Regular Expressions: Restrict Possible Usernames

Regular Expressions: Restrict Possible Usernames
Usernames are used everywhere on the internet. They are what give users a unique identity on their favorite sites.

You need to check all the usernames in a database. Here are some simple rules that users have to follow when creating their username.

1) The only numbers in the username have to be at the end. There can be zero or more of them at the end.

2) Username letters can be lowercase and uppercase.

3) Usernames have to be at least two characters long. A two-letter username can only use alphabet letter characters.


Change the regex userCheck to fit the constraints listed above.

Regular Expressions: Find One or More Criminals in a Hunt

Regular Expressions: Find One or More Criminals in a Hunt
Time to pause and test your new regex writing skills. A group of criminals escaped from jail and ran away, but you don't know how many. However, you do know that they stay close together when they are around other people. You are responsible for finding all of the criminals at once.

Here's an example to review how to do this:

The regex /z+/ matches the letter z when it appears one or more times in a row. It would find matches in all of the following strings:

"z"
"zzzzzz"
"ABCzzzz"
"zzzzABC"
"abczzzzzzzzzzzzzzzzzzzzzabc"
But it does not find matches in the following strings since there are no letter z characters:

""
"ABC"
"abcabc"

Write a greedy regex that finds one or more criminals within a group of other people. A criminal is represented by the capital letter C.

Regular Expressions: Match Single Characters Not Specified

Regular Expressions: Match Single Characters Not Specified
So far, you have created a set of characters that you want to match, but you could also create a set of characters that you do not want to match. These types of character sets are called negated character sets.

To create a negated character set, you place a caret character (^) after the opening bracket and before the characters you do not want to match.

For example, /[^aeiou]/gi matches all characters that are not a vowel. Note that characters like ., !, [, @, / and white space are matched - the negated vowel character set only excludes the vowel characters.


Create a single regex that matches all characters that are not a number or a vowel. Remember to include the appropriate flags in the regex.

Regular Expressions: Match Numbers and Letters of the Alphabet

Regular Expressions: Match Numbers and Letters of the Alphabet
Using the hyphen (-) to match a range of characters is not limited to letters. It also works to match a range of numbers.

For example, /[0-5]/ matches any number between 0 and 5, including the 0 and 5.

Also, it is possible to combine a range of letters and numbers in a single character set.

let jennyStr = "Jenny8675309";
let myRegex = /[a-z0-9]/ig;
// matches all letters and numbers in jennyStr
jennyStr.match(myRegex);

Create a single regex that matches a range of letters between h and s, and a range of numbers between 2 and 6. Remember to include the appropriate flags in the regex.

Regular Expressions: Match Letters of the Alphabet

Regular Expressions: Match Letters of the Alphabet
You saw how you can use character sets to specify a group of characters to match, but that's a lot of typing when you need to match a large range of characters (for example, every letter in the alphabet). Fortunately, there is a built-in feature that makes this short and simple.

Inside a character set, you can define a range of characters to match using a hyphen character: -.

For example, to match lowercase letters a through e you would use [a-e].

let catStr = "cat";
let batStr = "bat";
let matStr = "mat";
let bgRegex = /[a-e]at/;
catStr.match(bgRegex); // Returns ["cat"]
batStr.match(bgRegex); // Returns ["bat"]
matStr.match(bgRegex); // Returns null

Match all the letters in the string quoteSample.

Note
Be sure to match both upper- and lowercase letters.

Regular Expressions: Match Single Character with Multiple Possibilities

Regular Expressions: Match Single Character with Multiple Possibilities
You learned how to match literal patterns (/literal/) and wildcard character (/./). Those are the extremes of regular expressions, where one finds exact matches and the other matches everything. There are options that are a balance between the two extremes.

You can search for a literal pattern with some flexibility with character classes. Character classes allow you to define a group of characters you wish to match by placing them inside square ([ and ]) brackets.

For example, you want to match "bag", "big", and "bug" but not "bog". You can create the regex /b[aiu]g/ to do this. The [aiu] is the character class that will only match the characters "a", "i", or "u".

let bigStr = "big";
let bagStr = "bag";
let bugStr = "bug";
let bogStr = "bog";
let bgRegex = /b[aiu]g/;
bigStr.match(bgRegex); // Returns ["big"]
bagStr.match(bgRegex); // Returns ["bag"]
bugStr.match(bgRegex); // Returns ["bug"]
bogStr.match(bgRegex); // Returns null

Use a character class with vowels (a, e, i, o, u) in your regex vowelRegex to find all the vowels in the string quoteSample.

Note
Be sure to match both upper- and lowercase vowels.

RegEx: Extract Matches

Regular Expressions: Extract Matches
So far, you have only been checking if a pattern exists or not within a string. You can also extract the actual matches you found with the .match() method.

To use the .match() method, apply the method on a string and pass in the regex inside the parentheses. Here's an example:

"Hello, World!".match(/Hello/);
// Returns ["Hello"]
let ourStr = "Regular expressions";
let ourRegex = /expressions/;
ourStr.match(ourRegex);
// Returns ["expressions"]

Apply the .match() method to extract the word coding.

Regular Expressions: Using the Test Method

Regular Expressions: Using the Test Method
Regular expressions are used in programming languages to match parts of strings. You create patterns to help you do that matching.

If you want to find the word "the" in the string "The dog chased the cat", you could use the following regular expression: /the/. Notice that quote marks are not required within the regular expression.

JavaScript has multiple ways to use regexes. One way to test a regex is using the .test() method. The .test() method takes the regex, applies it to a string (which is placed inside the parentheses), and returns true or false if your pattern finds something or not.

let testStr = "freeCodeCamp";
let testRegex = /Code/;
testRegex.test(testStr);
// Returns true

Understand the Differences Between import and require

ES6: Understand the Differences Between import and require
In the past, the function require() would be used to import the functions and code in external files and modules. While handy, this presents a problem: some files and modules are rather large, and you may only need certain code from those external resources.

ES6 gives us a very handy tool known as import. With it, we can choose which parts of a module or file to load into a given file, saving time and memory.

Consider the following example. Imagine that math_array_functions has about 20 functions, but I only need one, countItems, in my current file. The old require() approach would force me to bring in all 20 functions. With this new import syntax, I can bring in just the desired function, like so:

import { countItems } from "math_array_functions"
A description of the above code:

import { function } from "file_path_goes_here"
// We can also import variables the same way!
There are a few ways to write an import statement, but the above is a very common use-case.

Note
The whitespace surrounding the function inside the curly braces is a best practice - it makes it easier to read the import statement.

Note
The lessons in this section handle non-browser features. import, and the statements we introduce in the rest of these lessons, won't work on a browser directly. However, we can use various tools to create code out of this to make it work in browser.

Note
In most cases, the file path requires a ./ before it; otherwise, node will look in the node_modules directory first trying to load it as a dependency.


Add the appropriate import statement that will allow the current file to use the capitalizeString function. The file where this function lives is called "string_functions", and it is in the same directory as the current file.

ES6: Use getters and setters to Control Access to an Object

ES6: Use getters and setters to Control Access to an Object
You can obtain values from an object, and set a value of a property within an object.

These are classically called getters and setters.

Getter functions are meant to simply return (get) the value of an object's private variable to the user without the user directly accessing the private variable.

Setter functions are meant to modify (set) the value of an object's private variable based on the value passed into the setter function. This change could involve calculations, or even overwriting the previous value completely.

class Book {
  constructor(author) {
    this._author = author;
  }
  // getter
  get writer(){
    return this._author;
  }
  // setter
  set writer(updatedAuthor){
    this._author = updatedAuthor;
  }
}
const lol = new Book('anonymous');
console.log(lol.writer);  // anonymous
lol.writer = 'wut';
console.log(lol.writer);  // wut
Notice the syntax we are using to invoke the getter and setter - as if they are not even functions.

Getters and setters are important, because they hide internal implementation details.


Use class keyword to create a Thermostat class. The constructor accepts Fahrenheit temperature.

Now create getter and setter in the class, to obtain the temperature in Celsius scale.

Remember that C = 5/9 * (F - 32) and F = C * 9.0 / 5 + 32, where F is the value of temperature in Fahrenheit scale, and C is the value of the same temperature in Celsius scale

Note

When you implement this, you would be tracking the temperature inside the class in one scale - either Fahrenheit or Celsius.

This is the power of getter or setter - you are creating an API for another user, who would get the correct result, no matter which one you track.

In other words, you are abstracting implementation details from the consumer.

Beat Astroids Game Using a Neural Network - JavaScript Tutorial

This complete JavaScript tutorial shows how to automate an astroids game using a neural network. This tutorial keeps the complex theory to a minimum and demonstrates how to use a neural network in a real-world situation. The tutorial builds off of a previous JavaScript tutorial that shows how to build an astroids game from scratch. However, viewing that tutorial first is not required.

ES6: Use class Syntax to Define a Constructor Function

ES6: Use class Syntax to Define a Constructor Function
ES6 provides a new syntax to help create objects, using the keyword class.

This is to be noted, that the class syntax is just a syntax, and not a full-fledged class based implementation of object oriented paradigm, unlike in languages like Java, or Python, or Ruby etc.

In ES5, we usually define a constructor function, and use the new keyword to instantiate an object.

var SpaceShuttle = function(targetPlanet){
  this.targetPlanet = targetPlanet;
}
var zeus = new SpaceShuttle('Jupiter');
The class syntax simply replaces the constructor function creation:

class SpaceShuttle {
  constructor(targetPlanet){
    this.targetPlanet = targetPlanet;
  }
}
const zeus = new SpaceShuttle('Jupiter');
Notice that the class keyword declares a new function, and a constructor was added, which would be invoked when new is called - to create a new object.


Use class keyword and write a proper constructor to create the Vegetable class.

The Vegetable lets you create a vegetable object, with a property name, to be passed to constructor.

ES6: Write Concise Declarative Functions with ES6

ES6: Write Concise Declarative Functions with ES6
When defining functions within objects in ES5, we have to use the keyword function as follows:

const person = {
  name: "Taylor",
  sayHello: function() {
    return `Hello! My name is ${this.name}.`;
  }
};
With ES6, You can remove the function keyword and colon altogether when defining functions in objects. Here's an example of this syntax:

const person = {
  name: "Taylor",
  sayHello() {
    return `Hello! My name is ${this.name}.`;
  }
};

Refactor the function setGear inside the object bicycle to use the shorthand syntax described above.

ES6: Write Concise Object Literal Declarations Using Simple Fields

ES6 adds some nice support for easily defining object literals.

Consider the following code:

const getMousePosition = (x, y) => ({
  x: x,
  y: y
});
getMousePosition is a simple function that returns an object containing two fields.

ES6 provides the syntactic sugar to eliminate the redundancy of having to write x: x. You can simply write x once, and it will be converted tox: x (or something equivalent) under the hood.

Here is the same function from above rewritten to use this new syntax:

const getMousePosition = (x, y) => ({ x, y });

Use simple fields with object literals to create and return a Person object.

ES6: Create Strings using Template Literals

ES6: Create Strings using Template Literals
A new feature of ES6 is the template literal. This is a special type of string that makes creating complex strings easier.

Template literals allow you to create multi-line strings and to use string interpolation features to create strings.

Consider the code below:

const person = {
  name: "Zodiac Hasbro",
  age: 56
};

// Template literal with multi-line and string interpolation
const greeting = `Hello, my name is ${person.name}!
I am ${person.age} years old.`;

console.log(greeting); // prints
// Hello, my name is Zodiac Hasbro!
// I am 56 years old.
A lot of things happened there.

Firstly, the example uses backticks (`), not quotes (' or "), to wrap the string.

Secondly, notice that the string is multi-line, both in the code and the output. This saves inserting \n within strings.

The ${variable} syntax used above is a placeholder. Basically, you won't have to use concatenation with the + operator anymore. To add variables to strings, you just drop the variable in a template string and wrap it with ${ and }. Similarly, you can include other expressions in your string literal, for example ${a + b}.

This new way of creating strings gives you more flexibility to create robust strings.


Use template literal syntax with backticks to display each entry of the result object's failure array. Each entry should be wrapped inside an li element with the class attribute text-warning, and listed within the resultDisplayArray.

Use Destructuring Assignment to Pass an Object as a Function's Parameters

ES6: Use Destructuring Assignment to Pass an Object as a Function's Parameters
In some cases, you can destructure the object in a function argument itself.

Consider the code below:

const profileUpdate = (profileData) => {
  const { name, age, nationality, location } = profileData;
  // do something with these variables
}
This effectively destructures the object sent into the function. This can also be done in-place:

const profileUpdate = ({ name, age, nationality, location }) => {
  /* do something with these fields */
}
This removes some extra lines and makes our code look neat.

This has the added benefit of not having to manipulate an entire object in a function; only the fields that are needed are copied inside the function.


Use destructuring assignment within the argument to the function half to send only max and min inside the function.