Basics

Install Visual Studio Code and add following extentions

Brackets pair
Live server
ES6 Code snippets

To find the time it took to execute the scripts:

    console.time('Hello')
    console.log('hello world')
    console.log('hello world')
    console.log('hello world')
    console.log('hello world')
    console.timeEnd('Hello')

    RESULT:
    ==>   Hello:
            0.05898324 sec

Declaration: Var, Let, Const

Var : variable declaration
Let : Similar to Var
Const: cannot reassign, but object’s property can be changed.

    const msg = "Hello world";
    Msg = "Welcome";  // not accepted
    
    const person = { name: "ABC", age: 30 };
    Person.age = 35;   // accepted

    Const num = [1,2,3,4,5];
    Num.push(6);   // accepted

Primitive datatypes:

1. String
2. Number
3. Boolean
4. Null
5. Undefined
6. Symbols (ES6)

Reference data types:

1. Array literals
2. Object literals
3. Functions
4. Dates
5. Anything else..

Dynamically typed language

The same variable can hold multiple types
We don’t need to specify types
Typescript - superset of JS - allow static typing

To find type of a variable use:

console.log(typeof age);

Conversion

To String 1:

    let a;
    a=String(5);
    a=String(4+6);
    a=String(true);
    a=String(new Date());
    a=String([1,2,3,4]);

To String 2:

    let a;
    a=(5).toString();
    a=(true).toString();

To Number:

    a = Number('5')
    a.toFixed(2); // 5.00

    b = Number(true);   //1
    b = Number(false);  //0
    b = Number(null);    //0
    b = Number('hello');      //NaN
    b = Number([1,2,3,4]);  //NaN

    b = parseInt('100');          //100
    b = parseFloat('100.30');  //100.30

Math Operations

    Math.PI
    Math.E
    Math.round(2.4); //2
    Math.ceil(2.4); //3
    Math.floor(2.8);  //2
    Math.sqrt(64);  //8
    Math.abs(-3);  //3
    Math.pow(8,2);  //64
    Math.min(3,7,2,5,6); //2
    Math.random();
    Math.floor(Math.random() * 20);

String methods

concat
toUpperCase
toLowerCase
length
concat
firstName[2];
firstName.indexOf(‘l’);
lastIndexOf(‘l’);
charAt(‘2’);
firstName.charAt(firstName.length - 1);
substring(0, 3);
slice(0,3)
slice(-3);
split(’ ‘);
replace()
includes(‘hello’) //whether word exist or not

Template strings

ES5:

    Html = '<ul> ' +
            '<li>Name: ' + name + '</li>' + 
            '<li>Age: ' + age + '</li>' + 
            '<li>Job: ' + job + '</li>' +
            '<li>City: ' + city + '</li>' +
    '</ul>';

ES6:

    Html = `<ul> 
            <li>Name: ${name} </li> 
            <li>Age: ${age} </li>
            <li>Job: ${job} </li>
            <li>City: ${city} </li>
            <li>City: ${4+6} </li>
            <li>City: ${ fnDisplay() } </li>
            <li>City: ${age>30 ? 'over 30' : 'under 30' } </li>
            </ul>`;

Arrays

    const num = [1,2,4,6,8,3];
    const num2 = new Array(23,5,6,34,75);
    var mix = [22,'apple', true, undefined, null, {a:1, b:2}, new Date()];

Functions

push();
indexOf()
isArray()
unshift()
Pop()
Shift()
Splice(1,3)
Reverse()
Concat()
Sort()
Sort( function(x,y) { return x-y; } );
Find ( function(num) { return num < 50; } }

Objects

    const person = { 
        firstName: 'Senthamilan',
        lastName: 'Semmai',
        age: 40,
        hobbies: ['nature', 'social'],
        address: {
            City: 'thanjavore',
            State: 'TN'
            },
        getBirthyear: function(){
            return 1978;
            }
        };

    Person.firstName;  //Result: Senthamilan

Date

    var d = new Date();

Conditions

    // Equel to (== vs ===):
    If (n == 100) // checks the value only.
    If (n === 150)  //checks the type & value

    //var id;
    if(typeof id == 'undefined'){
        console.log('msg: id undefined');
    }