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02-JS-Values-And-Variables

1. Goals & Primitives

JS BUILDING BLOCKS

Values & Variables

GOALS:

  • Work with primitive types
  • Understand let & const
  • Use String Template Literals
  • Work with common operators / methods

PRIMITIVE TYPES

  • Number
  • String
  • Boolean
  • Null
  • Undefined
  • Technically there are two others: Symbol and BigInt

2. Running Code in the JS Console

We'll start by using the chrome console to quickly run code without any setup. (just for this section)

  • Open Google Chrome and hit F12 to open the console and paste the code in the terminal and hit enter

console.log('Hello World!')

Play around with the 'Dock side'

3. Introducing Numbers

NUMBERS IN JS

50
7
3.874
0.99
-45
-777.23444
  • Javascrit has ONE Number type
  • Positive numbers!
  • Negative numbers!
  • Whole number (integers)!
  • Decimal numbers!

Play around with the numbers in the terminal

SIMPLE OPERATIONS

// Addition
50 + 5 = 55
1.5 + 8 = 9.5

// Subtraction
90 - 1 = 89
12 - 4.567 = 7.433

// Multiplication
11111 * 7 = 77777
4 * 1.2 = 4.8

// Division
400 / 25 = 16
4 / 2 = 2
10 / 3 = 3.3333333333333335

// Modulo
27 % 2 = 1
25 % 5 = 0
25 % 3 = 1
14 % 4 = 2

// Exponential
2 ** 3 = 8
5 ** 2 = 25
5 ** 5 = 3125

// We have all the basic math operations you would expect...

// creates a comment in a single line which JavaScript will ignore


// Order expressions
3 - 9 * 2 = -15
(3 - 9) * 2 = -12

4. NaN & Infinity

NaN: NOT A NUMBER
NaN is a numeric value that represents something that is not... a number

0 / 0 // NaN
1 + NaN // NaN
1 / NaN // NaN
1 / 0 // Infinity
-1 / 0 // -Infinity
-0 / -0 // NaN

5. Numbers Quiz

1.5 + 1.5 * 2 // 4.5
(10 % 6) ** 2 // 16
200 + 0 / 0 // NaN

6. VARIABLES

Variables are like "labeled jars" for a value in JavaScript.

We can store a value and give it a name, so that we can...

  • recall it
  • use it
  • or change it later on

BASIC SYNTAX

let someName = 'value';
let age = 35;
age // 35

RECALL VARIABLES

let hens = 4;
let roosters = 2;
hens + roosters // 6

UPDATE VARIABLES

let hens = 4;

// A raccoon killed a hen
// This does not change the value stored in hens
hens - 1; // 3
hens; // Still 4!

// To actually change hens:
// This does!
hens = hens -1;
hens // 3
hens = 100
hens // 100

RESERVED WORDS IN JS

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#reserved_keywords_as_of_ecmascript_2015

Naming Convention Camel Cased Names

let numOfHens = 6; // This is the standard

// Bad naming conventions
let NumOfHens = 6;
let avgrating = 9.7;
let avg_rating = 9.7;
let avg-rating = 9.7;

Redeclaring Variables

let avgRating = 0;
avgRating = 9.7;
let avgRating = 10; // Identifier 'avgRating' has already been declared

7. UNARY OPERATORS

let score = 0;
score = score + 10; // 10
score += 1; // 11
score += 10; // 21
score -= 11; // 10

let score = 30;
let bonusMult = 5;  
score *= bonusMult // 150

let counter = 0;
counter++;
counter // 1
counter--
counter // 0

8. CONST

Const works just like let, except you CANNOT change the value

const hens = 4;
hens = 20; // ERROR!
// NOT ALLOWED!
const age = 17;
age = age + 1; // ERROR!

WHY USE CONST?

const pi = 3.14159;
const daysInWeek = 7;
const minHeightForRide = 60;

Once we cover Arrays & Objects, we'll see other situations where const makes sense over let.

9. VAR

var tripDistance = 7.4;

Before let & const, var was the only way of declaring variables. These days, there isn't really a reason to use it, but you will see it a lot on older code.

10. Variables Quiz

let eggCount = 42;
eggCount + 2; // 44
eggCount // 42

const rating = 7.5;
rating = 8; // Error!
// Assignment to constant variable

// What's the minor issue with this code?
let wind_speed = 76;
wind_speed += 5;
wind_speed--; // 80
// It uses not standard camelCase syntax
// windSpeed