Javascript Notes

Variables

let is the preferred way in modern JavaScript.

Data Types

• Null = Null (type of null is an object: its a bug)

Null specifies that we have something to store value but that value is simply nonexistent.

• Undefined = undefined

The value is never set/declared .Undefined means that we don't have any of the variables or the value it's basically undefined that doesn't exist.

• Strings

backticks ``

It is used to embed different variables into strings

use: when we have to apply extended functionality

ex:

let backticks =` Hello ${username}`;

• Number

Javascript is untyped (Dynamically typed)

NaN - Not a Number (type of NaN is number)

It represents a computational error it is a result of an incorrect or undefined mathematical operation.

Operators and Equality

Strict VS Loose equality

• Strict equality

compares values & data types

return true only if both are the same console.log(20 === "20"); //False

• Loose equality

console.log(20 == "20"); //true

Doesn't compare DATA Types

The Good one : === !==

The Evil Twin's: == !=

Always use === (Strict Equality)

Decision-Making Code

Falsy values

False, 0 , ' ', null, undefined, NaN

you can check if the value is truthy or Falsy using !!

• Logical Operators

&& - scans for left to right

true && true -> true

true && False -> False

False && true -> False

False && False -> False

if all values are true returns the last value

eg.

console.log('truthy' && 1 && 'test' && 999);

-> 999

if any one value is false then return that value.

eg.

console.log('truthy' && 0 && 'test' && 999);

->  0

|| OR

Returns first true value.

eg.

console.log('truthy' || 1 || 'test' || 999);
-> truthy

if all values are false it returns the last value.

eg.

console.log(' ' || 0 || null || undefined);
-> undefined

Ternary Operator ? :

condition? block of code if true: block of code if false;

Function

A block of code => Performs a task

A function declaration (defining a function)

A function call (calling/executing a function)

// A function declaration
function name(params){
 // statements // have access to "this" keyboard
}
// A function expression
const name = function(params){
// Statements;
}
//An arrow function
const name = (params) => {
//statements;
}

Returns undefined

Arrow Function

const square = (number) => { return number * number; }

const square = (number) => number * number;

// Parameters - used when defining a function.

// Arguments - passed when making a function call.

Scope

Global

Function/Local scope

Block

Hoisting in Javascript

Bringing the variables to the top of the scope.

eg.

console.log(age);   |  var age;
var age = 20;          |  console.log(age);
undefined               |  age = 20;

let, const are not hoisted

In modern JavaScript, hoisting does not exist.

Closures

A closure gives access to the outer function scope

In JavaScript, closures are created every time function is created at a function creation time.

const init = () => {}
//local variable created by init
const hobby = 'Learning Javascript';
// Display hobby is the inner function 
const displatHobby = () => {
// using a variable created in the parent function
console.log(hobby);
}
displayHobby();
}
init();
}