();    }    setAttr(attr) {       this.atText = attr;    } }  new Node();               // will throw TypeError new AttributionalNode();  // works fine 

For a more detailed explanation of new.target see section 4.2 of this document.

Answer 2

This would work:

function Node() {      if (this.constructor === Node) {          throw new Error("Cannot instantiate this class");      }  }    function AttributionalNode() {      Node.call(this); // call super  }    AttributionalNode.prototype = Object.create(Node.prototype);  AttributionalNode.prototype.setAttr = function (attr) {      this.atText = attr;  };  AttributionalNode.prototype.constructor = AttributionalNode;    var attrNode = new AttributionalNode();  console.log(attrNode);  new Node();

Note: you cannot refer to this.prototype inside the constructor, as the prototype is only a property of the constructor function, not of the instances.

Also, see here for a good article on how to properly extend JS classes.