Objects & Methods in JavaScript

Question

What is the best way to make something like a class in JavaScript and create multiple instances of it?

I've tried the following:

function Test() {
    var currentState = -1;
    var stateArray = new Array();
}

Test.prototype.SetState = function(state) {
    this.currentState = state;

    stateArray.push(state);

    if (stateArray.length > 2) {
        stateArray.splice(0, 1);
    }
}

Test.prototype.GetState = function() {
    return this.currentState;
}

var Object1 = new Test();
var Object2 = new Test();

var EState = {
    ONE: 1,
    TWO: 2,
};

Object1.SetState(EState.ONE); //<< fails here

It works if I use it to create one object, but when I create multiple objects and use any of them get following error:

Uncaught TypeError: undefined is not a function 

What would cause this behaviour?

Answer 1

The issue is stateArray and currentState are private (local only to Test() ) So when you try to do stateArray.push in this context stateArray is undefined and therefore does not have a function called push. one way to fix this is make them a property of Test using the this key word

 function Test() { this.currentState = -1; this.stateArray = new Array(); } Test.prototype.SetState = function(state) { this.currentState = state; this.stateArray.push(state); if (this.stateArray.length > 2) { this.stateArray.splice(0, 1); } } Test.prototype.GetState = function() { return this.currentState; } var Object1 = new Test(); var Object2 = new Test(); var EState = { ONE: 1, TWO: 2, }; Object1.SetState(EState.ONE); console.log(Object1); 

if you would like these to be private you could only return the publicly available functions from the constructor and keep the private data hidden

 function Test() { //private var currentState = -1; var stateArray = new Array(); //public return { SetState: function(state) { currentState = state; stateArray.push(state); if (stateArray.length > 2) { stateArray.splice(0, 1); } }, GetState: function() { return currentState; } } } var Object1 = new Test(); var Object2 = new Test(); var EState = { ONE: 1, TWO: 2, }; Object1.SetState(EState.ONE); Object2.SetState(EState.TWO); console.log(Object1.GetState()); console.log(Object2.GetState());