help2.txt

*Iterables
Arrays
String
Array like objects that have length property

*map
person = new map();
1. person.set('key','value');
set data
2. person.keys()
get all keys
3. for(key of a.keys())
    {
        console.log(key);}
print all keys

it is an object
key can be of any type


*Sets
store iterable data like arrays | strings
store data in linear order
no index based access, no duplicates allowed
order is not guarrantee
a = new Set([1,2,3,], "abc");
add items
1. a.add("z");
2. a.has();
to check an item it has or not
returns true | false






*Functions
1. function declaration
function hello(){}
2. function expression
let hello = function(){}
3. arrow function
(a,b) => a+b


* iterate object
1. for in loop
for(key of person){} 
2. Object.keys
for(key of Object.keys(person)){}

*math formula
(n*(n+1))/2
it sums all the numbers


* Create Dates
new Date()
new Date(year, month, day, hours, minutes, seconds, milliseconds)
new Date(milliseconds)
new Date(date string)
Date.parse()

* Convert Date
1. toString()
2. toUTCString()
3. toDateString()
4. toISOString()
The ISO 8601 syntax (YYYY-MM-DD)
with time (YYYY-MM-DDTHH:MM:SSZ)
Date and time is separated with a capital T.
UTC time is defined with a capital letter Z.
"MMM DD YYYY"
"DD MMM YYYY"
month can be written in full (January), or abbreviated (Jan)

* Loops
	1. for of loop
	for(k of i){}
	
	2. for in loop
	for(k in I){}
	used for index


* Array Clone
1 … spread operator
a1 = […a2];
2 slice method
3 a = [].concat[a2]

* Array Iteration
1. forEach(myFunction)
2. map(myFunction)
return new array
a.map(func());
func(value,index,array){}
|| myFunction(value){}
3. filter(myFunction)
a.filter(myFunc);
myFunc(value){return value>18}
4. reduce(myFunc,initialvalue)
a.reduce(accumulator,currentvalue)
myFunction(total,value,index,array){
    return total+value;}
reduce(myFunction,100);
5. reduceRight(myFunction)
it starts from end
6. every()
all array values pass a test return boolean
7. some()
same as filter & return boolean

* Search in Array
1. indexOf(item,optional-Start);
return index of first occurance
2. lastIndexOf(item,optional-start);
return index of last occurance
3. find(myFunction)
myFunction(value,index,array){
    return value>1}
returns the first found element
4. findIndex(myFunction)
returns index of first found element
5. Array.from("abcdef")
create array object from anything
6. keys();
return iterator like
key = a.keys();
    for(k of key){
    console.log(k);}
7. entries()
returns an Array Iterator object
with key/value pairs like
b = a.entries();
for(e of b){
    console.log(e);}
8. includes()
check if an element is present
returns boolean
9. fill(value,start,end);
a.fill(#,0,5);





* Array Methods
1. length()
2. sort()
3. toString()
4. join()
5. pop()
returns value that popped out
6. push()
return length
7. shift()
returns value
8. unshift()
returns length
9. delete array[0]
leaves undefined holes
10. concat()
a.concat(b); it doesn't change array
returns a new array
can also take a string value
like a.concat("hello")
11. splice()
add new | remove items & returns deleted items
a.splice(start,delete, "c", "d");
12. slice()
returns new array
2nd argument optional
slice(index,count)

* Sorting Array
1. sort()
2. reverse()
sort with compare function
ascending
a = [5,4,3];
a.sort(function(a, b){return a - b});
descending
a = [1,2,3];
a.sort(function(a, b){return b - a});
random order
a.sort(function(a, b){return 0.5 - Math.random()});
3. Math.max.apply(null,array)
4. Math.min.apply(null,array)

* Sorting Array Objects
const cars = [
  {type:"Volvo", year:2016},
  {type:"Saab", year:2001},
  {type:"BMW", year:2010}];
1. compare the numeric property values
cars.sort(function(a, b){return a.year - b.year});
2. compare the string property values
cars.sort(function(a, b){
  let x = a.type.toLowerCase();
  let y = b.type.toLowerCase();
  if (x < y) {return -1;}
  if (x > y) {return 1;}
  return 0;
});




* Arrays
a = new Array();
a = [];
Array.isArray(name);    check
car instanceof() Array;


* Number Properties
Number.MAX_VALUE
Number.MIN_VALUE
Number.POSITIVE_INFINITY
Number.NEGATIVE_INFINITY
Number.NaN

*Number Methods
1. toString()
(123).toString();
(100 + 23).toString();
2.toExponential()
 returns a string
3. toFixed(width)
returns a string with a specified number of decimals
4. toPrecision(length)
returns a string with a specified number of length
5. valueOf()
returns a number as a number

* Converting Variables to Numbers
1. Number()
2. parseInt()
returns a whole number
3. parseFloat()
Spaces are allowed
Only the first number is returned
4. 



*Template Literals
uses ` back-tick
* String Interpolation ${}
* Variable Substitutions
text = `Welcome ${firstName}`;
* Expression Substitution
let total = `Total: ${a+b}`;


* Search Methods
1. indexOf()
returns first occurrence + CASE SENSITIVE
2. lastIndexOf()
returns last occurrence,
Both methods accept a second parameter
as the starting position as
str.indexOf("locate", 15);
3. startsWith()
4. endsWith()
5. search()
search() method cannot take a second start position argument.
indexOf() method cannot take powerful search values (regular expressions)
6. match()
return array of objects  || null
text.match(/asad/gi);
/g = global & i = case insensitive
7. includes()
return boolean


* JavaScript Search Methods
1. indexOf()
2. lastIndexOf()
3. startsWith(value,start)
4. endsWith(value,length)
5. search()
6. match()
7. includes(value,start)
8. 

* Common HTML Events
onchange
onclick
onmouseover
onmouseout
onkeydown
onload

* Converting a String to an Array
1. split()
text.split(",")    // Split on commas


* Extracting String Characters
1. charAt(position)
2. charCodeAt(position)
returns the unicode of the character
returns a UTF-16 code (an integer between 0 and 65535)
3. Property access [ ]
text[0] like arrays

* Strings Extracting
1. slice(start, end)
(end not included)
2. substring(start, end)
cannot accept negative indexes.
3. substr(start, length)
second parameter specifies the length

* String Methods
1. replace()
/i flag (insensitive)
/g flag (global match)
2. toUpperCase()
3. toLowerCase()
4. concat()
5. trim()
removes whitespace from both sides of a string
6. padStart()
text.padStart(4,0);
7. padEnd()
text.padEnd(4,0);
8. toString(base_number)
base 2 to base 36.


output
console.log("")
windows.alert("")
windows.print("")

input
prompt("message")
it inputs string only


"use strict"; at the top
to ensure there no variable without let or var

* ternary operator condition
condition ? true statement : false statement
use multiple or nested conditional operator



* convert to int
parseInt(str);
with radix or base
parseInt(str,2);


* random numbers
Math.random();
Math.floor();
between range formula
Math.floor(Math.random()* (max-min+1))+min;

* object properties
uses dot . or bracket [] operator
update property
add new property
delete property
hasOwnProperty      (check)
var dog = {
    "dog name" : "Alpha",
    "dog legs" : 4,
    "dog drink" : "water"
};


*comparison operator
strict equality operator ===
does not do the type conversion

strict inequality operator !==

formal logical operators
and && or ||

*Functions
nextInLine() to implement queue

* Array Functions
push       // atEnd
pop        // atEnd
shift     // atStart
unshift   // atStart

* create variables 
var
let
const
 

* data types
undefined
null
boolean
string
symbol
number
object