JavaScript Objects: Referencing Own Properties

Prerequisites

js-objects-modeling

Objectives

Contrast the defintions of "property", "attribute", and "method"
From within a method, access properties of the same object using this.

Preparation

Fork and clone this repository.
Create a new branch, training, for your work.
Install dependencies with npm install.

Kinds of Properties

Suppose we have the following object literal:

{
  foo: 'bar',
  baz: function () {
    // How do I access the value stored in 'foo' above?
  }
}

foo and baz are both properties, also known as keys. We typically use the term "keys" when object literals are used as dictionaries. When we mix data and methods, we should prefer the term "object" over "dictionary". So, we'll also call the accessor name a "property" instead of a "key".

foo is a kind of property known as an attribute. Attributes are properties that point to values. On the other hand, properties that point to functions are methods.

For the remainder of this talk, we will be focusing exclusively on how we can use this inside of a method to point to value defined elsewhere in an object literal. All other uses of this are out-of-scope.

The answer to the question posed in the previous code snippet is that we use this to refer to the object's own properties. That is to say, when we use this inside a method, that method is contained within an object. For the patterns used in this talk, the containing object is what this refers to.

{
  foo: 'bar',
  baz: function () {
    return this.foo;
  }
}

Do not over-generalize. We usually cannot know what this refers to until the code runs.

`this`

One way to break up the complexity of a problem is by using multiple kinds of objects together, and having each object be responsible for representing a small part of the problem. But these objects don't need to exist in isolation - objects can have other objects (or even collections of other objects) as properties.

Suppose that we wanted to create a simple program ('RunTracker') that helps people prepare for running a 5k. Each day that a person runs, they create a record of their run which contains:

the date and time of the run
the distance covered, in meters
the time taken, in seconds

The program also stores information about the user (the user's name and email address) and can perform some calculations (total distance run, longest run so far, and average speed).

Lab: Diagram and Model

Using the description of the program above, create an entity diagram.

Identify the entities (kinds of objects) needed in the program.
Draw a box for each entity and label it with the singular, capitalized entity name.
Connect any entities that are related using a line.
List attributes and methods of each entity separately within each entity's box.

Demo: Write Methods With `this`

let user = {
  name: "Person McFace",
  email: "wdi@personmcface.com",
  runs : [
    {
      date: "2016-05-25 15:00",
      distance: 1200,
      timeTaken: 600
    },
    {
      date: "2016-05-25 15:00",
      distance: 1400,
      timeTaken: 800
    }
  ],

  totalDistance : function () {},
  longestRun : function () {},
  averageSpeed : function () {}
}

When we start thinking about how the methods for 'User' will work, we run into a difficulty. A method for calculating the longest run so far needs to be able to see, and refer to, all of the runs associated with that particular user. How do we do that?

Follow along as I demonstrate how to complete writing each method.

Lab: Self-Referential Objects

In groups, you're going to work on a similar program to our previous one, this time for meal tracking. In particular, you're going to create an example 'User' object, complete with several 'Meals'.

A 'User' needs to have:

a name (name)
a date-of-birth (bornOn)
a target daily calorie intake (calorieTarget)
a list of 'Meals' that they've eaten (meals)

Every 'Meal' must have:

a title (title), e.g. 'breakfast', 'lunch', 'dinner'
a date (date), represented as a string e.g. "2016-06-25"
a description (description)
a number of estimated calories (calories)

Then, create the following methods for your instance of a 'User':

caloriesEatenOn, which accepts a date (in the format above) and calculates the total number of calories consumed on that date.
avgDailyCalories, which (as indicated), calculates the average number of calories consumed per day, rounded down to the nearest whole calorie.
onTrack, which compares averageDailyCalories to the User's target daily calorie intake, and returns true if average caloric intake is at or below the target (or false if the reverse is true).

Add your code to lib/meals.js, structured similarly to lib/runs.js.

Additional Resources

License

All content is licensed under a CCBYNCSA 4.0 license.
All software code is licensed under GNU GPLv3. For commercial use or alternative licensing, please contact legal@ga.co.

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