CODEWITHSUNDEEP
javascript
Drew
Drew

Reputation: 111

Adding a method to an object

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);

Upvotes: 2

Views: 102

Answers (4)

Telmo Dias
Telmo Dias

Reputation: 4168

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());

Upvotes: 0

Roko C. Buljan
Roko C. Buljan

Reputation: 206028

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

Upvotes: 5

epascarello
epascarello

Reputation: 207501

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);

Upvotes: 2

Patrick
Patrick

Reputation: 17973

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;
}

Upvotes: 2

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