diff --git a/.eslintrc.json b/.eslintrc.json
index 5ea8287780..b51915202c 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -20,7 +20,6 @@
     "@typescript-eslint/no-unused-vars": ["warn", { "argsIgnorePattern": "^_" }],
 
     "eqeqeq": "off",
-    "prefer-const": "off",
     "no-var": "off",
     "@typescript-eslint/no-empty-interface": "off",
     "@typescript-eslint/explicit-module-boundary-types": "off",
diff --git a/ui/analyse/src/control.ts b/ui/analyse/src/control.ts
index c988f99e10..6556847adc 100644
--- a/ui/analyse/src/control.ts
+++ b/ui/analyse/src/control.ts
@@ -24,7 +24,7 @@ export function first(ctrl: AnalyseCtrl): void {
 }
 
 export function enterVariation(ctrl: AnalyseCtrl): void {
-  let child = ctrl.node.children[1];
+  const child = ctrl.node.children[1];
   if (child) ctrl.userJump(ctrl.path + child.id);
 }
 
diff --git a/ui/analyse/src/crazy/crazyView.ts b/ui/analyse/src/crazy/crazyView.ts
index 0c9a55f113..c884dd59a3 100644
--- a/ui/analyse/src/crazy/crazyView.ts
+++ b/ui/analyse/src/crazy/crazyView.ts
@@ -13,7 +13,7 @@ export default function (ctrl: AnalyseCtrl, color: Color, position: Position) {
   if (!ctrl.node.crazy) return;
   const pocket = ctrl.node.crazy.pockets[color === 'white' ? 0 : 1];
   const dropped = ctrl.justDropped;
-  let captured = ctrl.justCaptured;
+  const captured = ctrl.justCaptured;
   if (captured) captured.role = captured.promoted ? 'pawn' : captured.role;
   const activeColor = color === ctrl.turnColor();
   const usable = !ctrl.embed && activeColor;
diff --git a/ui/analyse/src/study/studyCtrl.ts b/ui/analyse/src/study/studyCtrl.ts
index a8be4e2f61..b48a4fc623 100644
--- a/ui/analyse/src/study/studyCtrl.ts
+++ b/ui/analyse/src/study/studyCtrl.ts
@@ -631,7 +631,7 @@ export default function (
     nextChapter(): StudyChapterMeta | undefined {
       const chapters = data.chapters,
         currentId = currentChapter().id;
-      for (let i in chapters) if (chapters[i].id === currentId) return chapters[parseInt(i) + 1];
+      for (const i in chapters) if (chapters[i].id === currentId) return chapters[parseInt(i) + 1];
       return undefined;
     },
     setGamebookOverride(o) {
diff --git a/ui/analyse/src/study/studyMembers.ts b/ui/analyse/src/study/studyMembers.ts
index 475e6048b7..6597ee3b16 100644
--- a/ui/analyse/src/study/studyMembers.ts
+++ b/ui/analyse/src/study/studyMembers.ts
@@ -21,7 +21,7 @@ interface Opts {
 
 function memberActivity(onIdle: () => void) {
   let timeout: Timeout;
-  let schedule = () => {
+  const schedule = () => {
     if (timeout) clearTimeout(timeout);
     timeout = setTimeout(onIdle, 100);
   };
@@ -32,7 +32,7 @@ function memberActivity(onIdle: () => void) {
 export function ctrl(opts: Opts) {
   const dict = prop<StudyMemberMap>(opts.initDict);
   const confing = prop<string | undefined>(undefined);
-  let active: { [id: string]: () => void } = {};
+  const active: { [id: string]: () => void } = {};
   let online: { [id: string]: boolean } = {};
   let spectatorIds: string[] = [];
   const max = 30;
@@ -150,7 +150,7 @@ export function ctrl(opts: Opts) {
     },
     hasOnlineContributor() {
       const members = dict();
-      for (let i in members) if (online[i] && members[i].role === 'w') return true;
+      for (const i in members) if (online[i] && members[i].role === 'w') return true;
       return false;
     },
   };
diff --git a/ui/challenge/src/view.ts b/ui/challenge/src/view.ts
index d65b453dc3..4c0ea3b0b6 100644
--- a/ui/challenge/src/view.ts
+++ b/ui/challenge/src/view.ts
@@ -12,7 +12,7 @@ export function loading(): VNode {
 }
 
 function renderContent(ctrl: Ctrl): VNode[] {
-  let d = ctrl.data();
+  const d = ctrl.data();
   const nb = d.in.length + d.out.length;
   return nb ? [allChallenges(ctrl, d, nb)] : [empty(), create()];
 }
diff --git a/ui/dasher/src/dasher.ts b/ui/dasher/src/dasher.ts
index 587455069e..bded43ae90 100644
--- a/ui/dasher/src/dasher.ts
+++ b/ui/dasher/src/dasher.ts
@@ -50,7 +50,7 @@ export interface DasherOpts {
 export function makeCtrl(opts: DasherOpts, data: DasherData, redraw: Redraw): DasherCtrl {
   const trans = lichess.trans(data.i18n);
 
-  let mode: Prop<Mode> = prop(defaultMode as Mode);
+  const mode: Prop<Mode> = prop(defaultMode as Mode);
 
   function setMode(m: Mode) {
     mode(m);
diff --git a/ui/dgt/src/play.ts b/ui/dgt/src/play.ts
index 3d4f38d42d..f4c7b43227 100644
--- a/ui/dgt/src/play.ts
+++ b/ui/dgt/src/play.ts
@@ -123,7 +123,7 @@ export default function (token: string) {
         return new Promise<void>(resolve => {
           var output = '';
           for (let i = 0; i < arguments.length; i++) {
-            let arg = arguments[i];
+            const arg = arguments[i];
             if (arg == '*' || arg == ':') {
               output += arg;
             } else {
@@ -137,7 +137,7 @@ export default function (token: string) {
             }
           }
           //Added to keep on-screen log small
-          let maxLogBytes = verbose ? -1048576 : -8192;
+          const maxLogBytes = verbose ? -1048576 : -8192;
           let isScrolledToBottom = false;
           if (autoScroll) {
             isScrolledToBottom =
@@ -416,7 +416,7 @@ export default function (token: string) {
         //sleep 5 seconds and just listen to events
         await sleep(5000);
         //Check if any started games are disconnected
-        for (let [gameId, networkState] of gameConnectionMap) {
+        for (const [gameId, networkState] of gameConnectionMap) {
           if (!networkState.connected && gameStateMap.get(gameId).status == 'started') {
             //Game is not conencted and has not finished, reconnect
             if (verbose) console.log(`Started game is disconnected. Attempting reconnection for gameId: ${gameId}`);
@@ -468,7 +468,7 @@ export default function (token: string) {
       let index = -1;
       for (let i = 0; i < playableGames.length; i++) {
         //makeBoardFen return only the board, ideal for comparison
-        let tmpFEN = fen.makeBoardFen(gameChessBoardMap.get(playableGames[i].gameId)!.board);
+        const tmpFEN = fen.makeBoardFen(gameChessBoardMap.get(playableGames[i].gameId)!.board);
         if (verbose) console.log(`GameId: ${playableGames[i].gameId} FEN: ${tmpFEN}`);
         if (tmpFEN == lastLiveChessBoard) {
           index = i;
@@ -1152,7 +1152,7 @@ export default function (token: string) {
    */
   function padBeforeNumbers(moveString: string) {
     var paddedMoveString = '';
-    for (let c of moveString) {
+    for (const c of moveString) {
       Number.isInteger(+c) ? (paddedMoveString += ` ${c} `) : (paddedMoveString += c);
     }
     return paddedMoveString;
diff --git a/ui/lobby/src/setup.ts b/ui/lobby/src/setup.ts
index b2eae98dcb..1abe629599 100644
--- a/ui/lobby/src/setup.ts
+++ b/ui/lobby/src/setup.ts
@@ -115,7 +115,7 @@ export default class Setup {
   private hookToPoolMember = (color: string, form: HTMLFormElement) => {
     const data = Array.from(new FormData(form).entries());
     const hash: any = {};
-    for (let i in data) hash[data[i][0]] = data[i][1];
+    for (const i in data) hash[data[i][0]] = data[i][1];
     const valid = color == 'random' && hash.variant == 1 && hash.mode == 1 && hash.timeMode == 1,
       id = parseFloat(hash.time) + '+' + parseInt(hash.increment);
     return valid && this.root.pools.find(p => p.id === id)
diff --git a/ui/lobby/src/view/realTime/chart.ts b/ui/lobby/src/view/realTime/chart.ts
index f1c7b9dab6..51d10dd0a4 100644
--- a/ui/lobby/src/view/realTime/chart.ts
+++ b/ui/lobby/src/view/realTime/chart.ts
@@ -10,7 +10,7 @@ const ratingLog = (a: number) => Math.log(a / 150 + 1);
 function ratingY(e: number) {
   const rating = Math.max(1000, Math.min(2200, e || 1500));
   let ratio: number;
-  let mid = 2 / 5;
+  const mid = 2 / 5;
   if (rating == 1500) {
     ratio = mid;
   } else if (rating > 1500) {
diff --git a/ui/msg/src/view/interact.ts b/ui/msg/src/view/interact.ts
index 972115b152..9c7adbdb63 100644
--- a/ui/msg/src/view/interact.ts
+++ b/ui/msg/src/view/interact.ts
@@ -64,7 +64,7 @@ function setupTextarea(area: HTMLTextAreaElement, contact: string, ctrl: MsgCtrl
 
   // hack to automatically resize the textarea based on content
   area.value = '';
-  let baseScrollHeight = area.scrollHeight;
+  const baseScrollHeight = area.scrollHeight;
   area.addEventListener(
     'input',
     throttle(500, () => {
diff --git a/ui/nvui/src/chess.ts b/ui/nvui/src/chess.ts
index ebd11df5d5..3f925e8f08 100644
--- a/ui/nvui/src/chess.ts
+++ b/ui/nvui/src/chess.ts
@@ -304,7 +304,7 @@ export function renderBoard(
     return h('th', { attrs: { scope: 'row' } }, rank);
   };
   const doFileHeaders = (): VNode => {
-    let ths = files.map(file => h('th', { attrs: { scope: 'col' } }, file));
+    const ths = files.map(file => h('th', { attrs: { scope: 'col' } }, file));
     if (pov === 'black') ths.reverse();
     return h('tr', [h('td'), ...ths, h('td')]);
   };
@@ -344,14 +344,14 @@ export function renderBoard(
     }
   };
   const doRank = (pov: Color, rank: Rank): VNode => {
-    let rankElements = [];
+    const rankElements = [];
     if (boardStyle === 'table') rankElements.push(doRankHeader(rank));
     rankElements.push(...files.map(file => doPiece(rank, file)));
     if (boardStyle === 'table') rankElements.push(doRankHeader(rank));
     if (pov === 'black') rankElements.reverse();
     return h(boardStyle === 'table' ? 'tr' : 'div', rankElements);
   };
-  let ranks: VNode[] = [];
+  const ranks: VNode[] = [];
   if (boardStyle === 'table') ranks.push(doFileHeaders());
   ranks.push(...invRanks.map(rank => doRank(pov, rank)));
   if (boardStyle === 'table') ranks.push(doFileHeaders());
diff --git a/ui/palantir/src/main.ts b/ui/palantir/src/main.ts
index 5150fc68bc..e57d047a8e 100644
--- a/ui/palantir/src/main.ts
+++ b/ui/palantir/src/main.ts
@@ -145,7 +145,7 @@ export function palantir(opts: PalantirOpts) {
   }
   function closeDisconnectedCalls() {
     if (peer) {
-      for (let otherPeer in peer.connections) {
+      for (const otherPeer in peer.connections) {
         peer.connections[otherPeer].forEach(c => {
           if (c.peerConnection && c.peerConnection.connectionState == 'disconnected') {
             log(`close disconnected call to ${c.peer}`);
@@ -159,7 +159,7 @@ export function palantir(opts: PalantirOpts) {
   function allOpenConnections() {
     if (!peer) return [];
     const conns: any[] = [];
-    for (let peerId in peer.connections) {
+    for (const peerId in peer.connections) {
       const c = findOpenConnectionTo(peerId);
       if (c) conns.push(c);
     }
diff --git a/ui/puzzle/src/ctrl.ts b/ui/puzzle/src/ctrl.ts
index 018150a28c..61b58f64b6 100644
--- a/ui/puzzle/src/ctrl.ts
+++ b/ui/puzzle/src/ctrl.ts
@@ -24,7 +24,7 @@ import { Role, Move, Outcome } from 'chessops/types';
 import { storedProp } from 'common/storage';
 
 export default function (opts: PuzzleOpts, redraw: Redraw): Controller {
-  let vm: Vm = {
+  const vm: Vm = {
     next: defer<PuzzleData>(),
   } as Vm;
   let data: PuzzleData, tree: TreeWrapper, ceval: CevalCtrl;
diff --git a/ui/round/src/plugins/keyboardMove.ts b/ui/round/src/plugins/keyboardMove.ts
index e83ab7fb7a..5f5693bc50 100644
--- a/ui/round/src/plugins/keyboardMove.ts
+++ b/ui/round/src/plugins/keyboardMove.ts
@@ -122,7 +122,7 @@ function makeBindings(opts: any, submit: Submit, clear: () => void) {
 function sanToUci(san: string, legalSans: SanToUci): Uci | undefined {
   if (san in legalSans) return legalSans[san];
   const lowered = san.toLowerCase();
-  for (let i in legalSans) if (i.toLowerCase() === lowered) return legalSans[i];
+  for (const i in legalSans) if (i.toLowerCase() === lowered) return legalSans[i];
   return;
 }
 
diff --git a/ui/round/src/plugins/nvui.ts b/ui/round/src/plugins/nvui.ts
index a32c310322..a405c24da4 100644
--- a/ui/round/src/plugins/nvui.ts
+++ b/ui/round/src/plugins/nvui.ts
@@ -368,7 +368,7 @@ function destsToUcis(dests: Dests) {
 function sanToUci(san: string, legalSans: SanToUci): Uci | undefined {
   if (san in legalSans) return legalSans[san];
   const lowered = san.toLowerCase();
-  for (let i in legalSans) if (i.toLowerCase() === lowered) return legalSans[i];
+  for (const i in legalSans) if (i.toLowerCase() === lowered) return legalSans[i];
   return;
 }
 
diff --git a/ui/round/src/util.ts b/ui/round/src/util.ts
index 6c474c7e8d..b3c978b18d 100644
--- a/ui/round/src/util.ts
+++ b/ui/round/src/util.ts
@@ -80,7 +80,7 @@ export const noChecks: CheckCount = {
 
 export function countChecks(steps: Step[], ply: Ply): CheckCount {
   const checks: CheckCount = { ...noChecks };
-  for (let step of steps) {
+  for (const step of steps) {
     if (ply < step.ply) break;
     if (step.check) {
       if (step.ply % 2 === 1) checks.white++;
diff --git a/ui/site/src/captcha.ts b/ui/site/src/captcha.ts
index 096a41a136..8b82d2b8c4 100644
--- a/ui/site/src/captcha.ts
+++ b/ui/site/src/captcha.ts
@@ -11,7 +11,7 @@ lichess.load.then(() => {
         fen = cg.getFen(),
         destsObj = $board.data('moves'),
         dests = new Map();
-      for (let k in destsObj) dests.set(k, destsObj[k].match(/.{2}/g));
+      for (const k in destsObj) dests.set(k, destsObj[k].match(/.{2}/g));
       cg.set({
         turnColor: cg.state.orientation,
         movable: {
diff --git a/ui/site/src/checkout.ts b/ui/site/src/checkout.ts
index 1931dd0e33..0ade0556a8 100644
--- a/ui/site/src/checkout.ts
+++ b/ui/site/src/checkout.ts
@@ -67,8 +67,8 @@ export default function (publicKey: string) {
     $checkout.find('.service').html(lichess.spinnerHtml);
   });
 
-  let stripe = window.Stripe(publicKey);
-  let showError = (error: string) => {
+  const stripe = window.Stripe(publicKey);
+  const showError = (error: string) => {
     // TODO: consider a more sophisticated error handling mechanism,
     //       for now, this should work just fine.
     alert(error);
diff --git a/ui/site/src/coachForm.ts b/ui/site/src/coachForm.ts
index d28e9c81b5..afbeda1af7 100644
--- a/ui/site/src/coachForm.ts
+++ b/ui/site/src/coachForm.ts
@@ -28,7 +28,7 @@ lichess.load.then(() => {
       {
         html: 'Fill in basic information',
         check() {
-          for (let name of ['profile.headline', 'languages']) {
+          for (const name of ['profile.headline', 'languages']) {
             if (!$editor.find('[name="' + name + '"]').val()) return false;
           }
           return true;
diff --git a/ui/site/src/component/pubsub.ts b/ui/site/src/component/pubsub.ts
index dbca4a77f4..a44d7afdcb 100644
--- a/ui/site/src/component/pubsub.ts
+++ b/ui/site/src/component/pubsub.ts
@@ -1,4 +1,4 @@
-let subs: Array<() => void> = [];
+const subs: Array<() => void> = [];
 
 const pubsub: Pubsub = {
   on(name: string, cb) {
@@ -7,7 +7,7 @@ const pubsub: Pubsub = {
   },
   off(name: string, cb) {
     if (subs[name])
-      for (let i in subs[name]) {
+      for (const i in subs[name]) {
         if (subs[name][i] === cb) {
           subs[name].splice(i);
           break;
@@ -15,7 +15,7 @@ const pubsub: Pubsub = {
       }
   },
   emit(name: string, ...args: any[]) {
-    if (subs[name]) for (let i in subs[name]) subs[name][i].apply(null, args);
+    if (subs[name]) for (const i in subs[name]) subs[name][i].apply(null, args);
   },
 };
 
diff --git a/ui/tournament/src/sound.ts b/ui/tournament/src/sound.ts
index d21f306ae5..198091b1dc 100644
--- a/ui/tournament/src/sound.ts
+++ b/ui/tournament/src/sound.ts
@@ -7,14 +7,14 @@ function doCountDown(targetTime: number) {
   let started = false;
 
   return function curCounter() {
-    let secondsToStart = (targetTime - performance.now()) / 1000;
+    const secondsToStart = (targetTime - performance.now()) / 1000;
 
     // always play the 0 sound before completing.
-    let bestTick = Math.max(0, Math.round(secondsToStart));
+    const bestTick = Math.max(0, Math.round(secondsToStart));
     if (bestTick <= 10) lichess.sound.play('countDown' + bestTick);
 
     if (bestTick > 0) {
-      let nextTick = Math.min(10, bestTick - 1);
+      const nextTick = Math.min(10, bestTick - 1);
       countDownTimeout = setTimeout(curCounter, 1000 * Math.min(1.1, Math.max(0.8, secondsToStart - nextTick)));
     }
 
diff --git a/ui/tournamentCalendar/src/view.ts b/ui/tournamentCalendar/src/view.ts
index 125c78c489..651a07b00e 100644
--- a/ui/tournamentCalendar/src/view.ts
+++ b/ui/tournamentCalendar/src/view.ts
@@ -69,7 +69,7 @@ function fitLane(lane: Tournament[], tour2: Tournament) {
 function makeLanes(tours: Tournament[]): Lanes {
   const lanes: Lanes = [];
   tours.forEach(t => {
-    let lane = lanes.find(l => fitLane(l, t));
+    const lane = lanes.find(l => fitLane(l, t));
     if (lane) lane.push(t);
     else lanes.push([t]);
   });
diff --git a/ui/tree/src/ops.ts b/ui/tree/src/ops.ts
index 1a0b5db141..4f0918a87c 100644
--- a/ui/tree/src/ops.ts
+++ b/ui/tree/src/ops.ts
@@ -41,7 +41,7 @@ export function nodeAtPly(nodeList: Tree.Node[], ply: number): Tree.Node | undef
 
 export function takePathWhile(nodeList: Tree.Node[], predicate: (node: Tree.Node) => boolean): Tree.Path {
   let path = '';
-  for (let i in nodeList) {
+  for (const i in nodeList) {
     if (predicate(nodeList[i])) path += nodeList[i].id;
     else break;
   }
diff --git a/ui/tree/src/tree.ts b/ui/tree/src/tree.ts
index 3926a55049..3794eb6bb4 100644
--- a/ui/tree/src/tree.ts
+++ b/ui/tree/src/tree.ts
@@ -68,7 +68,7 @@ export function build(root: Tree.Node): TreeWrapper {
   function getCurrentNodesAfterPly(nodeList: Tree.Node[], mainline: Tree.Node[], ply: number): Tree.Node[] {
     var node,
       nodes = [];
-    for (let i in nodeList) {
+    for (const i in nodeList) {
       node = nodeList[i];
       if (node.ply <= ply && mainline[i].id !== node.id) break;
       if (node.ply > ply) nodes.push(node);