diff --git a/src/2015/day11.js b/src/2015/day11.js
index fd88d27c..f577095c 100644
--- a/src/2015/day11.js
+++ b/src/2015/day11.js
@@ -1,5 +1,5 @@
-let abc = "abcdefghijklmnopqrstuvwxyz";
-let subStrings = abc
+const abc = "abcdefghijklmnopqrstuvwxyz";
+const subStrings = abc
   .split("")
   .map((x, i) => abc.substr(i, 3))
   .filter(x => x.length === 3);
diff --git a/src/2015/day13.js b/src/2015/day13.js
index df1e3a49..96df4366 100644
--- a/src/2015/day13.js
+++ b/src/2015/day13.js
@@ -8,7 +8,7 @@ function longest(graph, curr, visited) {
 }
 
 function parse(input) {
-  let signs = { gain: +1, lose: -1 };
+  const signs = { gain: +1, lose: -1 };
   let graph = input
     .split("\n")
     .map(x =>
diff --git a/src/2015/day16.js b/src/2015/day16.js
index 4eb4644f..316d4828 100644
--- a/src/2015/day16.js
+++ b/src/2015/day16.js
@@ -1,4 +1,4 @@
-let expect1 = {
+const expect1 = {
   id: () => true,
   children: 3,
   cats: 7,
@@ -12,7 +12,7 @@ let expect1 = {
   perfumes: 1,
 };
 
-let expect2 = {
+const expect2 = {
   ...expect1,
   cats: x => x > expect1.cats,
   trees: x => x > expect1.trees,
diff --git a/src/2015/day21.js b/src/2015/day21.js
index 2f5fa946..1e8de1c5 100644
--- a/src/2015/day21.js
+++ b/src/2015/day21.js
@@ -1,5 +1,5 @@
 function calcOptions() {
-  let items = {
+  const items = {
     weapons: [
       { name: "dagger", cost: 8, damage: 4, armor: 0 },
       { name: "shortsword", cost: 10, damage: 5, armor: 0 },
diff --git a/src/2015/day22.js b/src/2015/day22.js
index bad77da6..5c29090c 100644
--- a/src/2015/day22.js
+++ b/src/2015/day22.js
@@ -1,4 +1,4 @@
-let spells = {
+const spells = {
   Missile: {
     mana: 53,
     effect: game => {
diff --git a/src/2015/day23.js b/src/2015/day23.js
index 6cd10632..079ee093 100644
--- a/src/2015/day23.js
+++ b/src/2015/day23.js
@@ -1,4 +1,4 @@
-let operations = {
+const operations = {
   hlf: (state, p1) => ({ ...state, [p1]: state[p1] / 2, next: state.next + 1 }),
   tpl: (state, p1) => ({ ...state, [p1]: state[p1] * 3, next: state.next + 1 }),
   inc: (state, p1) => ({ ...state, [p1]: state[p1] + 1, next: state.next + 1 }),
diff --git a/src/2016/day01.js b/src/2016/day01.js
index c04e27f7..055c6276 100644
--- a/src/2016/day01.js
+++ b/src/2016/day01.js
@@ -1,4 +1,4 @@
-let directions = [
+const directions = [
   { y: 1, x: 0 },
   { y: 0, x: 1 },
   { y: -1, x: 0 },
diff --git a/src/2016/day02.js b/src/2016/day02.js
index c563fc91..cf14033b 100644
--- a/src/2016/day02.js
+++ b/src/2016/day02.js
@@ -1,4 +1,4 @@
-let directions = {
+const directions = {
   D: { y: 1, x: 0 },
   R: { y: 0, x: 1 },
   U: { y: -1, x: 0 },
@@ -28,7 +28,7 @@ function solve(input, keypad, start) {
     .join("");
 }
 
-let keypad1 = [
+const keypad1 = [
   ["1", "2", "3"],
   ["4", "5", "6"],
   ["7", "8", "9"],
@@ -38,7 +38,7 @@ export function part1(input) {
   return solve(input, keypad1, { x: 1, y: 1 });
 }
 
-let keypad2 = [
+const keypad2 = [
   [NaN, NaN, "1", NaN, NaN],
   [NaN, "2", "3", "4", NaN],
   ["5", "6", "7", "8", "9"],
diff --git a/src/2016/day12.js b/src/2016/day12.js
index 3dca6454..31393d57 100644
--- a/src/2016/day12.js
+++ b/src/2016/day12.js
@@ -1,4 +1,4 @@
-let ops = {
+const ops = {
   cpy: (src, register) => state =>
     (state[register] = src.match(/^\d+$/) ? +src : state[src]),
   inc: register => state => state[register]++,
diff --git a/src/2016/day20.js b/src/2016/day20.js
index 20b4e7a6..c0fb1cf2 100644
--- a/src/2016/day20.js
+++ b/src/2016/day20.js
@@ -1,4 +1,4 @@
-let MAX_IP = 4294967295;
+const MAX_IP = 4294967295;
 
 function merge(ranges) {
   ranges.sort((a, b) => a[0] - b[0]);
diff --git a/src/2016/day21.js b/src/2016/day21.js
index 30a4768c..855553ff 100644
--- a/src/2016/day21.js
+++ b/src/2016/day21.js
@@ -1,6 +1,6 @@
-let regexp =
+const regexp =
   /^(swap position|swap letter|rotate based|rotate|reverse positions|move position) ([^\s]+)\s.*\s([^\s]+)$/;
-let ops = {
+const ops = {
   "swap position": (a, b) => str => {
     [a, b] = [+a, +b].sort((a, b) => a - b);
     let arr = str
diff --git a/src/2016/day23.js b/src/2016/day23.js
index 59995782..e8779cd5 100644
--- a/src/2016/day23.js
+++ b/src/2016/day23.js
@@ -1,4 +1,4 @@
-let ops = {
+const ops = {
   cpy: (src, register) => state => {
     if (state[register] !== undefined) {
       src = state[src] === undefined ? +src : state[src];
diff --git a/src/2016/day25.js b/src/2016/day25.js
index 5563b0a8..50cf6b4d 100644
--- a/src/2016/day25.js
+++ b/src/2016/day25.js
@@ -1,4 +1,4 @@
-let ops = {
+const ops = {
   cpy: (src, register) => state =>
     (state[register] = state[src] === undefined ? +src : state[src]),
   inc: register => state => state[register]++,
diff --git a/src/2017/day18.js b/src/2017/day18.js
index d74bc796..a716643a 100644
--- a/src/2017/day18.js
+++ b/src/2017/day18.js
@@ -10,7 +10,7 @@ export function getter(state, p) {
 }
 
 export function parse(input, ops2 = {}, debug = undefined) {
-  let ops = {
+  const ops = {
     snd: p1 => state => (state.sound = getter(state, p1)),
     set: (p1, p2) => state => (state[p1] = getter(state, p2)),
     add: (p1, p2) => state =>
diff --git a/src/2017/day19.js b/src/2017/day19.js
index 4db57c38..c5f5c556 100644
--- a/src/2017/day19.js
+++ b/src/2017/day19.js
@@ -6,7 +6,7 @@ function findEntryPoint(route) {
   return { x: route[0].indexOf("|"), y: 0 };
 }
 
-let next = {
+const next = {
   down: ({ x, y }) => ({ x, y: y + 1 }),
   up: ({ x, y }) => ({ x, y: y - 1 }),
   left: ({ x, y }) => ({ x: x - 1, y }),
diff --git a/src/2017/day21.js b/src/2017/day21.js
index 8c2f4db1..88e2b4c3 100644
--- a/src/2017/day21.js
+++ b/src/2017/day21.js
@@ -1,5 +1,5 @@
-let pattern = [".#.", "..#", "###"];
-let permutations2 = [
+const pattern = [".#.", "..#", "###"];
+const permutations2 = [
   [1, 2, 3, 4],
   [3, 4, 1, 2],
   [1, 3, 2, 4],
@@ -9,7 +9,7 @@ let permutations2 = [
   [4, 2, 3, 1],
   [2, 4, 1, 3],
 ];
-let permutations3 = [
+const permutations3 = [
   [1, 2, 3, 4, 5, 6, 7, 8, 9],
   [7, 8, 9, 4, 5, 6, 1, 2, 3],
   [1, 4, 7, 2, 5, 8, 3, 6, 9],
diff --git a/src/2017/day22.js b/src/2017/day22.js
index 24f5dad6..ba8ba7f7 100644
--- a/src/2017/day22.js
+++ b/src/2017/day22.js
@@ -1,18 +1,18 @@
-let turnLeft = {
+const turnLeft = {
   U: "L",
   D: "R",
   L: "D",
   R: "U",
 };
 
-let turnRight = {
+const turnRight = {
   U: "R",
   D: "L",
   L: "U",
   R: "D",
 };
 
-let turnBackward = {
+const turnBackward = {
   U: "D",
   D: "U",
   L: "R",
diff --git a/src/2018/day05.js b/src/2018/day05.js
index 271a25e9..2bb66aaa 100644
--- a/src/2018/day05.js
+++ b/src/2018/day05.js
@@ -1,6 +1,6 @@
-let u = x => x.toUpperCase();
-let abc = "abcdefghijklmnopqrstuvwxyz".split("");
-let kill = abc.reduce((arr, x) => arr.concat([x + u(x), u(x) + x]), []);
+const u = x => x.toUpperCase();
+const abc = "abcdefghijklmnopqrstuvwxyz".split("");
+const kill = abc.reduce((arr, x) => arr.concat([x + u(x), u(x) + x]), []);
 
 export function part1(input) {
   let len;
diff --git a/src/2018/day07.js b/src/2018/day07.js
index b4677daa..188be71a 100644
--- a/src/2018/day07.js
+++ b/src/2018/day07.js
@@ -1,4 +1,4 @@
-let abc = " ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+const abc = " ABCDEFGHIJKLMNOPQRSTUVWXYZ";
 
 function next(prerequisites, done) {
   let options = [];
diff --git a/src/2018/day13.js b/src/2018/day13.js
index 32a155ff..69571a11 100644
--- a/src/2018/day13.js
+++ b/src/2018/day13.js
@@ -1,13 +1,13 @@
-let position = cart => `${cart.x},${cart.y}`;
-let onSlash = { ">": "^", "<": "v", "^": ">", "v": "<" };
-let onBackSlash = { ">": "v", "<": "^", "^": "<", "v": ">" };
-let nextTurn = { left: "straight", straight: "right", right: "left" };
-let onTurn = {
+const position = cart => `${cart.x},${cart.y}`;
+const onSlash = { ">": "^", "<": "v", "^": ">", "v": "<" };
+const onBackSlash = { ">": "v", "<": "^", "^": "<", "v": ">" };
+const nextTurn = { left: "straight", straight: "right", right: "left" };
+const onTurn = {
   left: { ">": "^", "<": "v", "^": "<", "v": ">" },
   right: { ">": "v", "<": "^", "^": ">", "v": "<" },
   straight: { ">": ">", "<": "<", "^": "^", "v": "v" },
 };
-let onMove = {
+const onMove = {
   ">": c => c.x++,
   "<": c => c.x--,
   "^": c => c.y--,
diff --git a/src/2018/day15.js b/src/2018/day15.js
index 2487c521..e633229c 100644
--- a/src/2018/day15.js
+++ b/src/2018/day15.js
@@ -1,8 +1,8 @@
-let pos = ({ x, y }) => `${x},${y}`;
-let count = (units, type) => units.filter(u => u.type === type).length;
+const pos = ({ x, y }) => `${x},${y}`;
+const count = (units, type) => units.filter(u => u.type === type).length;
 
 function attackIfPossible(map, units, unit) {
-  let inRange = u => Math.abs(u.x - unit.x) + Math.abs(u.y - unit.y) === 1;
+  const inRange = u => Math.abs(u.x - unit.x) + Math.abs(u.y - unit.y) === 1;
   let attack = units
     .filter(u => u.type !== unit.type && inRange(u))
     .sort((a, b) => a.hit - b.hit || a.y - b.y || a.x - b.x)
diff --git a/src/2018/day16.js b/src/2018/day16.js
index dfdda684..b514bb70 100644
--- a/src/2018/day16.js
+++ b/src/2018/day16.js
@@ -1,4 +1,4 @@
-let ops = {
+const ops = {
   addr: (r, i1, i2, o) => (r[o] = r[i1] + r[i2]),
   addi: (r, i1, i2, o) => (r[o] = r[i1] + i2),
   mulr: (r, i1, i2, o) => (r[o] = r[i1] * r[i2]),
@@ -16,7 +16,7 @@ let ops = {
   eqri: (r, i1, i2, o) => (r[o] = r[i1] === i2 ? 1 : 0),
   eqrr: (r, i1, i2, o) => (r[o] = r[i1] === r[i2] ? 1 : 0),
 };
-let numbers = arr => arr.map(Number);
+const numbers = arr => arr.map(Number);
 
 function parseSamples(input) {
   let samples = input.split("\n\n\n\n")[0].split("\n\n");
diff --git a/src/2018/day19.js b/src/2018/day19.js
index 5aac2309..4edaf462 100644
--- a/src/2018/day19.js
+++ b/src/2018/day19.js
@@ -1,6 +1,6 @@
 import { divisors } from "../utils/divisors.js";
 
-let ops = {
+const ops = {
   addr: (r, i1, i2, o) => (r[o] = r[i1] + r[i2]),
   addi: (r, i1, i2, o) => (r[o] = r[i1] + i2),
   mulr: (r, i1, i2, o) => (r[o] = r[i1] * r[i2]),
@@ -18,7 +18,7 @@ let ops = {
   eqri: (r, i1, i2, o) => (r[o] = r[i1] === i2 ? 1 : 0),
   eqrr: (r, i1, i2, o) => (r[o] = r[i1] === r[i2] ? 1 : 0),
 };
-let numbers = arr => arr.map(Number);
+const numbers = arr => arr.map(Number);
 
 export function part1(input, reg0 = 0) {
   let lines = input.split("\n");
diff --git a/src/2018/day20.js b/src/2018/day20.js
index 1ada9e45..86a8f5de 100644
--- a/src/2018/day20.js
+++ b/src/2018/day20.js
@@ -1,11 +1,11 @@
-let walk = {
+const walk = {
   W: ({ x, y }) => ({ x: x - 1, y: y + 0 }),
   E: ({ x, y }) => ({ x: x + 1, y: y + 0 }),
   N: ({ x, y }) => ({ x: x + 0, y: y - 1 }),
   S: ({ x, y }) => ({ x: x + 0, y: y + 1 }),
 };
-let pos = ({ x, y }) => `${x},${y}`;
-let unique = arr =>
+const pos = ({ x, y }) => `${x},${y}`;
+const unique = arr =>
   arr
     .reduce((all, p) => all.concat(p), [])
     .sort((a, b) => a.x - b.x || a.y - b.y)
diff --git a/src/2018/day21.js b/src/2018/day21.js
index 8bac6207..9e372a68 100644
--- a/src/2018/day21.js
+++ b/src/2018/day21.js
@@ -1,4 +1,4 @@
-let ops = {
+const ops = {
   addr: (l, i1, i2, o) => `case ${l}: r${o} = r${i1} + r${i2}; break;`,
   addi: (l, i1, i2, o) => `case ${l}: r${o} = r${i1} + ${i2}; break;`,
   mulr: (l, i1, i2, o) => `case ${l}: r${o} = r${i1} * r${i2}; break;`,
diff --git a/src/2018/day22.js b/src/2018/day22.js
index 95c95380..c7444b5c 100644
--- a/src/2018/day22.js
+++ b/src/2018/day22.js
@@ -1,6 +1,6 @@
 let cache = {};
-let pos = ({ x, y }) => `${x},${y}`;
-let terrain = (point, depth) => erosion(point, depth) % 3;
+const pos = ({ x, y }) => `${x},${y}`;
+const terrain = (point, depth) => erosion(point, depth) % 3;
 
 function erosion({ x, y }, depth) {
   let geo;
diff --git a/src/2019/day11.js b/src/2019/day11.js
index 35b9636b..78044b20 100644
--- a/src/2019/day11.js
+++ b/src/2019/day11.js
@@ -2,7 +2,7 @@ import { execute } from "./day09.js";
 import { ocr } from "../utils/ocr.js";
 
 function move({ x, y }, direction) {
-  let directions = {
+  const directions = {
     "^": { x, y: y - 1 },
     "v": { x, y: y + 1 },
     "<": { x: x - 1, y },
@@ -11,14 +11,14 @@ function move({ x, y }, direction) {
   return directions[direction];
 }
 
-let left = {
+const left = {
   "^": "<",
   "<": "v",
   "v": ">",
   ">": "^",
 };
 
-let right = {
+const right = {
   "^": ">",
   ">": "v",
   "v": "<",