Assign function return value to value in Javascript function [closed]

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Here is a simplified example of the problem I am having.

function Part(price, qty) {
  this.price = price;
  this.qty = qty;
  this.value = function() {
    return this.price * this.qty
  }
}

var fancyPart = new Part(2, 3);


console.log(fancyPart.value)

I would like for this to print 6 to the console, instead it is printing:

function () {
    return this.price * this.qty
  }

Why is this happening?

I am trying to use this to create an "object" and was reading about the various methods over at this question: Which way is best for creating an object in javascript? is "var" necessary before variable of object?

console.log(fancyPart.value()) — you have to call the function. — Pointy
– Pointy, Commented Jan 19, 2018 at 1:03

RobG · Accepted Answer · 2018-01-19 01:23:18Z

1

You could use the class syntax and add value as a getter:

class Part {
  constructor (price, qty) {
    this.price = price;
    this.qty = qty;
  };

  // Implement value as a getter
  get value() {
    return this.price * this.qty
  };

}

var fancyPart = new Part(2, 3);

// No need for parameter list
console.log(fancyPart.value)

answered Jan 19, 2018 at 1:23

RobG

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1 Comment

blackandorangecat Over a year ago

Thanks. This is what I was trying to do - just using the incorrect terminology.

Firas Omrane · Accepted Answer · 2018-01-19 01:05:02Z

0

try console.log(fancyPart.value())... to invoke(call) the function

function Part(price, qty) {
  this.price = price;
  this.qty = qty;
  this.value = function() {
    return this.price * this.qty
  }
}

var fancyPart = new Part(2, 3);


console.log(fancyPart.value())

answered Jan 19, 2018 at 1:05

Firas Omrane

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KnightFox · Accepted Answer · 2018-01-19 01:10:23Z

0

You can from your constructor that value is being assigned as a function

this.value = function() { return this.price * this.qty }

Hence when you try to log fancyPart.value, you are actually logging the function and not it’s invocation.

If you as “()” after value, you will actually invoke the function and get the evaluation you expect (I.e 6 in case of fancypants.value)

answered Jan 19, 2018 at 1:10

KnightFox

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tay_rex · Accepted Answer · 2018-01-19 01:37:55Z

0

Or you can be different and use Function.prototype.call

function Part(price, qty) {
  this.price = price;
  this.qty = qty;
  this.value = function() {
    return this.price * this.qty
  }
}

var fancyPart = new Part(2, 3);


console.log(fancyPart.value.call(fancyPart))

answered Jan 19, 2018 at 1:37

tay_rex

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