adventofcode1.js

var input = document.getElementsByTagName('pre')[0].innerText.split('\n');
var values = input.map(e => e.split("   "));

var list1 = [];
var list2 = [];
var list2Counts = {};
var total = 0;
var similarityScore = 0;

values.forEach(([val1, val2]) => {
    list1.push(parseInt(val1));
    if (list2Counts[val2]) {
        list2Counts[val2] = list2Counts[val2] + 1;
    } else {
        list2Counts[val2] = 1;
    }
    list2.push(parseInt(val2));
});

list1.sort();
list2.sort();

list1.forEach((val1, indx) => {
    const val2 = list2[indx];
    const diff = Math.abs(val1 - val2);

    if (diff) {
        total += diff;
    }
    if (list2Counts[val1]) {
        similarityScore += val1 * list2Counts[val1];
    }
});