Learn.js

🇬🇧 This this is the english version of learn.js! Just keep scrolling :D
🇧🇷 Clique aqui para ir para a versão português: Aprenda.js
🇨🇳 点此查看中文版本：Learn.js

Click here to open the Table of Content

• Contributing

• Data types and structures
  • Numbers
  • Math Operators
  • Strings
  • Booleans and Comparisons
  • Empty values and Boolean conversion
  • Variables
  • Objects
  • Arrays
  • Merge Objects or Arrays
  • Functions
  • Conditional Operations
• Leveling up
  • Map
  • Filter
  • Chaining maps and filters
  • Reduce
  • Recursion and Loops
  • Mutability
  • Scopes and State Management
  • This is the secret of functions
• Interacting with the world
  • Asynchronous interactions
  • Callbacks
  • Promises
  • Promises and Maps
  • Async Await
  • Using foreign code
  • Learn more
    • What we didn't learn

Usage

You can copy any part of this guide inside the code blocks and paste it in a developer console and see the code output. To open a javascript developer console: open your web browser > press F12 > console tab. It's recommended to read this guide with the console open and fiddle with the code, this helps the learning process!

Click here to see a preview image of how the console looks like

Data types and structures

Programming always involve some kind of transformation of data. You can store data, count it, apply math operations, trigger other flows, but the basic mechanism of programing languages is always about reading some data and then executing some kind of logic with it.

In Javascript we have the following data types and structures:

Numbers

In javascript every number falls into the "Number" type and there is no distinction between integers and floating points

1
81072865287.331
19.35

// this is a comment, everything after // in the same line is ignored!

• Read More about Numbers

Math Operators

We can start to write some very simple programs if we know about Numbers and operators. Lets write some programs:

2 + 4 // 6
(2 * 4) // 8
// Parenthesis works as expected in math
2 / 4 // 0.5
2 ** 4 // 16 (2 * 2 * 2 * 2)
5 % 4 // 1 (the remainder of the integer division)

All 5 lines above could be seen as 5 really small programs. Each of them is very limited but they do perform a purposeful transformation in some initial data. These small programs are still quite unusable for and end user since a basic calculator would solve this types of problems, but the purpose of calling them programs is to try to consolidate the view that a program is simply a transformation of some given data. As we learn more complex data structures we'll quickly achieve some problem-solving techniques that no calculator can solve!

• Read More about Math

Strings

Sequences of alphanumeric characters, delimited by ', " or `

' or " are used for strings that has no literal linebrakes (you can add them with "\n" special character)

'pan'
"cake"
'pan cake'
"pancake cake"

Both phrases below are the same. ` is a newer version of ' and " that allows strings to have line breaks and also allows for us to add variables (a concept still to be explained) inside them later on.

"this makes\nme hungry"

`this makes
me hungry`

• Read More about Strings

Booleans and Comparisons

Some might say life is relative, but not for booleans. A boolean is a value that is either true or false

true
false

You can inverse a boolean value with the ! operator

!true // false
!false // true

You can also produce boolean values by making comparisons with these operators

1 === 1 // true (equals)
1 !== 1 // false (not equals)

4 > 3 // true (greater than)
3 >= 3 // true (greater than or equals)

4 <= 3 // false (smaller than)
4 < 3 // false (smaller than or equals)

Booleans are also used to make logical comparisons

true && true // true
true && false // false (in any order)
false && false // false

true || true // true
true || false // true (in any order)
false || false // false

Empty values and Boolean conversion

We can also say something is actually nothing, and one of the worst parts of javascript is that we have 2 ways to say that

undefined
null

Whats the difference? Roughly "undefined" happens when a value was not ever defined to begin with. "null" is a value that normally someone purposely added there because it will be filled later on. You shouldn't bother much with both now, just know that they exist and they act as "false" when used on boolean comparisons. Now that you know that, lets end this section with a reference table on what values are considered "truthy" or "falsy" when used as booleans:

The following values are always falsy, meaning that when used as booleans they are considered as false:

false
0 // (zero)
'' // (empty string)
""
``
null
undefined
NaN // Not a Number, shows up when you do math with a type that is not a number

ALL other values will always be truthy, meaning that they will be considered as true on boolean operations.

• Read More about Booleans

Variables

Now that we know about data types let's learn how to store those types in your computer memory so you can access them by your program later on

let x // (is the same as) let x = undefined
let y = 4
let someValue = "a string"
let anotherValue = 4 < 3 // in other words, anotherValue = false

y // 4
someValue // "a string"

We use the reserved keyword "let" in order to tell javascript that we want our variables to not only exist but also to have a initial value. After that we can read our variables by simply using their names. We can always reassign new values to our variables that were declared with "let":

y = 6
y // 6
y = y + 2
y // 8

someValue + " " + someValue // "a string a string"

It's highly not recommended to use math operators for any values that are not Numbers, but JS is really permissive at its core so this type of stuff exists, we can achieve this same result with a more modern fashion using ``

`${someValue} ${someValue}` // "a string a string"

• Read More about Variables

Objects

An object in javascript is a structure where you can have as many data types as you want aggregated in a single data type.

let exampleObject = {
  "property": "value",
  "anotherProperty": 2,
}

Objects are a bunch of Key:Values. Every key has a value, if a key doesn't exist you will receive undefined as a value when trying to access it.

exampleObject.property // "value"
exampleObject["property"] // "value"

Since keys are mostly always string, javascript has removed the need to use ' in Keys so its cleaner to declare objects:

exampleObject = {
  property: "value",
  anotherProperty: 2,
}

exampleObject.property // "value"

Object is a really powerful data type in JS, as you learn the next data types (Arrays/Lists and Functions) you'll see how easy it is to achieve high complexity solution with only these blocks. We use objects to aggregate data that has some kind of connection between them. Let's say you want to register a shirt on an e-commerce:

let shirt = {
  color: "blue",
  price: 19.90,
  description: "A blue t-shirt, you'll never need another one in your life!",
}

shirt.price // 19.90

Objects allow you to structure "new data types" that are specific to your problem's needs. Here is a secret: all the types we've seen until now are built on top of an "Object" because everything in JS is an Object.This will make more sense as you learn more concepts.

Since a Value in an object can be any data type, it can also be an object, this is how our structures start to quickly gain complexity:

let setOfClothes = {
  shirt: {
    color: "blue",
    price: 19.90,
    description: "A blue t-shirt, you'll never need another one in your life!",
  },
  pants: {
    color: "red",
    price: 19.70,
    description: "A red pant!",
  },
}

setOfClothes.shirt.color // "blue"
setOfClothes.pants.color // "red"

• Read More about Objects

Arrays

When using an object you store values in "named keys". Objects are an unordered list of values where you can assign keys to access those values. An Array (also called a List) is an ordered list of values where the key is the position of the element in the list.

let listOfFruits = ["apple", "banana", "mayonnaise"]

listOfFruits[0] // "apple"
listOfFruits[1] // "banana"
listOfFruits[2] // "mayonnaise"

Arrays have some properties because they are also objects, but you should not assign new properties to them

listOfFruits.length // 3

Arrays and Lists are extremely useful in many scenarios, specially when you want to apply operations to a large amount of data that has the same type We'll dive deeper into them later.

• Read More about Arrays

Merge Objects or Arrays

You can merge objects into other objects or arrays into other arrays. This is a largely useful operation that javascript has a sweet new syntax for you to do it:

let objectA = { name: "Avocado" }
let objectB = { name: "Orange", size: 1 }

let objectAB = {
  ...objectA,
  ...objectB,
} // { name: "Orange", size: 1 }

let objectBA = {
  ...objectB,
  ...objectA,
} // { name: "Avocado", size: 1 }

let anotherObject = {
  ...objectB,
  ...objectA,
  color: "green",
} // { name: "Avocado", size: 1, color: "green" }

and you can do the same with arrays

let arrayA = [1, 2, 3]
let arrayB = [4, 5, 6]

let arrayAB = [
  ...arrayA,
  ...arrayB,
] // [1, 2, 3, 4, 5, 6]

let arrayBA = [
  ...arrayB,
  ...arrayA,
] // [4, 5, 6, 1, 2, 3]

Functions

One of the most beautiful parts in JS are functions. Function is a data type that allow you to store a piece of code so you can then execute it later on. It allows you to easily customize the input value from the usage perspective and when you mix Functions with Arrays, Objects, and the other data types you can achieve any operation you desire. Let's understand the basic concepts of functions:

let addNumbers = function (someNumber, anotherNumber) {
  // someNumber and anotherNumber are variables that we are going to use inside
  // the function and will be passed by the code who calls this function with
  // (). This commonly called "argument" or "parameter". You can name parameters
  // as you like and have as many as you want.

  // you can do lots of things here
  return someNumber + anotherNumber
}

addNumbers // Function

In order to execute the code inside the function we need to invoke it with parenthesis and send the values for our variables declared in the first line of our function declaration

addNumbers(1, 2) // 3
addNumbers("a", "b") // "ab"

OMG we just went full illegal and used addNumbers to actually add strings! JS is a really permissive language and with great power comes great responsibility. Functions by default do not care about what types you send when you use them, since in js the + operator can also be used by strings our function will work, but it's our responsibility to build the tools that we want if we want to prevent unwanted scenarios (or if you want strong type safety in JS you can use something like rescript, elm, or typescript).

We can write functions in a shorter way thanks to the new versions of javascript:

let divide = (x, y) => {
  // you can do lots of things here
  return x / y
}

let multiply = (x, y) => x * y

This is a really neat way to write short functions, if you omit {} it will automatically return the value.

We use many functions that are standard from JS in order to write our programs. A classic one is:

console.log("Hi there!")

console.log is a default function that will print a value into your console. Until now we've been typing and returning values one by one, when running a program with many lines and values we often use console.log to debug or describe actions in our developer console.

• Read More about Functions

Conditional Operations

One last subject we need before being able to mix all concepts: Conditional execution of code. At many times you'll only want to execute code if a condition is met, and we do it like this:

3 === 4 below can be replaced for anything that returns a boolean, you can invoke function that returns a boolean for example

if (3 === 4) {
  console.log("yayy")
} else {
  console.log("nay")
} // if you run this it will only log "nay"

You can also assign values to variables given a condition. for example:

let someVar = 3 === 4 ? "yayy" : "nay" // someVar = "nay"

or you can use whitespace to help with readability

let otherVar = 4 === 4
  ? "yayy"
  : "nay" // otherVar = "yayy"

Both methods to execute code and assign values conditionally will be used as you progress.

• Read More about Conditional Operations

Leveling up

By now we have learned about complex types such as Functions, Objects, Arrays and simpler types like Number, String, Boolean, and emptiness. When developing in javascript you can always use any type anywhere. For example you could have an Array of Objects that has Functions inside them, or you can have a Function that receives another Function as a variable. There is truly no limits as to what you do with the types that you just learned, this is both really awesome and really dangerous because it's easy to lose track of what function deals with what part of your problem if you don't organize your code properly.

There is no one - right - way - fits - all to learn how to organize code, you will learn by writing it and applying to real cases and learning what works and what gives you headaches on maintenance time.

Map

Now lets dive into a scenario where we have an Array of numbers and a function that increments a number by 1:

let list = [1, 2, 3]

let increment = (x) => x + 1
// this is equivalent to:
increment = function (x) {
  return x + 1
}

Let's say we want to apply increment to all the values inside our list. It would be quite cumbersome to do it like this:

let newList = [increment(1), increment(2), increment(3)] // [2, 3, 4]

If this list had 10000 elements we would last forever to write this code. To help us, Arrays has 3 built-in functions that helps you to apply transformations in the whole list: map, filter an reduce. Map is the first one we'll learn and it solves exactly the case above:

newList = list.map(increment) // [2, 3, 4]

This may seem really magical but lets dive deeper into whats happening here

list is an Array and arrays have the map function by default. The map functions receives a function and applies it individually to the array values, the return of this function will be the new value, when finished it returns a whole new Array with tall new values. we can write the code above in other ways, such as

newList = list.map((element) => increment(element))

newList = list.map(function (element) { return increment(element) })

Let's see another example but this time using a more complex data type inside our Array and we'll also use some of our Merge knowledge

let anotherList = [
  {
    name: 'Apple',
    tastes: 'Really Good',
  },
  {
    name: 'Banana',
    tastes: 'Really Good',
  },
]

This time we have a list of Objects where an object represents a fruit and how do we think it tastes. Let's say that today we really hate fruits and we need to fix our list in order to match reality

let anotherNewList = anotherList.map((fruit) => {
  return {
    ...fruit,
    tastes: "Really Bad",
  }
})

console.log(anotherNewList) /*
[
  {
    name: 'Apple',
    tastes: 'Really Bad',
  },
  {
    name: 'Banana',
    tastes: 'Really Bad',
  },
]
*/

This can be rewritten in a shorter manner to achieve the same result:

anotherNewList = anotherList.map((fruit) => ({
  ...fruit,
  tastes: "Really Bad",
}))

Merging is extremely useful for creating new objects that inherit all old properties allowing you to manually override the values you want. We've use this in the examples above in order to use map to create a list of new objects given our list of old objects.

Map is used to apply transformation to all data inside a list. But what if we want to filter some elements out before applying transformations? We use a method called filter for that.

• Read More about Map

Filter

The usage of filter is really similar to map. It's a built-in array function that you also sends a function as input just like map, but instead of returning a new value you return either true or false to determine whether an element should be kept or thrown away. Here is an example:

let isEven = x => x % 2 === 0
// returns true when X is even and false when X its odd

let numList = [1, 2, 3, 4, 5]

let newNumList = numList.filter(isEven) // [2, 4]

And we could also have more complex types inside the array

let moreFruits = [
  {
    name: "Apple",
    tastes: "Really Good",
  },
  {
    name: "Banana",
    tastes: "Just Good",
  },
]

let newFruitList = moreFruits.filter(fruit => fruit.tastes === "Really Good")
// [ { name: "Apple", tastes: "Really Good" } ]

• Read More about Filter

Chaining maps and filters

Since both map and filter are guaranteed to always return a new array we can always chain another map or filter, this is a really sweet way to chain operations even if you are working only with 1 element inside the array. As you progress into programming you'll see how wonderfully this property really is, but for now let's just try to grasp the basic usage:

let products = [
  { name: "Skate", category: "Sports", price: 100 },
  { name: "Ball", category: "Sports", price: 10 },
  { name: "Shirt", category: "Clothes", price: 50 },
]

Let's say we want to generate a list that has only Sports products and we'll also apply a 20% discount on them.

products
  .filter(product => product.category === "Sports")
  .map(product => ({ ...product, price: product.price * 0.8 }))
// [
//  { name: "Ball", category: "Sports", price: 8 },
//  { name: "Shirt", category: "Clothes", price: 40 },
// ]

Remember: you can chain maps and filters as much as you want!

Reduce

Reduce, the parent of map and filter, is the most powerful function in our triad (map, filter, reduce) because you can simply write both map and filter with reduce and really you can do whatever you want with its power. Reduce allows you to produce any data type you want by reading an array and applying a function to all elements, but the difference between map and reduce is that the reduce function will maintain a variable that is always passed forward to the next iteration so you can build up anything you want by reading the array element by element and then building up this shared variable (called accumulator).

Let's see this in practice:

let moreProducts = [
  { name: "Skate", category: "Sports", price: 100 },
  { name: "Ball", category: "Sports", price: 10 },
  { name: "Shirt", category: "Clothes", price: 50 },
]

How would you sum the prices of all products? Let's do it with reduce:

moreProducts.reduce((accumulator, product) => accumulator + product.price, 0)
// 160

The execution of the above code happens like this:

// (0, { ... price: 100 }) =>  0 + 100
// (100, { ... price: 10 }) =>  100 + 10
// (110, { ... price: 50 }) =>  110 + 50
// 160

Note that the accumulator is always the result of the previous function invocation, and the initial accumulator is passed to the initial reduce function as a second argument.

In this case we reduced an array to a number but you could use any type and any kind of operation you want. Let's use reduce in a more complex scenario where we want to apply a series of functions to a value:

let someFunctions = [
  x => x + 2,
  x => x * 10,
  x => `The final result is ${x}`,
]

someFunctions.reduce((accumulator, func) => func(accumulator), 30)
// The final result is 320

Note that because reduce is not guaranteed to return an Array (it will return whatever type you want as the accumulator) you have to be careful when chaining maps and filters after using a reduce.

• Read More about Reduce

Recursion and Loops

Sometimes we need to repeat actions and if either map or reduce seems to not fit we can use both methods below

// Loop
let x = 0
while (x < 5) {
  console.log(x)
  x = x + 1
} // Will log 0, 1, 2, 3, 4 in separate lines in your console

// Recursion
let aFunc = x => {
  if (x < 5) {
    console.log(x)
    return aFunc(x + 1)
  }
}
aFunc(0) // Will log 0, 1, 2, 3, 4 in separate lines in your console

Loops take a more "mutable" approach while "recursion" looks more like the functions we learned. The only problem with recursion in JS is that for large loops (for example more than 10 thousand elements but it depends on many things) if you don't know what you are doing you might "explode the function call stack". There are many ways to avoid this, but the most efficient one I've seen simply convert your recursions into loops, this technique is called "trampoline" (also linked on a reference in the end of this guide), we wont dig further into this but feel free to use both loops and recursion, you'll learn better about the usage of both if you try them in practice and decide for yourself the best use-case for them!

• Read More about Recursion and Loops

Mutability

With all the tools above you are ready to transform anything into anything you want. I'd like to introduce you now to the concept of Mutability and Immutability so we can learn about state management, which will help us understand how to better manage our code.

Mutation is the act of changing an existing variable without creating a new variable. Thorough this guide there were a few examples where a variable was declared with "let" and then later on it was reassigned to another variable, that is a mutation. Here are some examples of mutations:

let a = 1 // No mutation happened here
a = a + 1 // This is a mutation

let b = { value: "something" } // No mutation happened here
b.value = "something else" // This is a mutation

let c = [1, 2, 3] // No mutation happened here
c[0] = c[0] + 1 // This is a mutation
c[1] = c[1] + 1 // This is a mutation
c[2] = c[2] + 1 // This is a mutation
console.log(c) // [4, 5, 6]

Here are the same examples avoiding mutations

let a = 1
let newA = a + 1

let b = { value: "something" }
let newB = { ...b, value: "something else" } // {value: "something else"}

let c = [1, 2, 3]
let newC = c.map(x => x + 1) // [2, 3, 4]

Note: map, filter, and reduce will always produce new values instead of mutating the old value, this is part of why they are amazing tools

We can clearly see that in the second example we had to create more variables and invent more names, but in the long run you'll be able to grasp how this allows you to write clearer and more debugable code.

One of the most common source of errors in programming happens by sharing a variable among many functions and mutating it, but this procedure is also a necessity for most of the programs. Let's learn now how to share a variable with functions to act as a storage and try to keep our mutations contained so our program's complexity doesn't outgrow us.

• Read More about Mutability and Immutability

Scopes and State Management

Our functions have access to all variables in the scope that it was declared. Let's see what this means in practice:

let x = 0

let someFunc = () => {
  console.log(x) // 0
}

See how someFunc can access x. Lets see how this wouldn't be possible

let someFunc = () => {
  let y = 1
}

let otherFunc = () => {
  console.log(y) // y is not defined (undefined)
}

otherFunc can't access y because its only visible inside someFunc scope. Let's see other example:

let someFunc = () => {
  let otherFunc = () => {
    let z = 2
  }
  console.log(z) // z is not defined (undefined)
}

someFunc cant access z because only otherFunc can. But now lets make an example where it works:

let a = 1
let someFunc = () => {
  let b = 2
  let otherFunc = () => {
    let c = 3
    console.log(a) // 1
    console.log(b) // 2
    console.log(c) // 3
  }
}

otherFunc can access everything. someFunc can access a, and b. The rest of the program will only be able to access a. Lets see how we use this in practice:

let createStatefulObject = (initialValue) => {

  let value = initialValue

  return {
    increment: (amount = 1) => {
      // "amount = 1" means that if amount is not sent it will be defaulted to 1
      value = value + amount
    },
    display: () => {
      console.log(`Value is: ${value}`)
    },
  }

}

let object = createStatefulObject(0)

object.display() // Value is: 0

object.increment()

object.display() // Value is: 1

object.increment(5)

object.display() // Value is: 6

The thing to note here is that we cannot access value inside our object without using the functions that we returned. This is a technique that uses the parent function scope to hold a variable that can be used by all inner functions but cannot be accessed by outside. This property of sharing scopes between functions is called "Closure".

• Read More about Scopes and State Management

This is the secret of functions

Functions have a secret they don't want you to know, but I'll tell you, this is the secret. There is an implicit argument that every function has by default named this, and the way it behaves depends on who calls your function or where did you declare it. this is normally used by some techniques to store scope like demonstrated on the section above, but we'll avoid using it in this guide since we can achieve the same results with other techniques. We can override this by using call and bind functions:

let thisFunc = function () { console.log(this) }
// Doesn't work with short arrow functions

thisFunc() // Window (In the browser this will log the Window object)

thisFunc.call(1) // 1
thisFunc.call("ayy") // "ayy"

let bindExample = thisFunc.bind(3)

bindExample() // 3
bindExample() // 3

let boundSomething = thisFunc.bind("something")

boundSomething() // "something"
boundSomething() // "something"
bindExample() // 3
bindExample() // 3

So we can use call to override this and execute a function or we can use bind to override this and create a new function that will remember that value for whenever we want to use it. The cool thing about bind and call is that they also accept the other variables of your function, you can ignore the this usage in order to use bind to simply store values in functions arguments for later usage. This technique is called "partial application" and it looks like this in practice:

let sum = (x, y) => x + y

sum(1, 2) // 3

let sum5 = sum.bind(null, 5)
// The first parameter is `this`, the second is `x`, the third is `y`, etc...

sum5(10) // 15
sum5(2) // 7

In the example above we used bind to simply store a number, but since this is JS you can use to store anything you want. Partial application is a handy way of creating new more specific functions based on any function that you already have, this is a really powerful tool! When you move on to start building larger applications you can use "partial application" to solve many common problems such as "dependency injection":

let dependencies = {
  database: {
    createUser: query => { console.log(`User Created: ${query}`) },
  },
}

let routes = [
  {
    name: 'createCharacter',
    handler: function (dependencies, query) {
      dependencies.database.createUser(query)
    }
  },
  {
    name: 'attackMonster',
    handler: function (dependencies, query) {
      // this would handle some logic on attacking a monster
    }
  },
]

let boundRoutes = routes.map(route => ({
  ...route,
  handler: route.handler.bind(null, dependencies)
}))

boundRoutes[0].handler("Blorms")
// User Created: Blorms

The example above might be really painful to understand if you are still learning the ropes, but it's an example on how you can use bind in order to partially apply a dependency object into a bunch of routes that will be used by us. This example is really rough and incomplete but the goal was to focus on the bind usage rather than providing a real dependency scenario, we'll need the next section in order to deal with those:

Interacting with the world

Asynchronous interactions

Until now everything we did was "synchronous". This means that the code executes each line as expected in the top-down order, but sometimes our functions are impossible to be able to return a value instantly (for example, imagine fetching data from a server, you have to request, wait for the server to compute, then finally you receive the response). Since Javascript was designed for the browser environment and many of the interactions happens Asynchronously (Async) it was born with tools to deal with those scenarios but the tools rapidly evolved, let's see how JS deals with async operations:

Callbacks

Callback is the fundamental async mechanic that builds all the other tools we'll see. Today we avoid using callbacks because most of the modern tools implement promises (and therefore async/await), but here is how a callback works:

let ding = () => {
  console.log("ding!")
}

setTimeout(ding, 1000)
// setTimeout is a default JS function that waits (in our case 1000 milliseconds)
// and then executes the callback function that you send to it after waiting

console.log("dong!")

In the example above we'll see dong! before ding! and this is only possible because ding was put on hold by setTimeout, so ding acted as a callback. With this we can better understand how callbacks help dealing with executing code that doesn't follow the synchronous structure, but they have a problem that as soon as you start using them you`ll find out.

Lets meet our friend "callback hell"

setTimeout(()) => {
  setTimeout(()) => {
    setTimeout(()) => {
      console.log("This will show up after 3 seconds")
      // say hello to callback hell!!

      // here we tried to chain up 3 functions that wait 1 second
      // but this will only go deeper and deeper the more
      // callbacks you need to chain :D so imagine chaining
      // up 20 times! We'll see how Promises avoid creating
      // deeper levels of indentation later on.
    }, 1000)
  }, 1000)
}, 1000)

• Read More about Callback

So lets move on to the structure that swore to destroy the callback hell but instead also brought it's own type of hell:

Promises

Promise is a structure that proposes to deal better with async interactions than callbacks, specially when you need to chain interactions. You can see a Promise as a "complex data type" just like arrays, objects, and functions.

let p = new Promise(resolve => {
  resolve(2)
})
// we won't go through "new" in this guide but you'll find
// reference on this guide's end if you want to learn about it

console.log(p) // Promise

p.then(x => console.log(`X is: ${x}`)) // X is: 2

The thing about promises is that once you throw a value inside a promise chain you can only access that value by using .then provided by the Promise type (much like map is provided by the Array type).

Let's see how to convert a callback into a promise:

let setTimeoutPromisified = waitTime => new Promise((resolve) => {
  setTimeout(() => resolve("any data"), waitTime)
})
// `new` is something we won't be looking at on this guide but basically
// it provides the functions `this` with many special properties that has
// no use for us at the moment in this example. You can think that
// `new Something()`
// could be internally rewritten to be used as something like
// `createSomething()`
// like we've done with `createStatefulObject` a couple sections back

setTimeoutPromisified(1000)
  .then((anyData) => {
    console.log(anyData) // "any data"
    return setTimeoutPromisified(1000)
    // "return x" works just like "resolve(x)" inside the promise chain
  })
  .then(() => setTimeoutPromisified(1000))
  .then(() => setTimeoutPromisified(1000))
  .then(() => {
     console.log("this will show up after 4 seconds passed")
    return setTimeoutPromisified(1000)
  })
  .then(() => setTimeoutPromisified(1000))
  .then(() => setTimeoutPromisified(1000))
  .then(() => setTimeoutPromisified(1000))
  .then(() => setTimeoutPromisified(1000))
  .then(() => setTimeoutPromisified(1000))
  .then(() => {
     console.log("this will show up after 10 seconds passed")
  })

  // look how we don't go many levels deeper to
  // achieve the same chain we had in the callback hell

Basically Promises solve the callback problem by giving you the structure of using .then() to declare what happens after something ends, and something ends by calling the resolve function (which is just a callback). Promises have many other properties that I won't go into now but they are worth looking into and will be referenced on this guide's end.

Let's see where Promises will make your life a bit difficult:

Promise.resolve(5) // This is a neat way to start a promise chain!
  .then(x => {
    console.log(x) // 5
    return "abc"
  })
  .then(y => {
    console.log(y) // "abc"
    return [1, 2, 3]
  })
  .then(z => {
    console.log(z) // [1, 2, 3]
    // what if we wanted to console log "x" or "y" here?
    // this is where promises "fail", you would have to either
    // keep passing down everything you want or you have to create a
    // mutable variable in the upper scope to keep what you want to use later.
    // both ways aren't neat solutions, so this is highly inconvenient.
    return
  })

• Read More about Promise

Promises and Maps

Before moving on to how to deal with async and solve the presented problem above I'd like to show you some cool programing moment. I'm gonna throw the code here and let you have your own thoughts:

Promise.resolve(5)
  .then(x => x * 2)
  .then(x => x + 10)
  .then(x => console.log(`X is ${x}`)) // X is 20

[5]
  .map(x => x * 2)
  .map(x => x + 10)
  .map(x => console.log(`X is ${x}`)) // X is 20

Can you spot the difference? Basically both Array and Promise are really special types that provides us with functions that work with its inner values and are always guaranteed to return the same type (Promise.then always return a Promise that guarantees to have .then, just like Array.map is always guarantees to return an Array). The only difference in both solution is that Promises accept async functions while array.map is strictly sync. In functional programming this concept of "types that are container to values and allow you to chain operations" is deeply explored along with the most obscure corners of math.

Async Await

async/await lets you write promises just like you would write synchronous code, and that's where we do the full circle from callbacks to being able to write code that looks mostly what all the code we wrote in the first 2 parts of this guide looked like. The cool thing about async/await is that it's just a more intuitive way to deal with promises, this means that all functions created with async are promises, and everything that is a promise can be awaited inside an async function. Let's see what this looks like:

let setTimeoutPromisified = waitTime => new Promise((resolve) => {
  setTimeout(() => resolve("bling"), waitTime)
})

let aFunction = async () => {
  let data = await setTimeoutPromisified(1000)
  let data2 = await setTimeoutPromisified(1000)
  let data3 = await setTimeoutPromisified(1000)

  // await can be used inside async functions and will not only wait
  // for the promise to resolve but will also unwrap the value inside
  // it and put it on the variable which we were not able to do before

  // using async/await we have no problem using
  // all the variables produced by previous promises
  console.log(`${data} ${data2} ${data3}`) // "bling bling bling"" will be logged after 3 sec

  console.log(await setTimeoutPromisified(1000)) // "bling" will be logged after 4 sec
  console.log(await setTimeoutPromisified(1000)) // "bling" will be logged after 5 sec
  console.log(await setTimeoutPromisified(1000)) // "bling" will be logged after 6 sec
}

aFunction() // This is just a Promise! "async function = Promise"

• Read More about Async Await

Using foreign code

In order to start getting serious and moving out of the console of our browser we should acquire some better coding tools. I recommend you go ahead and install:

• Visual Studio Code - A free and very powerful code editor that works for many languages and it's easy to hop into.

• Node.js + npm - This will install both Node.js and npm in your system. Node.js is used to run javascript outside of your browser, npm is used to both install code from other and share yours.

After doing that you should create a folder, open VSCode, and drag the folder into VSCode. This will make that folder become the project's root folder. Let's start by installing some package from npm that we want to use!

Let's say we want to fetch someone's address given their zipcode. Here in Brazil zipcode is called "cep" and we have npm package called "cep-promise" that gives us a neat promise-based interface to fetch our zipcode.

1. First on your VSCode click Terminal -> New Terminal
2. Then type npm init and fill up what you want but you can just spam enter
3. Then type npm install cep-promise

After that you'll see that some files were generated for us: package.json and a folder called node_modules.

• package.json contains information about your project (given in npm init), you can write some bash scripts there as well, and it also contains a short list of dependencies in case you delete node_modules and wants to reinstall it. npm install without a package name install everything on the list.

• node_modules contains all the files from the dependencies you installed. You normally don't send this file to other people, you simple send package.json and let them do npm install by themselves.

Now lets create a newFile.js to write or code:

const cep = require('cep-promise')
// 'require' is a predefined node.js function that
// allows us to import installed packages

cep('5010000') // this is our example zipcode
  .then(console.log)
// {
//   "cep":  "05010000",
//   "state":  "SP",
//   "city":  "São Paulo",
//   "street":  "Rua Caiubí",
//   "neighborhood":  "Perdizes",
// }

You can install pretty much any package you want and be happy integrating stuff! Sometimes packages are made for browser instead of node.js, this is why you should always read the readme's of the packages you use in order to understand if that package does run in node/browser.

Learn more

This guide is simply a lightning-speed introduction to many concepts. There are three references that I used to study JS myself and you'll find more information about the language either on them or at many different sources thorough the web. JS has grown so much and has so many tools that it's easy to find many wars on which is the better way to do something with JS, but I'd say you should focus on your own development as a developer and learn as many tools as you want to. The more tools you learn the better at solving problems you become, and eventually you'll get the hang by yourself on what tools are the best for what kind of scenario. I hope you had fun reading this and keep evolving your programming skills!

• Different ways to deal with Modules
  We've seen about "require" but modules are quite complex in JS since we have both browser and server environments so support, it's good to learn the different ways people use to export code depending on their target platform and available tools. The most common ones today are CommonJS (require/module.exports) and ES Modules (import/export).

• Eloquent JavaScript
  This book will guide you much more in depth about most of the tools presented on this guide.

• Javascript: The Definitive Guide
  This book is a huge guide that includes reference material on both Javascript and Browser-specific javascript.

• Javascript: The Good Parts
  Although this book is 80% about bad parts, when it does get to the good parts it helps you understand some of the direction that this guide took.

• The Modern JavaScript Tutorial
  I have not personally used this one in the past but it does look pretty neat!

• Safe Recursion with Trampoline in JavaScript

• You Don't Know JS Probably one of the most complete and popular free books on JS. It covers most of the language, and all of the dirty parts in detail like this, prototype, async, etc.

What we didn't learn

Here is a list of things that you might want to study in order to better understand javascript as a whole and wasn't in this guide:

• Hoisting

• Inheritance with the prototype chain

• DOM

• Loops

• Classes