alecgrey / fewpjs-iterators-fndcl-fnexpr-map-lab

Iterator Drill: Map

Learning Goals

• Define how the map() method works
• Demonstrate map() with Arrays
• Demonstrate map() with complex data structures
• Use map() to generate a new array

Introduction

As developers, we find ourselves responsible for all sorts of common, but tedious, tasks, such as iterating over arrays. Although a for loop or nesting in lists and collections will complete the task, we can take advantage of a method like map() to organize and optimize our code into building blocks of functions, which we can then chain together to create more readable and understandable functions.

Define How the map() Method Works

Array.prototype.map() is an array method that iterates over all elements, allowing you to apply a function to each element in that array, and changing them into something else. The result is then returned as a new array, leaving the original array unmodified (but remember, not the elements we modify, that requires defensive copying). This is super important, because it saves us from having to create a new array ourselves and copy stuff in there, or modifying the original elements in the array, which we may need later.

Demonstrate map() With Arrays

We use map() when we want to perform an action on each element in the collection, and "gather" the results into a new Array.

We'll also use this as a chance to demonstrate some of the power of functions in JavaScript. We'll write map() four times so you can see how the iterator functions allow us to write more expressive code.

for and for...of

In this example, we are using a standard bit of iteration code. Because for and for...of are general functions that can be used to do lots of things another programmer won't be sure if the inner workings return values are important or not.

let students = ["harry", "ron", "hermione", "ginevra"];
let rollCall = [];

for (const student of students) {
  rollCall.push( student + " the wizard" );
}

//=> rollCall = ["harry the wizard", "ron the wizard", "hermione the wizard", "ginevra the wizard"];

When we write .map() we are saying to other programmers: "Expect a new array to come out of this after each element is touched!"

map() With a Function Declaration

function studentRollCall(student) {
  return student + " the wizard";
}

let students = ["harry", "ron", "hermione", "ginevra"];
let rollCall = students.map(studentRollCall);
//=> rollCall = ["harry the wizard", "ron the wizard", "hermione the wizard", "ginevra the wizard"];

With map(), we have a list of n things and want that list of n things to be transformed. Here, we're passing a function as an argument. Arguments can be things like Number or String, but, in JavaScript, can also be work. Very few other programming languages allow that!

The iterator function map() expects to be passed a function as an argument which they will hand each of their elements off to. In the case of map() it hands each element to the function and stores the return value of the function into a new Array.

This code is more expressive because it lives up to the promise of map. It creates a new Array after each element is "touched" by a function.

One drawback to this code is that the studentRollCall function doesn't do much work. It just returns something that the student already knew how to do. What if we use a function expression ("anonymous function") instead?

map() With a Function Expression

let students = ["harry", "ron", "hermione", "ginevra"];
let rollCall = students.map(function(student) {
  return student + " the wizard";
});
//=> rollCall = ["harry the wizard", "ron the wizard", "hermione the wizard", "ginevra the wizard"];

That's much shorter. It has all the same advantages of the previous version.

map() With an Arrow Function

Thanks to arrow functions, we can shorten up the function expression to:

// When the parameter list is only one element, we can drop () !
let students = ["harry", "ron", "hermione", "ginevra"];
let rollCall = students.map(student => student + " the wizard")
//=> rollCall = ["harry the wizard", "ron the wizard", "hermione the wizard", "ginevra the wizard"];

The code now fits on one line! There is much less noisy JavaScript code so the expressiveness has increased: "rollCall is the result of map-ing students.

For the rest of of these examples, we'll use the arrow function.

Demonstrate map() With Complex Data Structures

Let's use the map() function on a trickier data structure — a list of robots. To start things off, we have an array of robots. Now, let's activate all of them. An activated robot needs to be marked as such using the isActivated boolean, as well as have its number of modes doubled:

const robots = [
  { name: 'Johnny 5', modes: 5, isActivated: false, },
  { name: 'C3PO', modes: 3, isActivated: false, },
  { name: 'Sonny', modes: 2.5, isActivated: false, },
  { name: 'Baymax', modes: 1.5, isActivated: false, },
];

const activatedRobots = robots.map(function (robot) {
  return Object.assign({}, robot, {
    modes: robot.modes * 2,
    isActivated: true,
  });
});

console.log(activatedRobots);

/*
 Result:
 [
   { name: 'Johnny 5', modes: 10, isActivated: true },
   { name: 'C3PO', modes: 6, isActivated: true },
   { name: 'Sonny', modes: 5, isActivated: true },
   { name: 'Baymax', modes: 3, isActivated: true }
 ]
*/

With the native map() function that is a property of Array's prototype, it gives us the exact same result! Now that we know how map is implemented, it holds no more secrets for us! We can discard our own map() function and just use the map() property on arrays.

Use map() to Generate a New Array

Let's put our newly acquired knowledge of map() to use! We just uploaded 10 coding tutorials online, but some of them have inconsistent casing. We want all the titles to be "title case", in other words, the first letter of each word is capitalized. Create a new array with the new names of each tutorial that is in the proper title case formatting.

For example, what does the this keyword mean? should become What Does The This Keyword Mean?.

const tutorials = [
  'what does the this keyword mean?',
  'What is the Contutorialuctor OO pattern?',
  'implementing Blockchain Web API',
  'The Test Driven Development Workflow',
  'What is NaN and how Can we Check for it',
  'What is the difference between stopPropagation and preventDefault?',
  'Immutable State and Pure Functions',
  'what is the difference between == and ===?',
  'what is the difference between event capturing and bubbling?',
  'what is JSONP?'
];

Run learn and ensure that tests are passing.

Conclusion

map() takes 2 arguments--a callback and the optional context. The callback runs for each value in an array and returns each new value in the resulting array. It returns a new array that is the same length as the original array and saves time while making the code simpler and easy to read.

Resources

• MDN: Array.prototype.map()