let array =["a","b","c"];

const duplicateArray = [...array];

[1, 1, 2, 2, 3].filter((element, index, array) => array.indexOf(element) !== index) // [1, 2]

function doubleValues(array) {

  var newArray = [];

  array.forEach(function (el) { newArray.push(el, el); });    

  return newArray;

}

​

console.log(doubleValues([1,2,3]));

const months = ['april','may','june','may','may','june'];

​

const countDuplicates = months.reduce((obj,month)=>{

    if(obj[month] == undefined){

        obj[month] = 1;

        return obj;

    }else{

        obj[month]++;

        return obj;

    }

},{});

console.log(countDuplicates);//output:{april: 1, may: 3, june: 2}

​

let chars = ['A', 'B', 'A', 'C', 'B'];

​

let uniqueChars = [];

chars.forEach((c) => {

    if (!uniqueChars.includes(c)) {

        uniqueChars.push(c);

    }

});

​

console.log(uniqueChars);

Code language: JavaScript (javascript)

let arr = [1, 7, 8, 9, 10, 20, 33, 0, 20, 7, 1]

console.log([... new Set(arr)]

// Output : [1, 7, 8, 9, 10, 20, 33, 0]