Javascript Variables

• Javascript Variables
  • Javascript types
    • Boolean
      • True Examples
      • False Examples
    • Objects
    • Enums
  • Defining and initialising variables
  • Related functions
    • TypoOf
    • Number
    • ParseInt
  • Casting
    • Explicit casting
    • Implicit casting (Coercion)
  • Structuring and destructuring variables

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Javascript types

Type	Description
number	a number from -infinity to +infinity
string	textual data
boolean	true or false.
undefined	A variable that has not been set with a value
null	Nothing. not the same as undefined.
date	A date/time object, stored in UTC (GMT 0) format
bigint	Larger number percision
Object	A complex data type which can include multiple primitive values (as above)
Array	A collection of items, which can be the same or different types

Boolean

Can also be represented as 1 and 0 respectively.

All values are considered true unless defined falsy

Falsy, being: false, 0, -0, 0n, “”, null, undefined, NaN

True Examples

- true
- {}
- []
- 42
- "0"
- "false"
- new Date()
- -42
- 12n
- 3.14
- -3.14
- Infinity
- -Infinity
- ( 1 == 1 )    // comparative

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False Examples

- false
- 0
- ""
- null
- NaN
- undefined
- 0n
- -0
- ( 1 == 2 )    // comparative

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Objects

A complex type which can include multiple other types

// creates an object that defines a set of pets and thier names
let myPets = {
    aninmal: "cat",
    pets: 5,
    names: [ "Gary", "Bruce", "Rojor", "Testeclese", "Spartacus"]
}
console.log(myPets.names); // prints the names of the pets

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Enums

Provide a set of choices for a given option. this is to improve the speed of development and limit options to allowed ones only.

// define a colours enum
const eColours = {
    "RED": 1,
    "BLUE": 2,
    "WHITE": 3,
    "PINK": "Ohhh!"
}

console.log(eColours.BLUE); // prints 2

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Defining and initialising variables

• let - Defines a variable where the value can be updated
• const - Defines a variable with the value is set and immutable
• var - Similar to let, but old. DO NOT USE

 // assigns a, but not initialised (will be 'undefined')
let a;   

// assigns a variable and initialises it
let b = 1;  
let name = "Hananna";

// assigns and initialises a variable which cannot be changed
const MY_CONST = "This-Is-The-Way"; 

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Related functions

TypoOf

typeof - Returns the type of data being stored in a variable

var c = "cat";
// prints 'string' to the console.
console.log(typeof c);

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Number

Number(string) - Tries to convert a string to a number. If there are any non-numeric characts, then NaN is returned.

// converts the string "5" to a number
let d = "5";
console.log(Number(d));

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ParseInt

parseInt(string) / parseFloat(string) - Tries to convert a string to a number. Will process the first valid number and ignore subsequent numbers after string characters

// converts the string "5" to a number
let d = "5 4";
// prints 5
console.log(parseInt(d));

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Casting

Turning a variable from one type to another.

Explicit casting

Explicit Type Conversion occurs when the programmer manually changes the type of the variables.

let myVariable = "5";   
// Explicitly change a string to number "5" to 5
myVariable = Number(myVariable);  
console.log(myVariable + 2);  // prints the number 7

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Implicit casting (Coercion)

Implicit Type Conversion occurs automatically by the JavaScript.

let myNumber = 5;
// Adding a string and number forces implicit conversion of number to string.
myNumber = myNumber + "1";  
console.log(myNumber);  // prints the string "51"

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Structuring and destructuring variables

To get values to and from Objects and arrays.

let m = 1;
let p = 10;
console.log(m,p);

// Structure in array
 let newStructure = [m,p]
console.log(newStructure);

 // reverse array elements by new structure assignment
newStructure = [p,m]
console.log(newStructure);

// pass array elements to variables
var [one, two, three] = ['orange', 'mango', 'banana'];
console.log(one); // "orange"
console.log(two); // "mango"
console.log(three); // "banana"

// Get user properties of an object using destructuring assignment,
var {name, age} = {name: 'John', age: 32};
console.log(name); // John
console.log(age); // 32

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