Asynchronous JavaScript
                

                
Once you get the hang of javascript promises, you might like async await, which is just "syntactic sugar" 
                    on topof promises. In the following example we create an async function and within that we await the greeter promise.
                
map, filter, reduce
                    
                    
 There is some confusion around the javascript array methods map, filter, reduce. These are helpful methods for
                        transforming an array or returning an aggregate value.
                    
 map: return array where each element is transformed as specified by the function
                    const arr = [1, 2, 3, 4, 5, 6];
                            const mapped = arr.map(el => el + 20);
                            console.log(mapped);
                            // [21, 22, 23, 24, 25, 26]
                    
filter: return array of elements where the function returns true
                    const arr = [1, 2, 3, 4, 5, 6];
                        const filtered = arr.filter(el => el === 2 || el === 4);
                        console.log(filtered);
                        // [2, 4]
                    
reduce: accumulate values as specified in function
                    const arr = [1, 2, 3, 4, 5, 6];
                        const reduced = arr.reduce((total, current) => total + current, 0);
                        console.log(reduced);
                        // 21
                
            

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Console
                

                
                    const marks = [80, 90, 70, 60, 50]
                    const names = ['rabin', 'shyam', 'sita', 'sita', 'sita']
                    const mixed = ['str', 10, true, [10, 20, 30]]
                
                    let arr = new Array(10, 20, 30, 40, 50)
                    console.log(arr.indexOf(20))
                
                    /// Mutating
                    arr.push(89) // last of array
                    arr.unshift(100) // front of array
                    arr.pop() // remove last element
                    arr.shift() // remove first element
                    arr.splice(2, 1) // remove element from index 2
                    arr.splice(2, 0, 'rabin') // add element at index 2
                    arr.reverse() // reverse array
                    arr.sort() // sort array
                    arr.sort((a, b) => a - b) // sort array in ascending order
                    arr.sort((a, b) => b - a) // sort array in descending order
                
                    arr.concat(names) // concat array
                    console.log(arr.concat(names)) // concat array
                
                    // object //
                    let myObj = {
                        'full name': 'rabin',
                        age: 23,
                        channel: 'youtube',
                        address: {
                            city: 'kolkata',
                            state: 'WB',
                            country: 'India'
                        },
                        marks: [80, 90, 70, 60, 50],
                    }
                    console.log(myObj.address.city)
                    console.log(myObj['full name']);
                
            

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