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JavaScript ... love it or hate it, here's how to use it

jbshipman profile image James Shipman ・5 min read

JavaScript

Ahhh JavaScript, the little DOM language that grew into a monster/beast/foundational web technical ... take your pick of terms.

It's light-weight, it can be fast and nowadays it can do almost anything on any operating system and/or browser.

"Quick" Reference Guide

Yeah, this is not going to be a short a list of functions or syntax. Below are my notes for mod 3 of my coding bootcamp that I refer back to often because I'm not going to memorize all of this anytime soon -- I will at some point because that's how I do.


DOM & Just Enough JavaScript

Document-Object Model

  • ask DOM to find or select HTML elements or elements in the rendered page
  • remove and/or insert element(s)
  • adjust a property of selected element(s)

JavaScript / DOM relationship

  • DOM only knows js, js and DOM are linked at hip from start
  • learning about DOM will require js
  • "sight words" to do this

"Sight Words"

  • basic vocab that forms foundation for more learning latter
  • js Sight Words = brief code snippets that are foundational

Just Enough JavaScript

  • learning sight words first is an interactive way of learning js
  • MDN's JavaScript Reference
  • code, code, code, code, code

Coding in Chrome DevTools

  • writing 1 + 1 is an expression
    • a valid unit of code that returns a value
  • math expressions are "raw" in console
  • literal text is wrapped in quotes

"Things" in JavaScript

  • number (integer or float) are things
  • string is a thing
  • reserved words var, let, const
    • var came first, now we have let and const
  • Template Literals
    • with template literals use back ticks, not single quotes

JavaScript is Object-Oriented

  • code i can talk to that know state and behavior
  • all Things in js
    • objects
    • arrays
    • strings
    • numbers
  • dot notation()

JavaScript has loops

  • for each character in slytherin collection, I would like the harry_potter object to invoke expelliarmus method on each wizard passed in as an argument
for (let i = 0; i <slytherins.length; i += 1)
  {
    harry_potter.expelliarmus(slytherins[i]);
  }

Variables

Multi-line Variable Assignment

var a = 5;
var b = 2;
var c = 3;
var d = {};
var e = [];

Equivalent To:

var a = b;
    b = 2;
    c = 3;
    d = {};
    e = [];

Can be Converted To:

var a = 5, b = 2, c = 3, d = {}, e = [];

Use typeof operator to check data type of value currently stored in a variable.

Using const and let instead of var

  • scope issues with 'var'
  • let will throw an error if declare same var more than once
  • const cannot be reassigned
    • assign a primitive value (any data type except object)
    • a const will always point to same object
    • const is self commenting
  • NEVER USE VAR
  • USE LET WHEN I KNOW A VALUE NEEDS TO CHANGE (COUNTERS, LOOPS, ETC...)
  • USER CONST FOR EVERY OTHER VARIABLE

The DOM Tree

document.querySelector(selector)

  • selector is like a query string lets me find things within HTML page

The DOM Tree:
(e) = element
(t) = text
(a) = attribute
Document

|-- <html> (e)
|---- <head> (e)
    |-- <title> (e)
      |-- <My title> (t)
  |-- <body>
      |-- <h1> (e)
          |-- <A heading> (t)
      |-- <a> (e) - <href> (a)
          |-- <Link text> (t)

Finding a Node

document.getElementById()
document.getElementsByClassName()
document.getElementsByTagName()

DOM Tree:

<main>
  <div>
    <div>
      <p>Hello!</p>
    </div>
  </div>
  <div>
    <div>
      <p>Hello!</p>
    </div>
  </div>
  <div>
    <div>
      <p>Hello!</p>
    </div>
  </div>
</main>

Find the <main> element:

const main = document.getElementsByTagName('main')[0]

Get the children of <main>

const div = main.children[1]

Get and update the <p>

const p = div.getElementsByTagName('p')[0]

Change <p>

p.textContent = "Goodbye!"


JS Fundamentals: Arrays

document.getElementsByClassName() => [...many elements...]

  • HTMLCollection, not actually an Array
  • HTMLCollection know its length, can be iterated over with for...in... loop
  • HTMLCollection not technically an Array

document.getElementsByTagName() =>[...multiple elements...]

document.getElementById() => single element


Looping and Iteration

for ([initialization]; [condition]; [iteration]) {
    [loop body]
}
  • use a for loop when I know exactly how many times I want/need the loop to run

querySelector Methods

Learning Goals

  1. Use document.querySelector() & document.querySelectorAll() to find nested nodes
  2. Change value of targeted DOM nodes

Introduction

  • essential skill, find elements in DOM
  • document.getElementById() & document.getElementsByClassName() are good, using tree structure & tags is better

Use document.querySelector() & document.querySelectorAll() to find Nested Nodes

document.querySelector()

  • 1 arg, CSS-compatible selectors
  • returns first instance of arg

example:

<body>
    <div>
      <ul class="ranked-list">
        <li>1</li>
        <li>
          <div>
            <ul>
              <li>2</li>
            </ul>
          </div>
        </li>
        <li>3</li>
      </ul>
    </div>
             
    <div>
      <ul class="unranked-list">
        <li>6</li>
        <li>2</li>
        <li>
          <div>4</div> **
        </li>
      </ul>
    </div>
</body>

const li2 = document.querySelector('ul.ranked-list li ul li')

const div4 = document.querySelector('ul.unranked-list li div')


document.querySelectorAll()

  • 1 arg
  • returns a NodeList collection
    • an Array, but not technically an Array

example:

<main id="app">
    <ul class="ranked-list">
      <li>1</li>
      <li>2</li>
    </ul>
             
    <ul class="ranked-list">
      <li>10</li>
      <li>11</li>
    </ul>
</main>

document.getElementById('app').querySelectorAll('ul.ranked-list li')

// => list of Nodes: <li>1</li>, <li>2</li>, <li>10</li>, <li>11</li>


Removing, Altering & Inserting HTML

document.querySelect('element-name').remove()

  • this is to remove an element

document.classOrIdThing.remove('thing-name-here')

  • removes a class or id from an element, leaves the element in place

Explicit and Safe .remove() pattern

 let x = document.querySelector('element#id');
 let y = x.parentNode;
 y.removeChild(x);

Explicit and Safe adding

We’ve been inundated by cat owners who are angry that we have missed their favorite species of pet. What JavaScript snippet will add an li with a span inside with the text "Nancy Drew (the cat)" inside to our ul?

 base = document.getElementsByTagName("li")[0].parentNode;
 item = document.createElement("li"); 
 s = document.createElement("span"); 
 s.id = "cat"; 
 s.textContent = "Nancy Drew (the cat)"; 
 item.appendChild(s); 
 base.appendChild(item)

JavaScript Everything

Different Types of User Events

  • mouse click
    • click, double-click, right-click, etc ...
  • key press
    • keypress, keydown, keyup
  • form submission
    • submit ton of browser events

Functions in js

  • most important thing in js
  • a series of multiple tasks can be pulled into a function if the series can be generalized
    • makes the tasks repeatable

Scope

  • global scope
    • accessible everywhere in js code
  • block scope
    • let & const
    • only within their function
  • variables declared w/out const, let, or (NO!!!!!) var are always globally-scoped
    • even within a block

Chain Scope

  • Chain Scope is using global vars in a local context
  • matters where it is declared, not invoked
  • local vars can reference outer scoped vars all the way up the levels

Lexical Scoping

  • scope based on where vars & blocks defined by developer at writing, solidified when code is processed

Hoisting

  • by time js engine reaches execution phase all functions are in memory so order is not important in the code itself
  • best way to avoid confusion, declare all functions at the top of my code
    • refactor as needed/on-going or otherwise
  • use let & const as js engine !allow referenced before initialized

Object Iteration

for ... of

  • use with arrays, actual ordered data
const myArray = ['a', 'b', 'c', 'd', 'e', 'f', 'g'];
         
 for (const el of myArray) {
    console.log(el);
 }

for ... in

  • use with objects where order is not required just the key/value pair
 const address = {
    street1: '11 Broadway',
    street2: '2ns Floor',
    city: 'New York',
    state: 'NY',
    zipCode: 10004
 };
 for (const key in address) {
    console.log(address[key]);
    console.log(key);
 };
 // console.log(address[key]) will return the value of the key
 // console.log(key) will return the key

Discussion

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Jessica Triana

Super informative!

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aaidenplays profile image
Aaidenplays

Wow what a great guide. Thank you!

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James Shipman Author

Glad you like it :)