Array Prototype Methods: Iterators

Lesson Goals

• Define what a prototype method is and what it allows you to do
• List several prototype methods for arrays
• Understand the use-cases for different prototype methods

Vocab

• prototype A model of something and how it should look or behave
• method A function that is defined on an object
• callback function A function passed into another function as an argument, which is then invoked inside the outer function

Prototype Methods

In order to understand what a prototype method is, let’s break down the terms individually:

• A prototype is a model of something and how it should look or behave. We can build off of prototypes to create multiple copies of similar models.
• A method in JavaScript is simply a function that’s defined on an object

Prototype methods are functions that allow you to manipulate the value of a particular data type or class. JavaScript comes with several built-in data types that each have their own prototype methods, that allow you to interact with them in certain ways. For example, you might want to add or remove an item from an array. Or inspect the properties on an object. Prototype methods allow you to perform these actions and manipulate your values.

Array Iteration Prototype Methods

These allow us to loop through an existing array and apply a callback function (similar to event listeners or setTimeout) to each element that might mutate each element and return a new value.

WAIT. Can’t we already iterate through arrays with a for loop?!

In Javascript, there are often many different ways to solve something, and many different tools to choose from. Yes, we can often accomplish the same thing using a for loop, but the array iteration methods do provide some good benefits!

• Cleaner syntax and easier to read
• Offers built-in functionality which in turn, DRYs up code
• More modern way of writing Javascript

There are occassional times when it makes more sense to use a for loop over something like a forEach. You can read more about why to use one over the other here.

Callback Functions

Iteration prototype methods (such as forEach, filter, etc.) take in a callback function as an argument. The callback function is what takes in the mandatory and optional parameters!

someArrayData.somePrototypeMethod(function callBack (mandatoryParameter [,optionalParameters... ]) {
  // some statements
  // often a return statement
})

Since these prototype methods were introduced as part of ES6, most of the time you will see them written using a fat arrow like below:

someArrayData.somePrototypeMethod((mandatoryParameter [,optionalParameters... ]) => {
  // some statements
  // often a return statement
})

There are many array prototype methods out there, but we are going to focus on some of these iterator methods:

• forEach()
• map()
• find()
• filter()
• reduce()

Array.forEach(callbackFunction)

Let’s start by exploring these prototype methods with some examples.

// increase prices by 1.00
const prices = [4.99, 5.50, 7.00, 10.25];
const increasedPrices = [];

const dogs = [ 
  {name: "Fido", numLegs: 4}, 
  {name: "Greg", numLegs: 5} 
];

// console output
=> 'Fido has 4 legs.'
   'Greg has 5 legs.'

Array.map(callbackFunction)

// increase prices by 1.00
const prices = [4.99, 5.50, 7.00, 10.25];
// const increasedPrices =  prices.map...

const degreesF = [67, 32, 55, 102]
// const degreesC...

// console output
=> [19.44, 0, 12.77, 38.88] 

Array.find(callbackFunction)

const numbers = [15, 6, 14, 2, 22, 9, 45];
// const foundNum = numbers.find...

let pets = [
  { name: 'harvey', age: 1 },
  { name: 'julius', age: 3 },
  { name: 'mishu', age: 3 },
];

// const foundPet...

Array.filter(callbackFunction)

const numbers = [1, 2, 3, 4, 5, 6, 7];
// const oddNumbers = numbers.filter...

var beatles = [
  { name: 'John', living: false, instruments: ['guitar', 'bass', 'piano'] },
  { name: 'Paul', living: true, instruments: ['bass', 'guitar', 'piano'] },
  { name: 'George', living: false, instruments: ['guitar', 'sitar'] },
  { name: 'Ringo', living: true, instruments: ['drums', 'bongos'] },
];

// const livingBeatles...

// const guitarPlayingBeatles...

Array.reduce(callbackFunction, initialValue)

Note that the reduce method is slightly different than the previous iterator methods. reduce takes two arguments:

Callback Function - Within the callback, we have access to the accumulator, the current element in the iteration, the current element’s index, and the original array we are looping over

Initial Value - The initial value to be used as the accumulator (the first argument to the first call of the callback). The accumulator is the single value that will eventually be returned. It’s called an accumulator because each iteration over the array will modify the accumulator value until the loop is complete.

const numbers = [1, 2, 3, 4, 5];
// const sum = numbers.reduce...

// console output
=> 15

const adjectives = ['fantastic', 'amazing', 'childish'];
// const wordLengths...

// console output
=> { fantastic: 9, amazing: 7, childish: 8 }

Extra Paired Practice

The only way to get better and more comfortable with these prototype methods is to continue practicing them. Here are a few more examples to work through.

Additional Resources

• Why and when to use forEach, map, filter, reduce, and find in JavaScript
• ForEach vs For Loops
• Finally Understand the JavaScript Reduce Method
• JavaScript Callback Functions - What are Callbacks in JS and How to Use Them
• Callback Functions MDN