Basics

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Variables

• We use let to declare variables and const to declare constants. We used to declare variables using var keyword.

• Problem with var

  • variables can be used before their declaration but it may resolve to undefined. This creates problems

• let and const initialises variable with block (within the curly braces) level scope whereas var has global and function scope

• var

  • No block scope
  • Can be redeclared anywhere
  • Can be used and reassigned anywhere

• let

  • Block scope
  • Can Not be redeclared within scope
  • Can be reassigned within scope

• const

  • Block scope
  • Can not be reassinged or redeclared
  • The value it references can be changed. Example - like a property in an object or array.

Data types

• Numbers
  • Unlike other programming language, there is no specification of Integer, Float or double.
  • By default,JS works with Float
    • Example
      • let price = 12; let price = 12.14;
• null and undefined
  • undefined basically mean variable is not initialized. JavaScript will set value to undefined for uninitialized variable whereas Programmer generally sets variable to null. We can set to undefined as well but it provides distinction

Truthy and Falsy

• Falsy
  • false
  • 0
  • "" or ''
  • null
  • undefined
  • NaN

-Truthy - Everthing NOT falsy - true - >0 - "0"

Just JS thing

• 1.1 + 1.3 !== 2.4 is true because JS uses inaccurate floating point number

Comparing === and ==

• 1 === '1' results in false since datatype are different but however in the below case it is true 1 == '1' JS attempts to convert first type into string

Functions

• Ways of decalring a function in JS

  • 1. function something() { //code }

  • 1. ``` let myValue = function() { }

    let myValue = function logMessage() {

    }

    myValue() //calls the function logMessage() // will give us error ```

Objects

• Symbol can be used inside an object to hide information

let mySymbol = Symbol();

let person = {
    name : "Vaibhav",
    [mySymbol] : 'secretInformation'
};

• We can have function as Object method

let person = {
    name : "Vaibhav",
    showInfo:  function() {
        showMessage(this.name)
    }
};

• Same as Java where when we use object we change the reference so it reflects even outside the methods. Same goes for JS Object. Object are passed by reference

Array

• Array can be initialized in 2-ways
  • let something = []
  • let something = Array.of(1,2,3)
• Array is not a datatype but an object

• Methods of Array

  • push
  • pop
  • shift & unshift
  • slice, used to delete elements from array
  • splice, can be used for deletion and insertion as well

• Filter(), can be used for searching purposes and it returns Array

Scope & Hoisting

• Global Scope

  • all the .js files in src are part of the global scope
  • Too many variable in the global scope can cause bugs

• Function Scope

  • Variable defined in the function scope can not be used outside of it

• var & Hoisting

  • Example of Hoisting. Lets us access the variable before its initialization productId = 456; console.log(productId); // prints 456 var productId = 123;
  • Hoisting is fine with the functions ``` showProduct();

  function showProduct() { console.log("A"); } ``` - Concept of Hoisting - Javascript file gets executed in two passes and in the first pass JS engine will know there's function called showProduct but in the second pass it will execute the file in top down manner

• Hoisting can be avoided by using let and const

Undeclared Variables and Strict Mode

• We need not declare the variable in order to use it. We can directy use the same.

• 'use strict'

  • Puts JS file in the strict mode and this forces you to declare the variable

Switch Case

• Default case can be placed anywhere in the switch block
• If break not there in one of the case then it will go on till the next case

• Switch does a strict comparison

  • Type and value must match

• By wrapping a block of code in bracesm any variable declared within that block is only visible within the block, and is released once the block ends.

For/in and For/of

• For/in

  • Iterates over elements of object (properties and methods)
  • Example - for(const key in product)

• For/of

  • Iterates over values in array, string etc.
  • Example - for(const item of _product)
  • Example - for(const char of productName)

• Label in JS

  • Label is any name you want followed by a colon
  • Not recommended
  • Example function labelSample(){ even: for(let index = 1; index <= 10; index++){ if(index % 2 == 1) { continue even; } console.log(index); } }

• we can delete variable and function in JS

Short Circuiting

• &&, if one first one is false then it wont execute next part
• ||, if first one is true then it wont execute the other side

Object Data Type / Constructor

• All object data types inherit from Object (not primitive)

• Object has constructor property, which returns a reference to the object itself

• Object literals and primitives are cast to objects for display

this

• this in the global scope refers to window object, but in the case of 'use strict' it will throw an error saying this is undefined.

• call() and apply()

  • call takes parameter to set it to this / what is passed to call()/apply() methods becomes 'this'

Spread opeartor

function stringToArray() {
    let productNumber = "ABCDEF";
    let values = [...productNumber] //has character array
}

• Shallow copy on Object Literal