duplicate_frequency.js

let listWithDuplicateValues = [1,1,1,1,2,3,4,5,6,5];
let frequencyMap = {};
/**
 * {
 *  1:4,
 *  2: 1,
 *  3: 1
 * }
 */
for(let i = 0; i < listWithDuplicateValues.length; i++) {
    let item = listWithDuplicateValues[i];
    let itemFromFrequencyMap = frequencyMap[item];
    if(itemFromFrequencyMap) { // this if statement will be true if the item 
        // is not undefined (which means that the item is already present in the frequency map)
        frequencyMap[item] = itemFromFrequencyMap + 1;
        // This means that the item is already present in the frequency map
        // and we need to increment the frequency
    }else {
        frequencyMap[item] = 1; // This will be the initial frequency
    }
}
console.log(frequencyMap)

Line 12:

let itemFromFrequencyMap = frequencyMap[item]; // Here we are trying to get
// the item from the frequency map
// here item is the key and frequencyMap[item] is the value