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Data Types & Fundamentals

1. JS intro

  • functional | lightweight | precompiled | scripting language for browser.
  • online playground: https://jsbin.com/
  • pollyfills
  • run on Server by Node.js
  • Hoisting - JS declares the variable and method inside Excecution stack before excution.
  • if var is not used, then it will be always global, irrespective wherver variable is defined.
  • scope - local | global | block
  • if (null == undefined) // true - both have internally same value.

2. JS engine

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3. Data Types

console.log(typeof a)

var a = Infinity, NaN, 0, -1, 1.1, - `number`
var a = undefined - `undefined`
var a = Null , {}, {a:'a', b:1}, [1,2,3] - `Object`
var a = "1" , '1' - `string`
var a = true -- `boolean`
var a = function() {} - `function`

null vs Undefined

  • Both have internally same value. hence if(null == undefined) : true.
  • Both have diff type as shown above. hence if(null === undefined) : false.
  • var a = undefined : technically assign it but no purpose.

3. NaN

  • its a error return by JS while mathematical error on number types.
  • NaN is of number type.
  • technically we can say its undefined number in JS world.

function type

  • functional declaration vs
  • function call vs
  • function assign (fn expression) vs
  • regular function vs
  • arrow function (short hand, but lexical this) 
    var a = function calc(var1, var2){
  return var1 + var2;
};

console.log( typeof a);  //"function"
console.log( a(2,3)); // 5
console.log( calc(2,3)); //"ReferenceError: calc is not defined

boolean

  • Number as boolean : false internally represents zero value, And true internally represent non-zero number. 

    if(0 == false){  console.log( 'true');} ;//true

if(1 == true){  console.log( 'true');} ;//true
if(-1 == true){  console.log( 'true');}; //true
if(100 == true){  console.log( 'true');} ;//true

if(1 == '1') - true
if(1 === '1') - false

  • string === true. 

    if("abc")
  console.log( 'true'); //output
else
  console.log( 'false');

  • NULL == false 

    if(null)
  console.log( 'true');
else
  console.log( 'false'); //output

array