Enable @typescript eslint/prefer nullish coalescing eslint rule (#1420)

## Description:

Fixes #952 
Enabled @typescript-eslint/prefer-nullish-coalescing rule and worked
through every error, introducing ?? and ??= operators or disabling
errors with inline comments where appropriate, to the best of my
ability.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

g_santos_m

src/client/LangSelector.ts

@@ -99,7 +99,7 @@ export class LangSelector extends LitElement {
   private async initializeLanguage() {
     const browserLocale = navigator.language;
     const savedLang = localStorage.getItem("lang");
-    const userLang = this.getClosestSupportedLang(savedLang || browserLocale);
+    const userLang = this.getClosestSupportedLang(savedLang ?? browserLocale);
 
     this.defaultTranslations = this.loadLanguage("en");
     this.translations = this.loadLanguage(userLang);
@@ -110,7 +110,7 @@ export class LangSelector extends LitElement {
   }
 
   private loadLanguage(lang: string): Record<string, string> {
-    const language = this.languageMap[lang] || {};
+    const language = this.languageMap[lang] ?? {};
     const flat = flattenTranslations(language);
     return flat;
   }

src/client/PublicLobby.ts

@@ -110,7 +110,7 @@ export class PublicLobby extends LitElement {
 
     const teamCount =
       lobby.gameConfig.gameMode === GameMode.Team
-        ? lobby.gameConfig.playerTeams || 0
+        ? (lobby.gameConfig.playerTeams ?? 0)
         : null;
 
     const mapImageSrc = this.mapImages.get(lobby.gameID);

src/client/Transport.ts

@@ -245,16 +245,14 @@ export class Transport {
   }
 
   private startPing() {
-    if (this.isLocal || this.pingInterval) return;
-    if (this.pingInterval === null) {
-      this.pingInterval = window.setInterval(() => {
-        if (this.socket !== null && this.socket.readyState === WebSocket.OPEN) {
-          this.sendMsg({
-            type: "ping",
-          } satisfies ClientPingMessage);
-        }
-      }, 5 * 1000);
-    }
+    if (this.isLocal) return;
+    this.pingInterval ??= window.setInterval(() => {
+      if (this.socket !== null && this.socket.readyState === WebSocket.OPEN) {
+        this.sendMsg({
+          type: "ping",
+        } satisfies ClientPingMessage);
+      }
+    }, 5 * 1000);
   }
 
   private stopPing() {

src/client/Utils.ts

@@ -172,7 +172,7 @@ export function getAltKey(): string {
 
 export function getGamesPlayed(): number {
   try {
-    return parseInt(localStorage.getItem("gamesPlayed") || "0", 10) || 0;
+    return parseInt(localStorage.getItem("gamesPlayed") ?? "0", 10) || 0;
   } catch (error) {
     console.warn("Failed to read games played from localStorage:", error);
     return 0;

src/client/graphics/SpriteLoader.ts

@@ -91,9 +91,9 @@ const getSpriteForUnit = (unit: UnitView): ImageBitmap | null => {
   const unitType = unit.type();
   if (unitType === UnitType.Train) {
     const trainType = trainTypeToSpriteType(unit);
-    return spriteMap.get(trainType) || null;
+    return spriteMap.get(trainType) ?? null;
   }
-  return spriteMap.get(unitType) || null;
+  return spriteMap.get(unitType) ?? null;
 };
 
 export const isSpriteReady = (unit: UnitView): boolean => {

src/client/graphics/layers/EventsDisplay.ts

@@ -137,7 +137,7 @@ export class EventsDisplay extends LitElement implements Layer {
   }
 
   private toggleEventFilter(filterName: MessageCategory) {
-    const currentState = this.eventsFilters.get(filterName) || false;
+    const currentState = this.eventsFilters.get(filterName) ?? false;
     this.eventsFilters.set(filterName, !currentState);
     this.requestUpdate();
   }

src/client/graphics/layers/RadialMenu.ts

@@ -298,14 +298,14 @@ export class RadialMenu implements Layer {
         const disabled = this.params === null || d.data.disabled(this.params);
         const color = disabled
           ? this.config.disabledColor
-          : d.data.color || "#333333";
+          : (d.data.color ?? "#333333");
         const opacity = disabled ? 0.5 : 0.7;
 
         if (d.data.id === this.selectedItemId && this.currentLevel > level) {
           return color;
         }
 
-        return d3.color(color)?.copy({ opacity: opacity })?.toString() || color;
+        return d3.color(color)?.copy({ opacity: opacity })?.toString() ?? color;
       })
       .attr("stroke", "#ffffff")
       .attr("stroke-width", "2")
@@ -341,7 +341,7 @@ export class RadialMenu implements Layer {
         const color =
           this.params === null || d.data.disabled(this.params)
             ? this.config.disabledColor
-            : d.data.color || "#333333";
+            : (d.data.color ?? "#333333");
         path.attr("fill", color);
       }
     });
@@ -405,11 +405,11 @@ export class RadialMenu implements Layer {
       path.attr("stroke-width", "2");
       const color = disabled
         ? this.config.disabledColor
-        : d.data.color || "#333333";
+        : (d.data.color ?? "#333333");
       const opacity = disabled ? 0.5 : 0.7;
       path.attr(
         "fill",
-        d3.color(color)?.copy({ opacity: opacity })?.toString() || color,
+        d3.color(color)?.copy({ opacity: opacity })?.toString() ?? color,
       );
     };
 
@@ -625,7 +625,7 @@ export class RadialMenu implements Layer {
       this.selectedItemId = null;
     }
 
-    this.currentMenuItems = previousItems || [];
+    this.currentMenuItems = previousItems ?? [];
 
     if (this.currentLevel === 0) {
       this.updateCenterButtonState("default");
@@ -645,11 +645,11 @@ export class RadialMenu implements Layer {
           const disabled = this.params === null || item.disabled(this.params);
           const color = disabled
             ? this.config.disabledColor
-            : item.color || "#333333";
+            : (item.color ?? "#333333");
           const opacity = disabled ? 0.5 : 0.7;
           selectedPath.attr(
             "fill",
-            d3.color(color)?.copy({ opacity: opacity })?.toString() || color,
+            d3.color(color)?.copy({ opacity: opacity })?.toString() ?? color,
           );
         }
       }

src/client/graphics/layers/RadialMenuElements.ts

@@ -352,9 +352,9 @@ export const buildMenuElement: MenuElement = {
           : undefined,
         icon: item.icon,
         tooltipItems: [
-          { text: translateText(item.key || ""), className: "title" },
+          { text: translateText(item.key ?? ""), className: "title" },
           {
-            text: translateText(item.description || ""),
+            text: translateText(item.description ?? ""),
             className: "description",
           },
           {
@@ -401,7 +401,7 @@ export const boatMenuElement: MenuElement = {
 
     params.playerActionHandler.handleBoatAttack(
       params.myPlayer,
-      params.selected?.id() || null,
+      params.selected?.id() ?? null,
       params.tile,
       spawn !== false ? spawn : null,
     );

src/client/graphics/layers/TeamStats.ts

@@ -52,7 +52,7 @@ export class TeamStats extends LitElement implements Layer {
     for (const player of players) {
       const team = player.team();
       if (team === null) continue;
-      if (!grouped[team]) grouped[team] = [];
+      grouped[team] ??= [];
       grouped[team].push(player);
     }
 

src/client/graphics/layers/UILayer.ts

@@ -317,10 +317,16 @@ export class UILayer implements Layer {
         if (constructionType === undefined) {
           return 1;
         }
+        const constDuration =
+          this.game.unitInfo(constructionType).constructionDuration;
+        if (constDuration === undefined) {
+          throw new Error("unit does not have constructionTime");
+        }
         return (
           (this.game.ticks() - unit.createdAt()) /
-          (this.game.unitInfo(constructionType).constructionDuration || 1)
+          (constDuration === 0 ? 1 : constDuration)
         );
+
       case UnitType.MissileSilo:
       case UnitType.SAMLauncher:
         return unit.missileReadinesss();

src/client/jwt.ts

@@ -104,9 +104,8 @@ export type IsLoggedInResponse =
   | false;
 let __isLoggedIn: IsLoggedInResponse | undefined = undefined;
 export function isLoggedIn(): IsLoggedInResponse {
-  if (__isLoggedIn === undefined) {
-    __isLoggedIn = _isLoggedIn();
-  }
+  __isLoggedIn ??= _isLoggedIn();
+
   return __isLoggedIn;
 }
 function _isLoggedIn(): IsLoggedInResponse {