Adding a method to an object

Question

I am having an issue with adding a method to an object in javascript. The following code should return a number but instead returns NaN. Hope you can help

function people(name, age){
    this.name = name;
    this.age = age;
    this.numYearsLeft = pension();
}

function pension(){
    numYears = 65 - this.age;
    return numYears;
}

var andrews = new people("Andrews Green", 28);

console.log(andrews.numYearsLeft);

Answer 1

You could use a prototypal model - making pension a Method of people :

function people(name, age){
  this.name = name;
  this.age = age;
  this.numYearsLeft = this.pension();  // note the `this`
}

people.prototype.pension = function(){ // note the `prototype`
  var numYears = 65 - this.age;
  return numYears;
};

var andrews = new people("Andrews Green", 28);

console.log(andrews.numYearsLeft);     // 37

Using prototype your pension method will inherit the constructor's ( people ) properties (allowing you to refer using the this keyword).
Another benefit of this is that, on every new instantiation of people you'll not recreate new instances / recalls of the pension method.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Introduction_to_Object-Oriented_JavaScript

Answer 2

JavaScript works on a "function scope", so in short you're in the wrong scope. You need to either bind the "this" variable or create a function on the people class using the prototype property.

You can define it as a prototype function instead

people.prototype.pension = function() {
    numYears = 65 - this.age;
    return numYears;
}

Answer 3

If you add a console.log() line inside of pension you will see that this is the window and not the people object. One way to change this would be to use call().

this.numYearsLeft = pension.call(this);

Example:

 function people(name, age) { this.name = name; this.age = age; this.numYearsLeft = pension.call(this); } function pension() { numYears = 65 - this.age; return numYears; } var andrews = new people("Andrews Green", 28); console.log(andrews.numYearsLeft); 

Other option is make it part of the people prototype.

 function people(name, age) { this.name = name; this.age = age; this.numYearsLeft = this.pension(); } people.prototype.pension = function () { numYears = 65 - this.age; return numYears; } var andrews = new people("Andrews Green", 28); console.log(andrews.numYearsLeft); 

Answer 4

In order to call a function you need to put (). console.log(andrews.numYearsLeft); should be console.log(andrews.numYearsLeft());

Also on

function pension(){
numYears = 65 - this.age;
return numYears;
}

this.age is undefined, thus the NaN.

(EDITED) Maybe try:

function people(name, age){
    var that = this;
    this.name = name;
    this.age = age;
    this.numYearsLeft = function(){
        numYears = 65 - that.age;
        return numYears;
    };
}
var andrews = new people("Andrews Green", 28);
console.log(andrews.numYearsLeft());