msosborne3/introduction-to-array-for-each-v-000

Introduction to Array.prototype.forEach

Objectives

1. Explain why we use forEach()
2. Explain how forEach() works

The weaknesses of for loops

Up until now, we've been doing our iterating with for loops. These are great for one-off tasks, and they're reasonably explicit: "I'm incrementing a variable, i, from 0 to myArray.length - 1, and using i to access the elements of the array."

var evens = [0, 2, 4, 6, 8, 10]

for (var i = 0, l = evens.length; i < l; i++) {
  console.log(`${evens[i]} is not odd!`)
}

But wouldn't it be nice if there were a way to access the elements directly, without having to type out evens[i]?

Well, it's our lucky day!

Follow along in index.js — you can run your code by copying it into your browser's console.

forEach()

Every array comes with the method forEach() in JavaScript.

This method lets us iterate over every element of the array directly, without writing out a whole for loop. Let's rewrite the above example with evens using forEach:

var evens = [0, 2, 4, 6, 8, 10]

evens.forEach(even => {
  console.log(`${even} is not odd!`)
})

Whoa! It's so much shorter! Even though this is just a tiny piece of code, let's make sure we understand what's happening.

On the array evens we call the method forEach by typing evens.forEach. The . tells JavaScript that we're going to access some property of the evens array. This property, forEach, happens to be a function, which, as we know, we call with parentheses around its arguments. forEach requires one argument, a function.

Whaaaaat? Did we read that right? We can pass a function as an argument? You bet your boots we can!

That function, in turn, accepts up to three arguments: the current element in the array, the index of that element in the array, and the array itself.

Let's rewrite the function and step through it with the debugger to get a feel for its arguments.

evens.forEach((even, index, array) => {
  debugger
  console.log(`${even} is not odd!`)
})

(As a reminder, debugger is globally available and opens up the debug console in your browser when it's evaluated. Clicking the arrow will let you jump around and inspect the arguments to forEach's function argument on as it iterates over the evens array.)

This is a super powerful idea that makes it much easier for us to build programs that can do many things. Instead of having to operate on each element using a for loop, we can instead very succinctly pass a function to Array.prototype.forEach and let the function do the work for us!

Functions as Arguments

When a language allows functions to be used as arguments, we say that the language treats functions as first-class objects. Oftentimes, these functions serve as callbacks, which do just what their name implies: they are called back, potentially with arguments, and do their work on bits of data that weren't available outside of the function.

For example, let's say that we have a function for logging the square of a number called, appropriately enough, square.

function square(n) {
  console.log(n * n)
}

If we wanted to square all of the elements of evens, we could just write evens.forEach(square). We'll see the squares of evens printed to console.

But what if we don't know ahead of time what we want to do to our evens array? We can make our little program even more generic by wrapping it all up in a function that accepts a function (a callback) as an argument:

// no need to redeclare if you've declared this above
var evens = [0, 2, 4, 6, 8, 10]

function doToEvens(callback) {
  evens.forEach(callback)
}

Now we can pass any function to doToEvens, and that function will be called on every element of evens!

Let's take it a step farther — what if we don't know beforehand what array we're going to be operating on? We can define a function that accepts two arguments: any array and a callback for forEach:

function doToElementsInArray(array, callback) {
  array.forEach(callback)
}

Our code is short and sweet; but it's also generic and enormously powerful.

function changeCompletely(element, index, array) {
  array[index] = (Math.random() * 100).toString() + '!!!'
}

var animals = ["dog", "fish", "cat"]

doToElementsInArray(animals, changeCompletely)

// log out animals -- pretty cool, right?
console.log(animals)

If you've been following along in index.js, you should now be able to run learn submit — all of the tests should pass!

Resources

• Array.prototype.forEach()

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