learn-co-students/js-advanced-functions-introduction-to-context-lab-london-web-012720

JavaScript Advanced Functions: Introduction to Context

Learning Goals

• Define the term "record"
• Define the term "record-oriented programming"

Introduction

Let's take a moment to appreciate where we are. We've reviewed the basic use and creation of functions. We've applied these skills in the context of collection-processing methods like map, reduce, and forEach. We're now ready to face one of the (infamously) most-challenging parts of JavaScript: working with execution context. We're going to start this sequence of lessons off by defining five key terms. Each will be addressed in depth elsewhere, but we want to give you a survey first so you can say "Oh, this is important" when you see the terms.

Definitions

1. Execution Context: When JavaScript functions run, they have an associated JavaScript Object that goes along with them which they can access by the keyword this.
2. this: Inside a function, this is the Object that represents the function's execution context
3. call: This is a method on a function that calls the function, just like (). You provide a new execution context as the first argument, traditionally called thisArg, and the arguments you want to send to the function after the thisArg. An invocation of call looks like: Calculator.sum.call(multilingualMessages, 1, 2)
4. apply: This is a method on a function that calls the function, just like (). You provide a new execution context as the first argument, traditionally called thisArg, and the arguments you want to send to the function as an Array after the thisArg. An invocation of apply looks like: Calculator.sum.apply(multilingualMessages, [1, 2])
5. bind: This method returns a copy of the function but with the execution context "set" to the argument that's passed to bind. It looks like this: sayHello.bind(greenFrog)("Hello") //=> "Mr. GreenFrog says *Hello* to you all."

Printing up these definitions is the only thing most JavaScript documentation does. People accept these as truth and shrug and muddle their way through, living in a state of fear when they have to apply these concepts in JavaScript — but that won't be you!

In this lab, we're going to practice what we've already learned about JavaScript to build a time card application, guided by tests. This application is an example of a "record-oriented" application, which is quite simply an application that is used to process records. For this lab, the records are the time cards for each employee. Once we have a working application, we'll show how execution context, this, call, apply and bind can DRY up our code.

With these capabilities, we hope you'll learn to love functions even more — maybe even as much as JavaScript does!

Define the Term "Record"

Back in the old days (the 1960s and earlier) computers didn't have much memory. Records were stored on, if you can even believe this, small paper cards called punch-cards. They looked like this:

These cards, or "records," often had information on them in "fields." In the first_name field, you'd find a first name, etc... An employee's card would also include a record of how many hours they worked on each day. So when a business needed to figure out how much to pay each person for a week's work, something like the following would happen:

• Load up all the employees' cards into a tray
• Feed the tray of cards into the computer
• The computer would read in each card and calculate the hours worked for the week per card
• The computer would emit a new card with all the old data but this card would have a new field added called something like wagesPaidInWeek33OfYear: 550
• The computer would also print out a table containing the employees' names and how much each of them was owed

ASIDE: Come to think of it, iterating over a collection, performing a transformation and emitting a new collection where every element has been transformed sounds an awful lot like map to us.

Then, the emitted pay ledger could be taken to the payroll department and the appropriate person could write (Write! With their hands! Using a pen and ink!) out paychecks to the employees.

Here's another use. If the executive team needed to know how much payroll cost the company in a given week, they'd (you guessed it!) load up all those punch cards in a tray and run them through a different program that calculated a total.

ASIDE: Come to think of it, iterating over a collection, performing an evaluation on each element and emitting a new value based on those elements sounds an awful lot like reduce to us.

Ultimately, the "punch card" was an intermediate step between paper records and digital records. But it was during the punch-card era that computing really got big, so a lot of our ways of thinking about programming started by thinking about "records."

Define the Term "Record-Oriented Programming"

Record-oriented programming is a style of programming based on accessing records and processing them so that they're updated (map-like) or so that their information is aggregated (reduce-like). "Record-oriented" isn't a buzzword that we hear used very much, but for these next few lessons, we'll use it. Ask any programmer who's worked in large scale billing (at phone companies, insurers, etc.) or at a university (50,000 grade point averages), and you can bet they'll understand what the term means, though.

The amazing thing is that in the 21st century this style of programming is back in vogue! We're not using punch cards, but the ability to spin up hundreds of little computers in a cloud, hand them each a bundle of records, and get answers back is cutting-edge!

In fact, a program to do map and reduce operations at scale on a cloud was standardized in the 2000s. Guess what it's called? mapReduce. It was pioneered and advanced as part of the secret sauce that made a small little company from Mountain View, California called Google become the giant it is today. Today you can use it under the name of Apache Hadoop

The "Go" programming language is built around building and processing records at scale. Record-oriented programming is not likely to go away any time soon. Maybe it'll be the hot job posting buzzword any minute now!

Lab

In this lab, we're going to build a time card and payroll application using the record-oriented approach.

The tests guide you to implementing a time card system: when someone enters the company's state of the art technical office, the employee has to insert their card in a time clock which will record the time they came in. When it's time to leave, the employee will "punch out."

For simplicity's sake, we'll make these assumptions:

1. Assume that employees always check in and check out.
2. Assume employees always check in and out on the hour
3. The time is represented on a 24-hour clock (1300 is 1:00 pm); this keeps the math easier and is the standard in most of the world
4. When timestamps are needed, they will be provided as Strings in the form: "YYYY-MM-DD 800" or "YYYY-MM-DD 1800" e.g. "2018-01-01 2300"
5. Employees will never work across days, i.e., in at 2200 and out at 0400 the next day.

The lab tests will guide you toward a solution. Keep in mind, the goal is to understand how to "grow" an application in "record-oriented" fashion in JavaScript, as well as pass the lab. Make sure you're learning about this app design while you code the solutions. When you encounter a failing test, look at how the test is calling the function that's missing or failing: how did it call the function, what arguments did it pass? What kind of thing did it expect back?

Take advantage of the collection-processing strengths you built up over the last few lessons.

Extending the Challenge

If you have the time, you can learn more about JavaScript and remove the simplifying assumptions we wrote above. You can expand your learning by:

• Raising an exception if a timeIn is found without a matching timeOut
  • Exception Handling in JavaScript
• Figuring out how to turn a time stamp into a construct that allows for you to handle cross-day and non-o'clock times
  • Date Class Documentation
• Raising errors if the time stamp is in an invalid format

While the bar set by the tests is at one level, you can turn this into a robust application, if you so desire!

Do your coding in index.js.

You should be guided by the tests as you work through the lab, but to help make the tests easier to read, we've also provided the signatures of the functions below. A function signature is the function name, the arguments it expects, and what the function returns.

createEmployeeRecord

• Argument(s)
  • A 4-element Array of a String, String, String, and Number corresponding to a first name, family name, title, and pay rate per hour
• Returns
  • JavaScript Object with keys:
    • firstName
    • familyName
    • title
    • payPerHour
    • timeInEvents
    • timeOutEvents
• Behavior
  • Loads Array elements into corresponding Object properties. Additionally, initialize empty Arrays on the properties timeInEvents and timeOutEvents.

createEmployeeRecords

• Argument(s)
  • Array of Arrays
• Returns
  • Array of Objects
• Behavior
  • Converts each nested Array into an employee record using createEmployeeRecord and accumulates it to a new Array

createTimeInEvent

• Argument(s)
  • An employee record Object
  • A date stamp ("YYYY-MM-DD HHMM")
• Returns
  • The employee record
• Behavior
  • Add an Object with keys to the timeInEvents Array on the record Object:
    • type: Set to "TimeIn"
    • hour: Derived from the argument
    • date: Derived from the argument

createTimeOutEvent

• Argument(s)
  • An employee record Object
  • A date stamp ("YYYY-MM-DD HHMM")
• Returns
  • The employee record
• Behavior
  • Add an Object with keys to the timeOutEvents Array on the record Object:
    • type: Set to "TimeOut"
    • hour: Derived from the argument
    • date: Derived from the argument

hoursWorkedOnDate

• Argument(s)
  • An employee record Object
  • A date of the form "YYYY-MM-DD"
• Returns
  • Hours worked, an Integer
• Behavior
  • Given a date, find the number of hours elapsed between that date's timeInEvent and timeOutEvent

wagesEarnedOnDate

• Argument(s)
  • An employee record Object
  • A date of the form "YYYY-MM-DD"
• Returns
  • Pay owed
• Behavior
  • Using hoursWorkedOnDate, multiply the hours by the record's payRate to determine amount owed. Amount should be returned as a number.

allWagesFor

• Argument(s)
  • An employee record Object
• Returns
  • Pay owed for all dates
• Behavior
  • Using wagesEarnedOnDate, accumulate the value of all dates worked by the employee in the record used as context. Amount should be returned as a number. HINT: You will need to find the available dates somehow...

findEmployeeByFirstName

• Argument(s)
  • srcArray: Array of employee records
  • firstName: String representing a first name held in an employee record
• Returns
  • Matching record or undefined
• Behavior
  • Test the firstName field for a match with the firstName argument

calculatePayroll

• Argument(s)
  • Array of employee records
• Returns
  • Sum of pay owed to all employees for all dates, as a number
• Behavior
  • Using wagesEarnedOnDate, accumulate the value of all dates worked by the employee in the record used as context. Amount should be returned as a number.

Conclusion

Congratulations! At the end of this lab, you should have built several incredibly simple functions that leveraged map and reduce to transform and aggregate data. Take a look at your code and see where you might be repeating yourself. Finding these repetitions will be where we can bring in the innovation of execution context. We'll learn how this can DRY up our code using execution contexts in the next lesson.

It's also worth your time to take a look at the tests in test/indexTest.js. Because of this application's design, it's incredibly easy to test the functions which drive the application. Some programmers consider this style of programming to be optimal for the ease of testing and simplicity of code.

Resources

• Record / Record-Oriented Programming
• JavaScript Error Class
• JavaScript Date Class