Duplicate checker

dupe-checker.js

const a = [10,20,30,40];
const b = [10,10,20,30,40];
const c = [10,10,20,20,30,30];
const d = [10,10,20,20,30,30,40,10];

const benchmark = [...a, ...b, ...c, ...d];


/*
	Using two for loops
	- first loop will determine which values are actually duplicates in the array
	- second loop iterates across the object counting the number of duplicates
*/
function hasDupes(input) {
	let count = 0;
	const holder = {};

	for (let i = 0; i < input.length; i++) {
		const currVal = input[i];
		holder[currVal] = holder[currVal] !== undefined;
	}

	for (let value in holder) {
		if (holder[value]) count++;
	}

	return count;
} 

console.time('first');
hasDupes(benchmark);
console.timeEnd('first');


/*
	Using forEach
	- eliminates need for a second for loop/uses `.forEach`
	- BUT -> `.lastIndexOf` has to check all of the values of the array
		on each iteration of the loop to determine if there is a duplicate
	- slightly more concise
*/
function hasDupesForEach(input) {
	let count = 0;	
	const holder = {};

	input.forEach((val, idx, inputArr) => {
		if (
			inputArr.lastIndexOf(val) !== idx && 
			!holder[val]
		) count++
		holder[val] = true;
	});

	return count;
} 

console.time('second');
hasDupesForEach(benchmark);
console.timeEnd('second');


/*
	Using reduce
	- no counter necessary
	- no holder object needed (technically the accumulator for reduce is the holder)
	- complicated, kinda hard to understand what it is actually doing.
*/

function hasDupesReduce(input) {
	return input.reduce((accumulator, currentValue, index) => {
		if (
			input.lastIndexOf(currentValue) !== index && 
			accumulator[currentValue] === undefined
		) {
			accumulator.count++;
		}
		accumulator[currentValue] = true;

		return accumulator;
	}, { count: 0 }).count
}

console.time('third');
hasDupesReduce(benchmark);
console.timeEnd('third');

/*
console.log(hasDupes(a));
console.log(hasDupes(b));
console.log(hasDupes(c));
console.log(hasDupes(d));
*/